Neta Bahcall

[Begin session 1]

De Swart:

Twenty-fourth of November. Professor Bahcall, thank you very much for having me.

Bahcall:

Nice to have you here.

De Swart:

So, I haven’t seen an interview with you yet. Is that correct? On the, of, an oral history interview you did?

Bahcall:

You mean something that was published earlier?

De Swart:

Yeah.

Bahcall:

Oh, there – I don’t know that there was any formal historical thing written but there were a whole bunch of, you know, shorter interviews and some video tapes that were shown, and like the Hayden Planetarium in New York, and those kind of interviews. But I’m not sure of any, I don’t know if there was any published.

De Swart:

No? Okay. So, yeah. I would love to begin where we really started last time, too.

Bahcall:

Yes.

De Swart:

And, maybe a little bit about your undergraduate, when you started to study, and then how you moved into, into your graduate, and that was in Tel Aviv?

Bahcall:

Yes. Yeah. So, I was an – I’m from Israel. I was an undergraduate at the Hebrew University in Jerusalem in Israel, in physics and math. Israel didn’t have any astrophysics at that time.

De Swart:

Oh, yeah. Yeah. Yeah.

Bahcall:

That was a long time ago, in the ‘60s. I then did my master’s degree in nuclear physics at the Weizmann Institute in Israel. And, after that – that’s where I met John, my husband, who was visiting from Caltech at that time, at the Weizmann Institute. And we were married shortly thereafter. So, then, I moved with him to Caltech. He was already a junior faculty at Caltech. So, we moved to Caltech. I finished my PhD working really nuclear astrophysics, because I connected my nuclear physics work with astrophysics that was very strong at Caltech. And I did it mostly at Caltech, but I received it formally from Tel Aviv University.

De Swart:

Oh, okay. And was it common to go into nuclear physics after doing your, doing your undergrad in physics?

Bahcall:

Nuclear physics, at those days in the ‘60s, was very active and, an active field. So, it was a popular and exciting field in Israel and elsewhere. They built the big accelerators. And so, I was part of that, again. It was…

De Swart:

And who was your…

Bahcall:

In the ‘60s. So, therefore, my master’s degree, my advisor was Professor Goldring on the accelerator, on the nuclear physics low-energy accelerator.

De Swart:

Okay.

Bahcall:

Then at Caltech, for my PhD, I combined my nuclear physics with astrophysics and my PhD advisor was Professor William Fowler, who received the Nobel Prize for nuclear astrophysics. So, that was a very exciting time.

De Swart:

And, but was the transfer to astrophysics, was that something you already were interested in when you were doing your masters?

Bahcall:

I was not really thinking much about astrophysics, because in Israel, at that time in the ‘60s, we had no astrophysics. So, I was not even thinking that way. I was interested in physics and particle physics, nuclear physics. Those were the active fields in physics at that time, especially in Israel. Astrophysics was, of course, active, but not in Israel. So, I did not really know much about it. But, once I moved to Caltech, and that was like ‘65 or ‘66, then the whole field of astrophysics opened up to me, because the astrophysics was strong at Caltech, and I got to talk to many, many of the astronomers, including even Zwicky I met, and, of course, Maarten Schmidt, and many of the other astronomers. So, then I got very interested in astrophysics.

De Swart:

And can you remember what your husband was working on when he gave the talk?

Bahcall:

Yeah. He was then starting to work on a very important topic that really occupied him for the rest of his life. Not all the time, of course. He did many different things. But he was starting this solar neutrino project to understand, to try to detect neutrinos from the sun. And that’s what he started doing in the ‘60s. And he was at Caltech then. And that was a very exciting project. And, of course, it then became known as the solar neutrino problem. John did all the theoretical calculations of the expected neutrino rates from the sun and how many we can expect to detect, and then Ray Davis did the measurements, also in the ‘60s. And he detected them but there was discrepancy of a factor of three with the theory that John predicted, and the observations. And that factor of three became well known as the solar neutrino problem that lasted over three decades, you know, like thirty-five years, until finally it was found that a big discovery that neutrinos have mass. They oscillate. So, solar neutrino oscillation and they have mass and that’s something that nobody has expected at that time.

De Swart:

Yeah. That’s interesting.

Bahcall:

So, that was a very exciting – yeah, it was actually awarded the Nobel Prize for that discovery. So, it was very, very important. It really changed the physics books. Now we know that neutrinos have mass. They oscillate. So, it’s a whole big industry.

De Swart:

Yeah. That’s interesting. So, that was kind of the connection that already was between particle and nuclear physics, and astrophysics?

Bahcall:

That’s right. So, John started from physics too, and then moved to the astrophysics as well.

De Swart:

So, when you, so when was it, 1970, you moved to…

Bahcall:

Sixties. ‘66 I moved.

De Swart:

To Caltech?

Bahcall:

In 1966 to Caltech.

De Swart:

Ah. Okay. Yeah.

Bahcall:

Yeah. From Israel.

De Swart:

Okay.

Bahcall:

So, that’s almost fifty years ago. (Laughs)

De Swart:

That’s a long time. And then you already knew who your supervisor was going to be there?

Bahcall:

No. I didn’t know. I moved there. I started working doing some just research work in the lab and then within a short time after that I started working with Willie Fowler for my PhD.

De Swart:

How was the nuclear physics?

Bahcall:

Very strong.

De Swart:

Also at Caltech?

Bahcall:

Extremely strong at Caltech. Yeah.

De Swart:

Okay.

Bahcall:

So, that’s a nice thing that both astrophysics and nuclear physics was very strong at Caltech.

De Swart:

But in the end you decided not to go? Because who were the people in nuclear physics at Caltech?

Bahcall:

Oh, they were very good people, but I really decided to work with Willie Fowler, who did more of the theory rather than the experiments. There were very strong experimentalists. There was Charlie, uhm, Charlie Lauritsen, and then there was Tommy Lauritsen, actually from the Netherlands, I think.

De Swart:

So, how do you spell it, say it?

Bahcall:

Charlie Lauritsen. L-A-U-R-I-T-S-E-N.

De Swart:

Okay.

Bahcall:

And, Tommy Lauritsen, and Charlie Bounds, and Cavanaugh. So, there were many very strong experimentalists, and some theorists. And, Willie Fowler did kind of an overall theory combining the data of his experiments with his theories. But, I also talked with many of the astrophysicists…

De Swart:

Okay.

Bahcall:

…and that’s how I got the connection.

De Swart:

And, so, and had, did John have any influence on that, that you also went into astrophysics, or this connection, maybe?

Bahcall:

Well, the fact that I was at Caltech, I was there because I got married with John, and their astrophysics was very exciting. So, I think, you know, that was the main input. Yeah. John had something to do with it, but it’s just sort of the whole environment. All of a sudden, I saw people working on topics that I thought were enormously important about, you know, what’s out there in the universe. At that time Zwicky already suggested dark matter, which I found—and I got a chance to talk to him—and I found that very exciting. And Maarten Schmidt, also from Holland, just discovered quasars in the ‘60s. So, I talked to him and that was very exciting. People were trying to understand the structure of galaxies and what’s out there in the universe. So, those topics seemed enormously exciting to me. And still do.

De Swart:

And they are. Yeah.

Bahcall:

Yeah.

De Swart:

Yeah. And then, what was the first things you started to work on with him?

Bahcall:

So, for my PhD I did the nuclear astrophysics calculations about energy generation in stars, different nuclear excited levels, contribution of nuclear excited levels and how much they can affect the nuclear reaction rates that produce the energy in the stars. So, that was basically my PhD. So, we had several papers on that, again back in the middle of the ‘60s.

De Swart:

Yeah. I saw those. With…

Bahcall:

With Willie Fowler.

De Swart:

Yeah.

Bahcall:

So, but at the same time, I also worked some with Maarten Schmidt. We had some papers in the ‘60s totally on astrophysics and astronomy. He was working on quasars, and it was still a time that one didn’t understand exactly–well, we understood that quasars were, most people understood quasars were at cosmological distances, but really to understand better what type of galaxies, or how they related, or what were quasars. So, we did maybe two or three papers together with Maarten Schmidt and also with, I had some papers with Geoff Burbidge, who was visiting Caltech a lot. He was a close friend of my adviser, Willie Fowler.

De Swart:

Oh, okay.

Bahcall:

So, he visited Caltech a lot of that time.

De Swart:

Because… Where was he at the time?

Bahcall:

He probably was already in San Diego.

De Swart:

Okay.

Bahcall:

I think he was in San Diego, but he would come frequently to Caltech. I’m not sure, but I think that’s where he was. I don’t remember. But he would come frequently to Caltech to visit. He and Margaret Burbidge, his wife, to visit with Willie Fowler. They were very good friends. They wrote this very famous paper together, B2FH, which was like the bible of the nuclear astrophysics, Burbidge, Burbidge, Fowler, and Hoyle.

De Swart:

Yeah.

Bahcall:

So, we all used it all the time.

De Swart:

Okay.

Bahcall:

So, I talked to him quite a bit about science and we had some paper together. Maybe also with Maarten Schmidt. I don’t remember. But it was Burbidge and Maarten Schmidt.

De Swart:

And that was during your graduate, during your PhD?

Bahcall:

That was during my PhD and then I stayed one more year as postdoc at Caltech. So, it was partly during the PhD and partly during the postdoc, a one-year postdoc, at Caltech. So, with Maarten Schmidt we worked on quasars and trying to find out where they were located. So, I remember Maarten Schmidt took the images of quasars in three-color bands so you can try to find quasars, because they were strongly a UV excessive. So, I was looking at all these three-image colors trying to find all the quasars based on the UV image.

De Swart:

Oh, okay.

Bahcall:

So, I did. Found a whole bunch of quasars and they were even named NAB some numbers, 1053. NAB was Neta Assaf-Bahcall. So, when we wrote the paper he said, “Just, well we’ll name them NAB.” (Laughs) So, there’s a whole bunch of quasars that are named with my name, which is really funny. (Laughs)

De Swart:

Oh, really? I didn’t know that. That’s very funny.

Bahcall:

That’s very funny.

De Swart:

Yeah.

Bahcall:

And then people will follow it up to take spectra, so they would frequently see papers NAB1053 plus something. And, you know, it was very funny.

De Swart:

And, so when was this, the quasar?

Bahcall:

It must have been in the ‘60s, maybe later ‘60s.

De Swart:

And what was the status of the quasars?

Bahcall:

So, one thing we did is we found the quasars. So, we had a list of newly found quasars, based on the UV excess.

De Swart:

Okay. Which you published?

Bahcall:

Which we published with Maarten Schmidt. And then, we used those quasars and I tried to look to see where they were located specifically, like in groups of clusters of galaxies, or were they like isolated, no galaxies around it. That’s why I got interested in the clusters.

De Swart:

Oh, okay. That’s interesting.

Bahcall:

Then went from there. So, and we have a paper on where the quasars are located. Also with Maarten Schmidt.

De Swart:

And the redshift, was it already clear what this, what the redshift of these quasars was?

Bahcall:

So, that was followed up and it’s possible that – I don’t remember now – but Maarten Schmidt may have obtained the redshift, and it may be in the papers. I found it from the imaging, and he may have then gone to take the spectra on the big telescope. And I think the redshift was probably published in that paper.

De Swart:

Yeah. But no theories? Just data? No theory about what this quasar might be?

Bahcall:

Well, the whole idea of what quasars are came… I think it came after that, with a supermassive black hole equation. I think it must have come shortly thereafter.

De Swart:

Okay.

Bahcall:

It was either shortly before or shortly thereafter.

De Swart:

Okay, the…

Bahcall:

I think it was probably later.

De Swart:

Uhm-hmm. That’s interesting. So…

Bahcall:

Of course, that was in, still in the early days of quasars. It was still somewhat controversial. Some people still kept talking about quasars not being at cosmological distances. But…

De Swart:

Well, that was, still at the time you were working on it?

Bahcall:

Not really, but there were a few people who fought for a very long time, like Chip Arp. Most people pretty clearly understood they were cosmological distances, and they were something happening in galaxies. But that’s what we were trying to find. “Are they really in galaxies? Are they clustered like galaxies? Are they clusters or not?” So, that’s the work that I did.

De Swart:

Oh, that’s really interesting. Because, you said from this kind of spectacular discovery of quasars and that you were looking into this with Maarten Schmidt, you got interested in clusters?

Bahcall:

Yeah. That was one way that I got into clusters. And I was also taking, a little bit, not so much, with Zwicky, who was still there. He was already retired, but he would still come, and he was older, and I would talk to him and he would tell me about the clusters. You know, he did the whole big survey of clusters with the Palomar Sky Survey. So, the Zwicky group and cluster catalog. So, we talked about that. And he would stop by and see what I was doing. And, when I told him, “I’m looking for quasars in the clusters,” so we would talk some about groups and clusters. So, we talked about his catalog. He talked some about dark matter that he, of course, really discovered in clusters in the 1930s. People didn’t pay attention. Well, they paid attention to it, but they didn’t believe in it, many people, for several decades, until the ‘70s. But we talked about that. So, all of that got me interested in clusters and really to try and understand what’s out there.

De Swart:

So, and Zwicky’s office was near the – that was at the time you had a PhD?

Bahcall:

Yes.

De Swart:

So, you were with the graduates in the office somewhere?

Bahcall:

So, I was, again, my adviser, Willie Fowler, was in the physics building, but the astrophysics was very close by. It’s all in the same division. So, I would frequently go there for colloquium, and for talking with people. And when I started doing some of the work with Maarten Schmidt, I would sit down in the basement, partly with the graduate students, but partly where they had this measuring thing that you could look. It was photographic plates. It’s not like today on the computer. So, it was photographic plates and we looked with a microscope to look at things on the photograph. So, I did a lot of that there, too. So, that’s how I found all these quasars by the UV axis, because there were like three images in different colors, one next to each other.

De Swart:

Oh, okay.

Bahcall:

So, I would have to look to see which one of all of them had UV excess. They were strong here but less strong here, compared to. And that’s how I found all these NAB quasars.

De Swart:

Oh, wow.

Bahcall:

So, it’s a whole list of them.

De Swart:

So, you were constantly comparing these images?

Bahcall:

Yes. So, I would locate – it’s called a scanning microscope. So, it was a microscope, or traveling microscope. I don’t remember. So, looking at the whole big plate to find them, and then looking to see if there were clusters around them and so on. So, I was working doing all of that and Zwicky’s office was in the basement there. I think it was in the basement, close to it. So, he would stop by from time to time to ask what I was doing and check. So, that’s what happened.

De Swart:

That’s interesting.

Bahcall:

Interested.

De Swart:

Yeah.

Bahcall:

He was always very—even at his older age—he was very interested to know what people were doing, and talk about what he was doing.

De Swart:

Because he would go to all graduates? And…

Bahcall:

I don’t remember if he went to all graduates, but he certainly came to talk to me and I’m sure other graduates. Yeah.

De Swart:

So, I heard this from Joe Kormendy, who also said that.

Bahcall:

Oh, really? That he did the same thing?

De Swart:

That he visited in his office. Yeah.

Bahcall:

Yeah. Yeah. Yeah, he was just very, a real scientist—very, very, smart scientist. Of course, he made many of the earliest discoveries, well before anybody else would think about it. So, he was a very, very smart scientist, and a lot of intuition, and he was very curious, and he wanted to know about, understand science all the way, you know, even to the older age.

De Swart:

Yeah. And how was his book received? Were you there? This is the Morphological Astronomy.

Bahcall:

Yes. Well, I used it quite a bit. I think it was very popular. You know, Zwicky had a reputation of being extremely bright, a very good astrophysicist who made some of the most brilliant discoveries, in many things, but he was a little, at least the reputation was that he could be a little abrasive and sort of attack people if they didn’t agree with him. You know, I don’t know that for a fact, because I never had that experience with him. He was always very nice and kind to me and interested in what I was doing. But that’s the reputation that people talk about. When I am sometimes asked to talk about these early days, I talk about Zwicky in a very different way.

De Swart:

Yeah. That’s interesting that he was so – and do you remember any talks at the time that were given on this subject that you got more interested in clusters?

Bahcall:

Well, there were certainly many talks given on quasars because that was when quasars were discovered. And, yeah, I’m sure I don’t remember, but I’m sure there were talks about it.

De Swart:

Because you would visit both physics and…

Bahcall:

Yeah. I was constantly between the two, and maybe even for the postdoc year I may have spent a little more time in astro. But, you know, I still was in both of them.

De Swart:

Yeah. But your office was in physics?

Bahcall:

I think when I was doing the PhD, my office was in physics. But I probably also – well, you know, certainly I remember in astro, I was in the basement doing the work on, I guess it was the lab, or where many of the students were. So, I certainly spent quite a bit of time there.

De Swart:

So, let me check. So, the clusters, do you remember your first work on it, specifically? What were you interested in when you started looking at the clusters? Was there a specific…

Bahcall:

I think the first one that I remember – again, I would, I will take a look at it to refresh my memory, and I didn’t have a chance to do that. But my earliest papers on clusters, I think I was interested and that followed from what I did with Maarten Schmidt, trying to understand what the galaxies, where the quasars are located and are they in clusters. So, I saw a whole bunch of, tried to find them, look at the clusters and I tried to understand how the galaxies are distributed in the clusters. It was not yet well known. So, what I did is tried to measure the density, how galaxies are distributed in the cluster, the density profile, and measured it for a whole bunch of clusters and tried to see the similarities of difference among the different clusters in the density profile from small-scale, how far the galaxy distribution goes, where does it sort of cut down to the edge of the noise, to the background. So, just to understand the density profile in clusters. And that was some of the first, and I did it for a whole bunch of clusters.

De Swart:

Ah. And, then with this you used the…which catalog were you using for this? Was it Zwicky’s catalog? Were there any others?

Bahcall:

I was measuring it directly on the Palomar, again with this traveling microscope. And, in fact, when I moved here around 1970 when John took the professorship at the Institute for Advanced Study, and I started here as a postdoc. I remember even getting… we built a little of this traveling microscope, because that’s how we did things at that time, looking still at plate. So, I had a whole bunch of plates. We had the whole Palomar Sky Survey plates here. So, I…

De Swart:

In Princeton?

Bahcall:

In Princeton. Many places had them. It was a set of, people, you know, universities bought that, because that’s where the data was. Just like today you have in the computer. But we had the whole set here and I was measuring – I don’t know if they had the glass plates or just the prints – but, I was measuring on glass plates. So, we built here the little traveling microscope, just a light bulb, with light coming under and then a traveling microscope where you could look at it and then measure the exact position. It’s amazing to think about it. But those were the days then. So, I measured the density profile of the galaxies in the clusters, the shape of the density profile, looking at bright and faint galaxies, and how they are distributed and so on, which was very interesting.

De Swart:

And you knew…

Bahcall:

I did a whole bunch of papers on that.

De Swart:

Uhm-hmm. And you knew about these clusters, where they were from?

Bahcall:

Yes. I think that must have been – well, there were two famous catalogs at that, recent famous catalogs. One was a Zwicky catalog, and the other one was the Abel, George Abel catalog, who was also at Caltech at that time. And he did his catalog, I think, in ‘58, and Zwicky was about – I mean, it was comparable time. I forgot the dates for the Zwicky catalog. So, Abel just looked at the richest clusters, and actually cataloged them extremely well for what one could do by eye, you know, the microscope. He did an amazingly good job. And, in fact, I used the Abel catalog for quite a long time after that. In fact, some of the big discoveries that we made were based from the Abel, using clusters in the Abel catalog. Now, of course, we use all the automated surveys like Sloan and so on. But the results are pretty consistent with what we obtained many years ago from the Abel catalog, at least the basic results. You cannot do today precision measurements or cosmology from them, but the basic results we found from the Abel catalog is enormously impressive, that agree with what we did later. There’s much more precision with automated catalogs.

De Swart:

And who was, at the time you started working, who were the big names working on clusters of galaxies?

Bahcall:

Well, Zwicky was, I don’t think was doing very much, or he was maybe giving – I don’t remember exactly when Zwicky passed away. But Abel was working on it. His, Abel’s, biggest contribution was his catalog. That was in the late ‘50s. But then he did a bunch of other things, trying to get the luminosity function of the clusters, and so on. So, he did some more things. And, unfortunately, he died at a young age. Yeah. So, he was a very good astronomer. He was at UCLA later on. So, he did the work on clusters. And, well people who looked at clusters or at least at the clustering of galaxies, not necessarily clusters (inaudible) where there was the Shane and Wirtanen big catalog from the Lick Observatory.

De Swart:

Shane and…

Bahcall:

Shane and Wirtanen. That’s what the map, it’s the map – oh, I used to have this big map of the Shane and Wirtanen from the Lick galaxy map. That’s what Jim Peebles used to try and determine – I used to have it on my office wall for a long, long time. It’s a big map with all the little galaxy dots. And that’s what Jim Peebles used, I think, in the ‘80s already, maybe late ‘70s, to measure some of the clustering of galaxies, galaxy correlation function. That maybe came a little bit later.

De Swart:

And other institutes at the time, or universities where people were working on cluster of galaxies, or was it mainly at Caltech where this was happening?

Bahcall:

Caltech was, UCLA was George Abel. Probably the Lick Observatory, they did some work, more of the clustering of galaxies. They didn’t necessarily call it “clusters”. Then there was some work on superclustering. There was a big question on clustered, superclustered, and that’s something that we did quite a bit of work on later on. De Vaucouleurs worked on it and, of course, Zwicky worked on it. Some people at the Lick Observatory. Jim Peebles did. I think those were some of the main places.

De Swart:

Because it’s a…

Bahcall:

It was a new, relatively new field.

De Swart:

That’s interesting. Yeah.

Bahcall:

Yeah.

De Swart:

So, new that there were only a few institutes or people even?

Bahcall:

Jim Gunn did some work on it. Bev Oke at Caltech. Caltech people worked on it. At Caltech it started with Zwicky and then Abel.

De Swart:

Yeah. So, at Caltech it was a very fashionable topic?

Bahcall:

It was. I mean, there were many topics fashionable. It was not the – I mean, quasars probably were the biggest thing at that time at Caltech. And, of course, very fashionable at Caltech were stars.

De Swart:

Okay.

Bahcall:

At that time, you know, Jesse Greenstein. There was a lot of work on stars, stars and stellar evolution, the whole stellar observation. But Zwicky is the main one who worked on it. And they did the big Palomar Sky Survey. So, from that Zwicky started getting to see how the distribution of these groups and clusters, and then Abel did the cluster catalog, and I came a little after that. So, that’s where I got very interested.

De Swart:

And when was the Sky Survey? Do you remember that? I can look it up.

Bahcall:

Yeah. It was, at that time I think, in the ‘50s, ‘60s. That’s when it was. You know, it took quite a while to complete it. It was mapping the whole image. Then, they published the plates and then there was Sky Survey prints. And, I know we had the whole collection of prints. I’m not sure that we had the whole plates, but I may have ordered some of the plates.

De Swart:

Uhm-hmm. So, and then you moved to Princeton?

Bahcall:

Yeah. Around ‘70 or ‘71.

De Swart:

And, you were, was there a specific cluster you were looking at at that time, or whom did you start working with here?

Bahcall:

Well, we moved here basically because John was offered a professorship at the Institute for Advanced Study, and we visited and he liked the place. It was a very good position, very good research possibility, very good place to build up astronomy. In fact, the one, the only astronomer that, the astronomer before him at the Institute for Advanced Study was Strömgren.

De Swart:

Okay.

Bahcall:

And he, at the end, I think may have left, or retired, or something and they had no astronomy. So, they recruited John to come at a very young age. He was only early thirties.

De Swart:

Oh, wow.

Bahcall:

So, he was very, very – now that I think about it, you know, he was very young to come as a professor there. So, and then I was offered a postdoc position here, because I just finished my PhD plus one year of postdoc. So, I started as a postdoc here.

De Swart:

At the – here?

Bahcall:

At this department and I’ve been here ever since. (Laughs) So, I’ve been here more than forty years.

De Swart:

Wow. And, and…

Bahcall:

For a few years I commuted. I took a position at the Space Telescope Science Institute, Hubble Space Telescope Science Institute, when they first opened it in ‘83, to head the science observation probe on there.

De Swart:

Oh. Okay.

Bahcall:

So, I was there commuting from here about from ‘83, for about six years.

De Swart:

Okay. Where was that?

Bahcall:

That’s in Baltimore.

De Swart:

Oh, okay.

Bahcall:

That’s on the campus, right touching the Johns Hopkins University campus. So, I commuted. Not everyday, but I commuted, stayed a few days and then came back. And, then came back here on the faculty.

De Swart:

And who were, so here they did have an Astronomy Department close to the Institute? So, who were the main people here?

Bahcall:

Right. So, the Institute, they didn’t have astrophysics. And, you know, there it’s only theoretical, no observation, and they got John and he really created one of the best theoretical astrophysics groups. It’s partly postdocs and visitors. There were no students then. It’s probably ranked number one in the world. And many Dutch astronomers. You know (inaudible) was a postdoc of his. Howie van der Laan would visit frequently. So, he had a lot of connection with, with Holland. Actually, the new director of the Institute now is Robert…

De Swart:

Robert Dijkgraaf.

Bahcall:

How do you say it?

De Swart:

Dijkgraaf.

Bahcall:

Dijkgraaf.

De Swart:

Dijkgraaf, which is…

Bahcall:

Dijkgraaf.

De Swart:

“Dikj” means the dikes, which hold water, and “graaf” means “count”.

Bahcall:

Okay.

De Swart:

So, it’s the count of…

Bahcall:

So, say it again.

De Swart:

Dijkgraaf.

Bahcall:

Dijkgraaf. Yeah. I never know how exactly to pronounce it. Dijkgraaf. (Laughs) Okay. So, yeah, he’s wonderful. He is great. Yeah. So, I started here, and the department was very small. I don’t think there was anybody working on clusters.

De Swart:

Oh, wow.

Bahcall:

So, I was just working on that. We had Lyman Spitzer here. Of course, he was doing interstellar medium and plasma astrophysics. And there was Martin Schwarzschild, who was doing stellar evolution, and then later galaxy dynamics. And Jerry Ostriker was here. I don’t remember what he was doing at that time. But it was not, not clusters, or not cosmology.

De Swart:

Probably rapidly eroding stars.

Bahcall:

Yeah. I think it was the stars. It was neutron stars. It was – yeah. And then he, he did many things in his life. And so was John. He did many – solar neutrinos were kind of one thing that stayed, but he did many, many things, stars, and galaxies, and dark matter, and quasars. Many things.

De Swart:

So…

Bahcall:

So, yeah. I was just working. I was a postdoc. I was working, talking with the other people, but I was really working on my own and, and maybe with some other collaborators when I, you know, from other institutes.

De Swart:

Okay. Yeah. Yeah. And so looking at some of your papers on the clusters. So, let me – for example the, you did a study of the Coma Cluster?

Bahcall:

Yes.

De Swart:

In ‘73, I found.

Bahcall:

Yes.

De Swart:

Do you remember what were the specifics? Were you interested in the mass or what you said, the distribution of it?

Bahcall:

I think I recollect – again, I would need to look at it again. My recollection is initially I wanted to look at the just overall distribution of galaxies and of light, and see how it compares from the different clusters. And, at that time, we represented it by an isothermal distribution to see if it fit it, and how the parameters fit. And there was one parameter that is called “core radius”. So, the density profile, if you know the density is a function of scale, of course, the density profile falls down with scale. So, if you have high density in the regions, low density of galaxies, and of stars, and of light, in the center, in the middle. So, it falls down. But there was some flattening near the center, which was called a core radius. So, there was introduced a parameter – the thermal distribution, it just falls like out of the -2. So, this was called the King Distribution, or the Negative Thermal Distribution, with a core radius. So, I was trying to fit it and get a core radius for the different clusters. And for a while, I tried to see if the core, or thought, if that was very popular at that time. But I thought, and other people thought too, for probably a decade, maybe longer, that the core radius was very similar in all clusters.

De Swart:

Oh, okay.

Bahcall:

So, we tried to see if that can be used. If you have a real fixed core radius, if it can be used for cosmology. And that’s where I started getting interested in cosmology.

De Swart:

Oh, okay.

Bahcall:

Well, both from the dark matter but also, we used core radius. Now we understand that that’s not – it may be similar but it’s not, not a standard candle. It’s not a good cosmological trace. But, for a while there, in the ‘70s, I did a lot of this core radius and gave a lot of talks about, “Can we use core radii to determine cosmology?” We even used it to try to put some estimates on cosmological parameters.

De Swart:

In what sense? Because you knew that it was constant?

Bahcall:

Yes. If we think we measured it to be constant for many clusters, and then even those slightly higher where just it seemed the same, then even x-ray emission from clusters came about. And they also measured it and followed what we said for the core radius was constant. So, then we used it as constant scale. If you know a constant scale you can get the cosmology. That’s like what the supernovae have done recently. Not with scale, with luminosity, but you could do it with scale.

De Swart:

Yeah.

Bahcall:

Like the baryon acoustic oscillations I’m doing now.

De Swart:

So, and how was the status of cosmology at that time? Were people, were people…

Bahcall:

Not very good. (Laughs)

De Swart:

No?

Bahcall:

No. No. We didn’t really know very much. You know, that was early ‘70s. We didn’t even really know that dark matter existed, except for Zwicky and those who believed in that. So, many people thought, “Well, we just have the baryons, very little amount of matter, and that’s it.” So, we didn’t know about dark matter. We, of course, didn’t know about dark energy. We really didn’t understand how structure formed.

De Swart:

And the Big – how was the evolution of the universe? Was it already dominated by the theory of the Big Bang?

Bahcall:

Yeah. The Big Bang was already reasonably well established. You know, you always have some people that question it. But because the cosmic microwave background radiation was already discovered also in the ‘60s. So, that sort of confirmed the whole Big Bang for most people.

De Swart:

Yeah.

Bahcall:

So, we knew the whole Big Bang, but we really did not understand how structure formed. You know, if you don’t know that there’s dark matter you don’t know that – well, dark energy is a whole separate sort –even if you don’t know dark matter, you don’t understand how structure formed because you don’t have enough gravity. So, the state of cosmology, people were trying to measure, you know, q0, which Sandage suggested, again for, not Caltech, from the Mount Wilson Observatory, a very famous cosmologist. And I knew him when I was at Caltech.

De Swart:

Oh, okay.

Bahcall:

It’s also in Pasadena. So, I talked to him many times as well. And he was kind of one of the fathers of cosmology at that time. So, he suggested various cosmological tests to measure what used to be called “heat reduce” I think, or q₀, which is deceleration parameter. So, people tried to, we tried to use this core radius of – I tried to use core radius of clusters to measure q₀. People didn’t know what, what q₀ was. I mean, and it was way too early to measure this. So, it was a, still at a very primitive age, but still much more advanced than twenty years before when we knew nothing. So, it really was a very exciting time to try and understand what’s going on in the bigger universe.

De Swart:

And what would you say was the most important thing that happened that people were doing cosmology again and that you started to do cosmology again?

Bahcall:

Yeah. So, I started doing cosmology roughly then in the ‘70s, I think. I think we tried to use the core radius, which really – we used it to get cosmological parameters, but now looking back, no, it was not a standard candle. So, whatever you get it’s not, not very good.

De Swart:

Yeah.

Bahcall:

But that’s when I started working in using clusters for cosmology and that has developed quite strongly over the next decade, two decades. Even now I use not only clusters, but all, many things, distribution of galaxies, distribution of light, the evolution of structure, to try and constrain cosmology. And the clusters have been probably the most powerful tool in constraining cosmology. Especially in the early days, it was the main tool to constrain cosmology.

De Swart:

And do you have…

Bahcall:

In the ‘80s, starting in the ‘80s, the ‘80s and ‘90s that was the best tool at that time. And then, of course, came the supernova and the microwave background, and that together really made it much more precise, putting it all together. They are still extremely powerful tool for cosmology, but in the ‘80s and ‘90s we used them as that was the most powerful tool for cosmology, and we used them to get Omega matter, which we showed them that it was only about .25, twenty-five of the critical density, which is, many people didn’t believe it and many talks that I gave all these people said, “But it has to be one. It’s impossible. How can it?” Well, so now we, no one can say that all of our measurements with clusters, the cluster correlation function, the cluster mass function, tracing mass versus light, we show that all of those show that Omega matter is only about between .2 and .3. And there were many people who didn’t believe or questioned it, or…

De Swart:

Do you have any examples of the talks you gave or people who you met?

Bahcall:

Oh, many. Many. There are many papers with those talks.

De Swart:

Yeah.

Bahcall:

Yeah. So, many, some of the papers on the talks are on that. It was big debates in the ‘80s and ‘90s, until people finally started accepting it. After the – even before the supernova, people still started agreeing that Omega matter, based on our measurements, that Omega matter is probably low. And then, the supernova came and then it fit very well. And then later came the CMB and then it fit everything. I have this paper, which I don’t know if I have still, many calls. It’s a really kind of popular paper. People like it. That’s from the ‘90s of a cosmology, “The Cosmic Triangle”. And, you can have a copy if you want it.

De Swart:

Oh, wow. Thank you.

Bahcall:

That’s “The Cosmic Triangle” we wrote, I think it’s 1999. So, that’s before the CMB.

De Swart:

Yeah.

Bahcall:

And we showed that the Lambda CDM is the best model. So, the CMB didn’t really discover Lambda CDM. They just confirmed it, just with much higher precision, but we show here. And that was a popular thing in the ‘90s. So, many of our papers showed that Lambda CDM is the best model.

De Swart:

Oh, yeah.

Bahcall:

Before the CMB.

De Swart:

And would you say that – or, let me put it this – how important was cosmology in making, like other interests in cosmology, in making clusters of galaxies interesting?

Bahcall:

Very important. Very, very important. People got very interested in clusters of galaxies, sort of, you know, in the ‘70s and ‘80s. The ‘70s, what happened is the first x-ray satellite, I think it was Uhuru, was starting to discover a lot of x-ray emission from clusters. Then it became – and, I worked on that, too – so, it became clear and just hot gas in all clusters. So, it became clear that clusters were interesting. It’s not just the galaxies, but you have all this hot gas that weighs much more than the galaxies. And then, in the ‘70s, the dark matter became clearer, so clusters, you know, became clear that we have a lot of dark matter. So, the ‘70s were very important and I did a lot of work in the ‘70s, both on the x-ray emission in clusters, tried to understand what it’s coming from, on the dark matter in clusters, and then, in the ‘80s, we did the cluster correlation function, which was, that was a big discovery in ‘83, with Ray Soneira, who is at Princeton. He was a student of Jim Peebles, and then he was a postdoc with John at the Institute. And he did very famous work with John on understanding the structure of our galaxy. There was Bahcall, Soneira, a famous paper on the structure of galaxies. And we did a very famous paper, Bahcall and Soneira, on the cluster correlation function.

De Swart:

Oh, yeah. Yeah. Yeah.

Bahcall:

He’s very good, a very good scientist.

De Swart:

And, where he is now?

Bahcall:

He left astronomy, unfortunately. He was very interested in computers, and he has his own company, a very famous image processing or image interface with computers.

De Swart:

Oh, okay.

Bahcall:

But we are in touch and we’re still good friends.

De Swart:

Oh, that’s nice. So, okay, so now I’m going into this dark matter notion.

Bahcall:

Let me just say that I have – I don’t what your plans are, but I have student appointment at three.

De Swart:

Oh, okay.

Bahcall:

At three. And then at three-thirty.

De Swart:

Oh, okay.

Bahcall:

So, I don’t know what your plans are. I’ll be free after those, okay, but I’ll need to finish because I’m advising some students.

De Swart:

Okay. No. We could, we could also go further, actually that – because what time are we now?

Bahcall:

It’s about five minutes.

De Swart:

Yeah. Five? Oh, okay.

Bahcall:

To three.

De Swart:

So, if you’re okay with that, maybe we can just, we can finish up afterwards?

De Swart:

Sure.

Bahcall:

So, we can go on until three, but then I’ll have to stop and stalk with a student. And then if you want, maybe around four o’clock or so?

De Swart:

Four? I’ll be back.

Bahcall:

Yeah. We can, we can finish up then.

De Swart:

Perfect. Thanks. So, now, we can…

Bahcall:

So, this paper summarizes some of the work in the ‘90s on clusters and other matter. But it has a little bit of a summary.

De Swart:

So, so…

Bahcall:

And it’s a very popular paper. It has a lot of… many people were reading it and using it, and we introduced this cosmic triangle as a way to put the picture of cosmology-based, constraints on cosmology based on the various methods.

De Swart:

Which got pretty famous, right? The…

Bahcall:

It got famous. Of course, yeah, many people use it. They show it. It’s in some of the books. They show it in classes. Yeah.

De Swart:

Presentations? And yeah. Yeah.

Bahcall:

Uhm-hmm.

De Swart:

I’ve seen it. Yeah.

Bahcall:

Yeah. I showed it a lot in many presentations, and talks, and papers. It’s an interesting, if you read it, you’ll understand, it’s an interesting way to show all the main three parameters: Omega matter, Omega Lambda, and Omega curvature, in a triangle. Because when you put each point there it satisfies Einstein’s equation, Freedman’s equation. Because the sum of all the three is one.

De Swart:

Ah, yeah.

Bahcall:

So, you start getting to understand the physics of when the universe is expanding forever or collapsing, or, you know, accelerating. So, it’s a, it’s a really neat way of pulling it together.

De Swart:

And, this “fate of the universe” topic, was that already interesting in the beginning of the ‘70s when you were starting here in Princeton?

Bahcall:

Yes. I think that that was when we were trying to measure, I was trying to measure, and others, q₀.

De Swart:

Oh, yeah.

Bahcall:

The q₀ is the deceleration, whether the universe decelerates or expands forever. So, yeah. So, that was already getting popular in the ‘70s, although nobody understood, even at that time, about dark matter. It was just starting to understand dark matter.

De Swart:

And this, but this notion of…

Bahcall:

And that’s where Mort Roberts and Rubin, and so on, but of course Zwicky started that way before.

De Swart:

And this notion of virial discrepancy was this used often when you started to, to look into clusters of galaxies?

Bahcall:

Yes, it was, again it was very controversial until Roberts and others showed the flat rotation curves.

De Swart:

When, would you say, was that?

Bahcall:

That was in the early ‘70s it started. So, there was a flowing of work by Roberts and, (inaudible) from the radio Shostak and (inaudible), and then Vera Rubin with the optical rotation curve.

De Swart:

How did you get to know these, these papers? Did you just get them as preprints? Or did you…

Bahcall:

Yeah. From preprints, from the APJ, from literature, from talks, from conferences. So, that’s when it became very popular.

De Swart:

It’s interesting that astronomy and astrophysics and cosmology were so cohesive...

Bahcall:

Yes.

De Swart:

In a sense, in the beginning, that you knew all this. And that…

Bahcall:

Well, it was partly connected. I mean, a lot of this was astronomy. Because you’re measuring all of that. But, you know, once you understand that there is dark matter, when it’s like confirmed or more strongly believed, then I started thinking, “Well, how else can we measure it much better? And, if clusters have a lot of dark matter, how do we measure it?”

De Swart:

Oh, yeah. Yeah. Yeah. Yeah. And, then back to this notion of virial discrepancy, which was used independently...

Bahcall:

Right.

De Swart:

Of course, when…

Bahcall:

So, Zwicky originally suggested that. But then, of course, in the ‘70s, this new revelation from the x-rays came about, and I think it was, at that time in the ‘70s, that you started finding – it took a while to, maybe a few years to understand what this, all of these x-rays were. But, once we, and I worked on that so that it’s, and others did too, that it’s probably from hot gas, thermal (inaudible). Once we showed that, it became clear that you have a lot of hot gas. The mass of the gas was much bigger than the mass of the luminous galaxy, of the stars. So, that became clear that clusters have much more matter. Still, it was not dark matter, even though, you know, some of us were, of course, believed Zwicky and we thought there was dark matter. But it was clear that there was much more than just stars. So, then it started becoming a little more interesting and a little closer to believing for the dark matter, and then came the rotation curves. So, it kind of came together.

De Swart:

Yeah. So, shall we…

Bahcall:

Very interesting.

De Swart:

Shall we?

Bahcall:

Yeah. Let’s…

De Swart:

Shall we stop for now?

[End session 1]

 

[Begin session 2]

De Swart:

November 25. Tuesday.

Bahcall:

So, – thank you. So, it was really nice that you presented at the lunch.

De Swart:

Yeah.

Bahcall:

And people got a chance to hear about it and make some comments. And I think Freeman Dyson will, even though he doesn’t work in that area, but you know he’s, he knows, I mean he remembers the time.

De Swart:

Yeah.

Bahcall:

He’s like ninety now.

De Swart:

Yeah.

Bahcall:

Very, very sharp still.

De Swart:

Yeah. Wow.

Bahcall:

Very impressive. Yeah. So, and you know, so he will, he would be a good person to talk about it.

De Swart:

Uhm-hmm.

Bahcall:

It would be a different perspective.

De Swart:

Yeah. Yeah. Yeah. Do you know where he was at the time you were studying?

Bahcall:

In the ‘60s? You know, I think he was already here at the Institute.

De Swart:

Oh, yeah. Yeah.

Bahcall:

He is the one who brought, actually, John to the Institute.

De Swart:

Oh, yeah.

Bahcall:

He’s the one who – well, you know, it’s the whole Institute but he was in the physics, in the Natural Science and he’s the one to push to have astrophysics. The whole Institute wanted it after Strömgren has left, and I know he was the head of the committee or something, because he was the main physics guy.

De Swart:

Oh, that’s very interesting.

Bahcall:

And he selected to bring John, and he frequently, I hear him give talks about it. He said, you know, he likes to exaggerate, but he said, that was his biggest achievement at the Institute, that he brought John to the Institute. (Laughs)

De Swart:

Yeah. I should talk to him.

Bahcall:

He likes to talk that way.

De Swart:

Yeah. Yeah. But I should talk with him about that. That’s really interesting, why they thought that they should.

Bahcall:

Oh, do you know who that guy is? You might want to meet him. He’s a very famous science writer, Michael Lemonick. Do you know him?

De Swart:

No.

Bahcall:

Do you want to meet him?

De Swart:

Maybe I will. Yeah. Yeah.

Bahcall:

Michael? Oh. If you have a minute.

Lemonick:

I’m in the middle of a class.

Bahcall:

Oh, you’re in middle of class. Okay.

Lemonick:

Yeah.

Bahcall:

Okay.

De Swart:

Okay.

Bahcall:

Okay. Well, I can introduce you to him.

De Swart:

Ah, okay.

Bahcall:

He’s a very good and very nice guy, writer. He writes for science. He writes about science for Time Magazine.

De Swart:

Oh, wow.

Bahcall:

But he also wrote several books. I don’t know if he wrote something about dark matter. Maybe not.

De Swart:

And what was his last name again?

Bahcall:

Lemonick.

De Swart:

Okay.

Bahcall:

His father was the dean of the college here. Uhm-hmm. And so he’s very good and he’s teaching some courses.

De Swart:

Oh. I’ll…

Bahcall:

So, he’s not on the faculty.

De Swart:

Ah.

Bahcall:

He’s a lecturer.

De Swart:

Oh, yeah.

Bahcall:

But he’s a writer. He’s a science writer.

De Swart:

Oh, very interesting.

Bahcall:

He does science writing. So, he’s teaching a course now. But, if you remind me, we’ll introduce you.

De Swart:

Yeah.

Bahcall:

Oh, I just wanted you to meet.

Lemonick:

One minute.

Bahcall:

If you’re in the middle of class. This is the famous writer that I told you, Mike Lemonick. (Laughs) This is Jaco, who is visiting from Holland, and he’s writing, he’s doing his PhD.

Lemonick:

Very nice to meet you.

Bahcall:

…Or masters in science writing on dark matter.

Lemonick:

Oh.

Bahcall:

So, he’s visiting here just for a few weeks, interviewing a bunch of people.

De Swart:

On the history of dark matter.

Bahcall:

History of dark matter.

Lemonick:

Okay. So, you’re writing on the history of dark matter?

De Swart:

Yeah.

Lemonick:

Okay. Okay.

Bahcall:

History. Yes.

Lemonick:

Yeah. Yeah. So, this is…

Bahcall:

The history of dark matter.

Lemonick:

This is a good place to be.

Bahcall:

It’s what?

Lemonick:

This is a good place.

Bahcall:

It’s a good place to be.

De Swart:

It’s the place.

Lemonick:

Yeah. Yeah.

Bahcall:

He’s a physicist, but he’s doing now the project in the history of science.

Lemonick:

Okay.

Bahcall:

On dark matter. So, I just wanted him to meet you as a famous science writer.

De Swart:

Yeah. Wow.

Bahcall:

Just to (inaudible).

Lemonick:

Well, you, so do you know Govert Schilling?

De Swart:

Yeah.

Lemonick:

Yeah. He’s a respected colleague. I admire him a lot.

De Swart:

Okay. Yeah. I’ve met him before Yeah. Yeah. So, that was, that’s interesting. Yeah.

Bahcall:

You did not yet write about dark matter, right?

Lemonick:

Well, not in book length.

Bahcall:

Not a book?

Lemonick:

But, I mean, of course I’ve written about it.

Bahcall:

Right. Oh, yeah, you’ve written articles but not a book, yet, on dark matter?

Lemonick:

No.

Bahcall:

Well, you may want to look at some of the articles that Mike Lemonick – if you just Google it you can find it.

De Swart:

I actually would be really interested in the amount of popular articles throughout the ‘80s and ‘90s on dark matter itself.

Lemonick:

Yeah.

De Swart:

That would also be really interesting.

Lemonick:

Sure.

De Swart:

Yeah.

Bahcall:

He’s looking at the scientific publication and there was a huge peak in the ‘80s when inflation came.

Lemonick:

Right. Right.

Bahcall:

A huge peak about dark matter.

Lemonick:

Right. A guy named Dennis Overbye, at the New York Times, has written a lot about it. He wrote about it for Discover Magazine, also.

Bahcall:

Yeah.

De Swart:

I have read some of those.

Lemonick:

Yeah.

De Swart:

Yeah. Definitely.

Bahcall:

Yeah.

Lemonick:

Yeah.

Bahcall:

He’s an excellent writer, too. Very good. Yeah.

Lemonick:

Well, I would, you know, if you’re around maybe we could talk? I mean…

De Swart:

Yeah. I’ll be around here for two more weeks.

Lemonick:

Okay.

Bahcall:

So, I can give you…

Lemonick:

Yeah. I was going to say.

Bahcall:

Mike’s email and just feel free to…

De Swart:

Thank you, so much.

Lemonick:

Okay. Yeah. Cool.

De Swart:

Okay.

Lemonick:

See you later.

Bahcall:

Okay.

De Swart:

Yeah. Thanks.

Bahcall:

So, that’s a different thing. It’s popular now in science.

De Swart:

Yeah. Yeah. Yeah. So, I have done some outreach things. I did some things on Dutch National Television. I’m doing some outreach talks. So, that’s, it’s really nice. It’s really, it kind of overlaps with doing history and looking at…

Bahcall:

That’s right.

De Swart:

… science. But, it’s, you, you have to…

Bahcall:

It’s different. Yeah.

De Swart:

Yeah.

Bahcall:

It’s a different…

De Swart:

You’re not doing science.

Bahcall:

Yeah. It’s a different perspective but it may be interesting to see the publication related. I’m sure there is a strong correlation.

De Swart:

Yeah. So, so actually, so I was at the American Institute of Physics last week and also Physics Today is there.

Bahcall:

Yes.

De Swart:

And I gave a talk and they actually asked me this, and it was a really interesting question. “Did you look at when were the first popular publications on dark matter?”

Bahcall:

Oh, okay.

De Swart:

Which was really interesting.

Bahcall:

So, I would look up his name, Michael Lemonick, and Dennis Overbye, who is very famous. He writes for the New York Times. He’s excellent.

De Swart:

So, I…

Bahcall:

And Lemonick, you can, you know, I’ll give you the email or you could look him up, look him up. He’s in, you know, he teaches so he’s, you know, he’s here frequently.

De Swart:

Well, that’s nice.

Bahcall:

Okay.

De Swart:

Yeah. So, I, I didn’t meet, and I would love to meet him, is David DeVorkin.

Bahcall:

Oh, yes.

De Swart:

Yeah.

Bahcall:

Yes. Yeah.

De Swart:

So, he’s doing a lot of history of astronomy.

Bahcall:

He does a lot history of astronomy. He runs the, something in the…

De Swart:

The Air & Space Museum.

Bahcall:

The Air & Space Museum.

De Swart:

Yeah.

Bahcall:

Yeah.

De Swart:

So, that, I would, I met some Postdocs of his at fellowships at…

Bahcall:

He’s probably difficult to get. He’s always running around, and busy. Yeah. Because he does the Air & Space Museum and so it keeps him very busy. But I know him well. He knows me and John. Very well. Yeah. Good. So, you’re really getting to go places and talk to – that’s excellent.

De Swart:

Yeah. That’s really…

Bahcall:

Okay.

De Swart:

Yeah. Okay. Yeah. Well, there were, I think, the two more themes that I wanted to just look at, the first one was something which kind of came about during the talk too, and that was that the problem itself as maybe the virial discrepancy.

Bahcall:

Yes.

De Swart:

When was the first time that you heard of it and what context, and maybe also the connection with Ambartsumian’s work.

Bahcall:

Yeah. It’s interesting that many people don’t really remember Ambartsumian. So, he was not Estonian. He was Armenian. I forgot. Yeah. Well, the first time heard about the mass discrepancy was from Zwicky.

De Swart:

Yeah. That’s…

Bahcall:

At the time, in the ‘30s. Well, not in the ‘30s. (Laughs)

De Swart:

No.

Bahcall:

In the ‘60s, when I started doing a little more astronomy. So, that’s when I heard about it, but of course it had been discussed since the ‘30s and I read about it then and so on. But I heard it from Zwicky in the ‘60s.

De Swart:

And, then you started to read it, about it?

Bahcall:

And then I started. Because before that I really was not doing astronomy. I was doing more nuclear physics. So, in the ‘60s, at Caltech, when, as we said, there was a lot of astronomy. I met and talked with Zwicky. As Freeman Dyson said, he was in the cellar, in the basement. (Laughs) That’s where he was sitting. Because, he was, not that he was put down there because people didn’t like him. I think it was, he was doing all this work with the Palomar Sky Survey plates, and searching, similar to what I did, for all these, finding all the groups of clusters to create his catalog. And all the equipment was there. The Palomar Sky Survey plates were there, these microscopes were there. The whole thing was set up for it. So, that’s why he was there. It was like a lab. So, I started hearing about it when I was there in the late ‘60s, and read about it, and it was very interesting. And then, of course, followed the discussion. So, the discussion, I don’t know when it started with Ambartsumian. I suspect it started right after the ‘30s. But I sort of picked up on that in the ‘60s. That’s when I started getting it. So, from the ‘30s until the ‘60s also, I don’t know when Ambartsumian died, but there was a discussion, and in meetings people talk about. It was not hot, hot, hot. People didn’t talk about it all the time, but it was also not fully neglected. I mean, people did talk of the clusters, virialized. Can you use velocity? So, it was discussed.

De Swart:

And?

Bahcall:

It would surprise me, now that I look back, if, I don’t understand why people didn’t measure – or maybe they did – low velocities, to really get to see if it’s a full Gaussian fate, how the distribution is. Not that that by itself will tell you if it’s virialized, but at least it would suggest that things are not just kind of exploding for falling in.

De Swart:

Yeah.

Bahcall:

So, I don’t know that people have followed up very much on that. They probably should have.

De Swart:

And, and do you know of any specific – maybe that’s a very specific question, but was Ambartsumian the only name which was connected to it, and Zwicky, or do you have any other names which were, of people which were interested in this virial description?

Bahcall:

Description? Oh, there were. There were others on – you know, I don’t remember. Now, those were the two big names, Zwicky was, of course, the main guy for, and Ambartsumian he kept saying, I guess for reasonable reasons, you know, that you don’t know. But, Zwicky had much better intuition. I cannot remember now. I’m sure many of the cosmologists at that time were interested. I’m sure Sandage was, but I’ll have to look back. I don’t remember.

De Swart:

So, actually there is a…

Bahcall:

Sandage was probably interested. He was there, too. Not at Caltech, but in Pasadena, I think, other, at a Carnegie institution.

De Swart:

So, actually, there is a really interesting article from ‘69, from Sandage and Geoffrey Burbidge.

Bahcall:

Oh. Yeah?

De Swart:

Which is called…

Bahcall:

Sixty-nine?

De Swart:

Sixty-nine, which is called The Case of the Missing Mass.

Bahcall:

Yeah. Yeah.

De Swart:

But…

Bahcall:

Oh, Burbidge was very interested in it. Burbidge was. I don’t think he did much work on it, but he was very interested. He probably did write some papers, but he was very interested. I think many of the cosmologists, at that time, and people who were really broadly interested were interested. Sandage and Geoff Burbidge. I’d have to remember who else was there.

De Swart:

I have seen a paper of [Karl Hensev].

Bahcall:

[Karl Hensev]. Yes. That’s a Russian. Yes. That’s correct. Yeah. He was a big name at that time, in doing missing mass, but also a lot of large construction, clusters, and groups, and large construction.

De Swart:

Ah. Okay.

Bahcall:

At that time, he was…

De Swart:

You knew about his work?

Bahcall:

Oh, yeah. Yes. Yes. We knew about it and we used it.

De Swart:

Oh, that’s interesting.

Bahcall:

I had read about it. Yeah.

De Swart:

Because, I see there, that probably there was kind of a time delay between?

Bahcall:

There was a little time delay, but, you know, we tried to look – there was not a huge amount of work on clusters and groups, and even the missing mass. So, we looked and found out who was working on it. But yes. And, of course, people in Russia, cosmologists, Zel’dovich. Probably Sunyaev at that time, too. You know, I’m sure they were all interested with Sunyaev – with Zel’dovich I even remember meeting in Russia. That was later. But, and we did talk about it, but that was much later. But we talked about cosmology in general, including dark matter and other things. But that was much later. So, the Russian group was interested, and in this country of the names you just mentioned.

De Swart:

Did you also….

Bahcall:

And, of course, Jim Peebles.

De Swart:

Yeah. Yeah. And did you have any letter contacts with, or just publications with Russia?

Bahcall:

It was mostly publications, and then at some conferences. They could not come here.

De Swart:

Oh, yeah.

Bahcall:

But, at some point we were at a conference in Bulgaria, I think, and the Russians could come, and we could come, and that’s where we all met, which was really nice.

De Swart:

Ah. Yeah.

Bahcall:

So, that’s when I remember Zel’dovich, and Sunyaev.

De Swart:

It’s so interesting that in astronomy this, the Cold War barrier wasn’t that big. Right? The papers just kept on being translated?

Bahcall:

Yeah. And we could follow. Again, you know, it’s not as easy as just publication here, but if you make an effort, you can get it. You can find it. You can try to talk to them. And, then there were conferences in Eastern, I mean like Bulgaria, Romania, or Hungary, where we could meet with them.

De Swart:

Maybe…

Bahcall:

They would come there, and we could come there.

De Swart:

Uh huh. The IAU conferences?

Bahcall:

Oh, I don’t know if it was IAU or some other special conference. But there were some IAU and I think there were some other special conferences.

De Swart:

And I’ll go on to the next thing. When was the – could you remember when the galaxies got started to get involved? So, that you were interested in clusters? So, when did this problem of galaxies and mass in galaxies appear in research you were doing?

Bahcall:

So, the galaxies, you know, as we discussed today and yesterday, started coming up in the ‘70s. I think even the earlier one guy, Rogstad, and Shostak, and Robert, I think it was still ‘70. So, maybe the very early ‘70s. Rubin was ‘74, I think, the first one.

De Swart:

The first was in ‘70.

Bahcall:

Was it ‘70?

De Swart:

Yeah.

Bahcall:

With the actual rotation curve?

De Swart:

Not – so, so the, the very flat one was in – I can show you the paper.

Bahcall:

Okay.

De Swart:

But…

Bahcall:

I remember the flat one was like ‘74.

De Swart:

It was later. The very flat one was kind of a – let me see. Let me check.

Bahcall:

Yeah. There was M31. I remember that paper.

De Swart:

Yeah. So, in the same time you have Ken Freeman.

Bahcall:

Yes.

De Swart:

So, of course, I can’t find it. Oh, here it is. Let’s see if you can find the curve. You can take this.

Bahcall:

Yeah. I remember that paper. Yeah. Yeah. Oh, this is in ‘70.

De Swart:

Yeah. And the one who is most often used is the one who she published in 1980.

Bahcall:

In ‘80? Yeah.

De Swart:

Yeah. Right.

Bahcall:

Because that one…

De Swart:

Where she combined it with, with the data of Roberts.

Bahcall:

Yes. Yes. Okay.

De Swart:

But, now, so I was wondering, in the context of your research, if this was…

Bahcall:

So, this does not yet discuss dark matter?

De Swart:

No. No. No. No. No.

Bahcall:

Yeah. So, that’s when, starting around 1970, that’s where more activities, and that’s because there were new instruments. That’s frequently what happens, of course, in science. Once you start having new telescopes, new instrumentation that they can look in either different wavelengths or higher resolutions or lower threshold, or something that was, could not be seen before, then you start finding new things. And that’s what happened around the ‘70s.

De Swart:

And do you remember something specifically for your research, technologically?

Bahcall:

Not, not for my research. I know what I used was for, generally in the ‘70s, the x-ray emission for clusters. That was the first time x-rays were looked at in the universe. So, they found it for stars, for binaries, for supernovae, and for clusters, and clusters was not quite expected. Well, it was not expected at all. You know, binaries, you know, supernovae one expected it, but not from clusters. So, that I remember was a big surprise. That was one of the biggest surprises or discoveries, and that was in the ‘70s I think, early ‘70s. And that’s when we started paying much more attention.

De Swart:

To clusters?

Bahcall:

To clusters and to the fact that there was much more matter there, and tried to understand the x-rays, and “Is it due to just point sources in the galaxies?” You know, which, because it’s not so interesting. But it was a lot of it, until we and others finally showed that it’s not. The spectrum looked more like just hot gas, (inaudible). And once it’s hot gas you can calculate just from the x-ray and velocity how much mass in the gas, and that was huge, much more than the mass in the stars. Not as much as the dark matter, but much more than – so, that became clear that something’s going on.

De Swart:

Uhm-hmm. So, and if we then kind of maybe skip a few years, to a paper, which was quite famous of you, the Annual Review you did?

Bahcall:

That Annual Review.

De Swart:

And, in 1977.

Bahcall:

Yeah. Maybe that’s the one that somebody, at lunch Abe Jenkins mentioned that maybe I had one reference with a page number wrong.

De Swart:

Oh, really?

Bahcall:

And, maybe, I don’t know, because he told me later it may have been my review paper. So, I don’t know. But many people—that paper has been used a lot. So, when people want to use clusters – I had two reviews in the Annual. One is clusters. One is large construction. And both of them have been used a lot, cited a lot. So, what people do is when they talk about clusters, rather than go find all the references, they just look at my paper. They cite my paper and then whatever reference, few references (inaudible), so without really looking at them. So, if I had something of a typo there, they might have copied the typo.

De Swart:

Yeah. That’s really funny.

Bahcall:

It is. Yeah.

De Swart:

And, so there, in this paper there’s a pretty large section on missing, oh, “Masses and Mass-to-Light Ratios of Clusters”?

Bahcall:

Yes.

De Swart:

And even a section called “The Missing Mass”?

Bahcall:

Oh, yeah. So, I wrote a lot of that from, at least from the ‘70s, maybe even before. Maybe even before. Even one of our recent papers – I think – oh, I didn’t give it to you – one of our recent papers looks at that mass-to-light ratio in a much, much, much, infinitely better way now. From the Sloan Survey we can get the mass from lensing and we look to see how the mass is distributed relative to the light. So, that’s something that I already was thinking about then. And then in the ‘90s we did a paper on that that really changed the way we see how the mass and the light trace each other, compared to what other people have been thinking. And, most recently, we redid it with much better data, with lensing. The ‘90s we didn’t have much lensing.

De Swart:

Oh, yeah. Of course.

Bahcall:

And it looks beautiful. I just gave a big talk about it and, when I was away at Carnegie-Mellon. I got some big award there and gave a big talk, and I talked about it.

De Swart:

Oh. Oh.

Bahcall:

Did I give you a paper?

De Swart:

No. No. No.

Bahcall:

So, that was just published recently, and that is really a very, I’m very excited about this paper because it really showed how the dark, the dark and light matter trace each other and they trace each other remarkably well.

De Swart:

So, this is also – oh, yes. You have, you did a short lunch talk on this a couple of weeks ago, right?

Bahcall:

Oh, that’s right.

De Swart:

I was there.

Bahcall:

That’s right. Oh, you – okay. Yeah. I just gave the, yeah, they asked me to give a little short highlight.

De Swart:

It was very short, but very interesting. Oh, thank you, so much.

Bahcall:

But I gave a big talk about – yeah. I gave three talks about it. And the week that I was gone several places, several places.

De Swart:

So, yeah. So, that’s actually really interesting. So, looking back, the first try, or the first paper, you, it seemed that you collected a lot of references.

Bahcall:

Yes.

De Swart:

As you said, you had a lot of references on the masses and luminosities of clusters of galaxies?

Bahcall:

Yes. Yes. Yes.

De Swart:

And, and you already seemed to…

Bahcall:

I also had, I don’t know if you know that, the, in the book, like the Allen book on astrophysical quantities, I wrote a – you know, this book, Allen’s Astrophysical? That, like the bible, everybody uses it for astrophysics. So, in this revised thing, Allen wrote it many years ago, but we did a revised, a revised one and I was asked to write a chapter on clusters. So, probably a lot of – here. Probably a lot of, about the mass here too.

De Swart:

When was this?

Bahcall:

Mass function. Yeah. That was, the revision was in maybe ninety-something…ninety, oh, ‘99.

De Swart:

So, what’s the title of the…

Bahcall:

It’s called…

De Swart:

Astro…

Bahcall:

Here, here is a chapter.

De Swart:

Chapter twenty-five.

Bahcall:

In Allen’s Astrophysical Quantities.

De Swart:

Ah. Yes. Yes. Oh, that’s interesting to see how it developed.

Bahcall:

It’s what?

De Swart:

Yeah. Yeah. Yeah.

Bahcall:

Yeah. So, so it’s, I’ve written a lot, as you can, you can, you know, you can (inaudible) there…

De Swart:

Yeah. Yeah. Yeah. Yeah.

Bahcall:

More than I can remember. But I’ve written a lot about the clusters, and then super clusters, and then large construction, and they’re use for cosmology. So, I’ve concentrated my work both on trying to understand clusters as systems by themselves, but also for the dark matter, and for use as a tool in cosmology, in getting the mass density of the universe and some of the other cosmological parameters. So, that’s something that I spent quite a bit of the ‘90s, getting omega matter.

De Swart:

Right. Yeah.

Bahcall:

And so that came later. But we used it because that’s connected, of course, to the dark matter. But we looked at various methods, devised some methods, some tools where we can use clusters to determine omega matter, because omega matter, as we said, became very controversial in the’80s and ‘90s. Some people thought it was one. That was the standard quota.

De Swart:

Yeah. Yeah. Yeah.

Bahcall:

So, we devised different methods that we discuss in this paper I gave you, The Cosmic Triangle.

De Swart:

Yeah. Yeah.

Bahcall:

How to use clusters and other methods to measure it. And, we showed, by a few different methods, that it was not one. It could not possible be one. One of them was mass to light, because it just flattens out. Mass starts following light above a few hundred kilogausses. And that means there’s no more dark matter. That was the simplest way to say that. But we used other methods too. So, and that was started in the ‘80s and then mostly in the ‘90s.

De Swart:

Yeah. So, so what I was wondering about, about this 1977 paper, is how did these different scales, galaxies and clusters of galaxies, get connected?

Bahcall:

Well, that’s a very, very good question. That’s a very important question. I know how it’s connected in recent years.

De Swart:

Yes. Of course.

Bahcall:

Because that recent paper that I gave you really is an attempt to connect everything from the galaxies to groups of clusters, and we show that the groups of clusters are just – and we showed it in the ‘90s, too. No. I think ‘95 paper on the mass-to-light. I think we asked, the title is “Where Is the Dark Matter”? And we showed that within most of the dark matter just comes with the galaxies into the groups and clusters.

De Swart:

Yeah. Yeah. Yeah.

Bahcall:

But, in those early days, in the ‘70s, the idea in the ‘70s, and that’s what I actually gave, gave in my talk, gave in my talk when I was (inaudible), the idea in the ‘70s was very different than what we are proposing now. Some people still think about the ideas from the ‘70s. The idea in the ‘70s was developed and showed that if you measure the mass-per-unit light for luminous galaxies, it’s small. It’s maybe about ten. You go a little farther with the rotation curves, so it increases a little bit. But nobody thought about the halos being big, so big. Then, groups, you measure the mass-to-light and in the ‘70s it was measured already, and we discuss it there, it was maybe a hundred to one. The clusters were 300. So, the concept, and that was done first by a paper of Ostriker, Peebles, and Yahil in ‘74, I think.

De Swart:

Seventy-four. Yeah.

Bahcall:

And it shows that the mass-to-light keeps increasing as you go to a larger and larger scale. And, of course, you have galaxies. You have groups. You have clusters. And it keeps rising. What does it mean? What does mass-to-light mean? It means that as you go to larger scale systems, you go from galaxies, to groups, to clusters, you are more dominated by dark matter than light. So, you have much more dark matter than you have light. Obviously, as you go to big system groups and clusters you have lots more dark matter, but you also have a lot more light. So, but the concept was that when you go to bigger scales you have more dark matter than you have light. So, it’s more dominated by dark matter. And, even today, some people think that that’s the case. It’s kind of the intuition is that you’re more dominated by dark matter as you go to bigger and bigger system. What we show in this paper that that’s not the case. That’s the case only if you go up to the big halos around individual galaxies about 300 kiloparsecs. That’s about the size virial radius of a typical L-star or bigger or normal galaxy, like our galaxy. But, beyond that it mostly flattens out. Mass and light follow each other.

De Swart:

Uhm-hmm. And do you remember this paper of Ostriker, Peebles, and Yahil?

Bahcall:

Oh, yeah.

De Swart:

When it got published?

Bahcall:

Yes. Yes. Yes. And that was a very famous paper. In fact, in my talks I show that. That’s one of the first viewgraphs I show that this is the historical idea that people felt, is mass-to-light keeps increasing. And that was connected to Roberts, and Rubin, and all that. That was the rotation curve, the mass keeps rising. But then they did galaxies and they did groups, and that’s from the velocities. And that’s when the connection came, and the clusters, from the velocities, same as Zwicky did. And you put it together and you see, you see that you have, it keeps increasing.

De Swart:

And that was the first time you heard of this, these two being the same?

Bahcall:

What do you mean “the same?” Well, that’s, that’s the first time that people tried to – in the early ‘70s is when people tried to understand what was going on with this dark matter, because it became clear that something is going on from all the rotation curves, mostly, but also more and more data on groups of clusters came in. Also, the x-rays of clusters came in. So, all of that together started tying in, you have dark matter in groups, in clusters, and now in galaxies. And “how do they relate to each other?” and so on.

De Swart:

And that, you would say, was the paper of Ostriker, and Peebles, and Yahil?

Bahcall:

I don’t know if that was the – I mean, people talked about some connection between the two, but I think that might be the first one that put it in those terms. Zwicky, of course, used mass-to-light in his ‘30s. I mean, that’s what he said. “I have much more mass from the virial mass compared to the light.” So, that’s a classical way to do that. So, they’re not the first one to do it, but they kind of put it with the galaxies and clusters, and so on. But came to the sort of conclusion, which I think is not good, not exactly – it’s not wrong, but it’s not right either – that as you go to bigger system you have more dark matter. And, what we saw, showed in the ‘90s and then more, more carefully here, is that that’s true only after a few hundred kiloparsec scale, and that’s a scale of individual galaxies. Once you got to groups of – so, it’s not true that groups have, I think, more dark matter than galaxies. It’s the same amount of dark matter per unit. Yeah. And that’s not, not the concept or intuition that most people think about.

De Swart:

And so, in ‘77 it was quite clear that missing mass in galaxies and clusters, and binaries, and all?

Bahcall:

Yes. In the middle of the ‘70s it was pretty clear that you have dark matter— or missing mass. I don’t know when “dark matter” being, started being used in such a name, but originally it was “missing mass.”

De Swart:

Yeah. Yeah. So, you still call it, in this article you still call it “missing mass.”

Bahcall:

Oh, it’s still called “missing mass”?

De Swart:

Yeah. Yeah.

Bahcall:

Yeah. I don’t know when “dark matter” was called. Anyway, but it’s the same thing.

De Swart:

Yeah.

Bahcall:

So, in the ‘70s it was already pretty clear, and maybe even a little before, before the groups and clusters probably have something there, but in the ‘70s it was clear that all those systems have some dark matter.

De Swart:

And how were the articles received that said that, “You don’t need missing mass to make sense of rotation curves.” For example, Geoffrey Burbidge publishing still about that?

Bahcall:

Not very well. It was not – you know, Geoffrey Burbidge was an extremely bright scientist, very smart, but very controversial, a little bit like Zwicky. Although, Zwicky had much more successes and insights. And, well, Burbidge did important work. But anyway, he liked to be, you know, in English they say, “the devil’s advocate.” You know what that is?

De Swart:

Uhm-hmm.

Bahcall:

Geoff Burbidge always liked to be the devil’s advocate. He always kept talking – we talked about quasars yesterday – that quasars are not at cosmological distances. He just liked to cause problems. You know? And I think he just wanted an argument, people to argue with him. I’m not sure if he believed in it or he just wanted to get people to discuss it. You know what I mean?

De Swart:

Yeah.

Bahcall:

Some, though some people like Chip Arp, who really believed in it. But Geoffrey Burbidge was too smart, I think, to believe that quasars were not there. (Laughs) But, he always tried to argue the opposite, you know. And I think with dark matter he also tried to argue that there was no dark matter. His wife, Margaret Burbidge, also obtained rotation – she was the observer – she obtained rotation curves well before Vera Rubin, and Roberts, and so on. But didn’t notice that it was flat. I mean, or at least they saw it a little bit declining and just assumed it was going as expected.

De Swart:

Yeah. So, what Michael Strauss mentioned, and Jim Gunn, is that they were still using the Brandt Curve to fit the data and, which it still fit well?

Bahcall:

Yes. Exactly. Exactly.

De Swart:

And was that also his argument in the ‘70s, that you could make sense of the rotation curves?

Bahcall:

Probably. You know, Geoff would just throw in all kinds of arguments. He would just throw in all kinds of arguments. (Laughs) But, he was, I think some of it was on purpose because, you know, he was the editor of – I don’t know if it’s the APJ. He was the editor of one of the journals. And I remember one evening I would sit and talk to him and he would have an argument, “Oh, you are working on clusters and dark matter, and none of that makes sense.” And he’d just, you know, that’s how he would talk. And then, the next day he’d send me a paper to referee about dark matter in clusters. (Laughs) And, then he would talk a lot of sense. He said, “Well, what do you think about it? Do they have it right?” You know, so that’s why I think he didn’t really, you know, he was kind of maybe split mind or something.

De Swart:

Yeah. Yeah.

Bahcall:

But, maybe partly because Margaret, his wife, missed the flat rotation curves. Maybe that was part of it.

De Swart:

Uhm-hmm. And…

Bahcall:

But, yeah, so some people missed it early on, but, but then there, mostly I think it was the, the radio actually, and they don’t get enough credit. They’re the one who really first got the flat rotation curves farther out. Vera Rubin got it, like Margaret Burbidge got it, with optical from the start, but you could not go very far out with those. That’s why Burbidge didn’t, she didn’t see infrared, because, you know, it was just a little bit beyond. So, it’s hard to tell. If it’s light coming down, you cannot see that. But the radio really could see it much further out.

De Swart:

And could you – so, this is maybe a weird question, but, could you read radio astronomy papers? Was that the nomenclature of radio astronomy?

Bahcall:

It was a little different, but we did. Because, you know, we always tried to, or I tried to work in, well, in the optical, but you always try, if you have a big fundamental question like dark matter, well, it has nothing to do with the optical. It’s, you have to put multi-wavelengths to see what it tells you. So, the radio, and I always reference them. They don’t always get referenced, the radio, and I don’t understand. You know, that’s unfortunate because they were the first to really show that. That’s not very good.

De Swart:

No.

Bahcall:

So, I always reference them.

De Swart:

Yeah.

Bahcall:

And, so, yeah. I did look at it carefully. I put it into consideration.

De Swart:

A lot of these references in your 1977 is, and your review, are radio astronomy. So, you have Roberts, and Ross, and Shostak.

Bahcall:

Roberts and Ross, Shostak. Yes. Yes. Yes.

De Swart:

But somebody who – so, it’s very apparent in, especially when you are discussing the mass of clusters, is, are papers of Herbert Rood.

Bahcall:

Oh, yes. Yes. Yes. Herbert Rood was a very interesting case. He worked on clusters and dark matter, and the large-scale structure, and did some of the earlier work on all of those things. And I reference him a lot. He was an interesting guy. Unfortunately, he’s not alive anymore. He was disabled.

De Swart:

Oh?

Bahcall:

Couldn’t walk very – I mean, walked with two canes, and, you know, just really was not, well, not well. And, had some, I don’t know, some, many problems. But very nice guy. And he, maybe he had a job originally, but then didn’t have a job because he had too many other complications.

De Swart:

Uhm-hmm. So, in the mental sense too?

Bahcall:

I don’t know if mental. I mean, he was thinking smart. I mean, but, but he was maybe – I don’t know what it was. Maybe, what do you call it? Not panic sort of, but…

De Swart:

Anxiety?

Bahcall:

Well, anxiety or – I don’t know what it was.

De Swart:

Fear? Yeah.

Bahcall:

It was something that–not that he was confused. He was just – or depression. I don’t know. I don’t know what. But, he was disabled physically, also. Couldn’t walk well. So, at the end he didn’t have a job.

De Swart:

Because – where was he?

Bahcall:

I don’t remember where he originally was. Maybe Michigan. But I don’t remember. He had a job when he was young, but then he didn’t have a job. Maybe he didn’t get tenure. I don’t remember. And, what happened is that John, at the Institute, I don’t know somehow, talked to – gave him an office.

De Swart:

Oh, wow.

Bahcall:

So, you know, he was not, not a faculty or member but he gave him an office and he participated and went to lunch with them. So, I actually talked to him a lot because he was in Princeton. And he did, and I reference him a lot, he did a lot of the early work on, kind of in parallel with me, on dark matter in clusters and the velocity distribution, the virial theorem, and then he did some of the earliest work, and he also doesn’t get much credit, for the large-scale, you know, the large-scale, you know, the CFA Redshift Survey that gets a lot of publicity, where they’ve found voids and filaments, and cellular structure. He and a couple of other people did that first, from a smaller area. Don’t get much credit for that.

De Swart:

Oh.

Bahcall:

All the credit goes to Hooker and Galileon. And, but they did it first and I always reference them. I always feel bad for him.

De Swart:

So, when you, so was he already here when you – no. No. So, you moved here…

Bahcall:

No. In the ‘70s, I don’t remember when he came here. One can look from his papers to see where he writes his… .

De Swart:

So, I think many…

Bahcall:

He came here either in the late ‘70s, maybe ‘80s. I don’t remember.

De Swart:

Yeah. Because many of your papers, I think, say Wesleyan.

Bahcall:

Oh, Wesleyan?

De Swart:

Yeah.

Bahcall:

Maybe. So, I don’t remember where he was, but he, at the end he lost his job, or quit that.

De Swart:

Hmm. Yeah.

Bahcall:

So, after some time he did have a job, then he didn’t have a job, and that was either the late ‘70s or in the ‘80s, and then John gave him an office.

De Swart:

And?

Bahcall:

And he was here until he died.

De Swart:

But did you know about his work before you…

Bahcall:

Oh, yeah. I knew about him.

De Swart:

Oh, okay.

Bahcall:

Yes. Yeah. Yeah. Oh, yeah.

De Swart:

Because he was working on clusters?

Bahcall:

Because he was working in the general field of clusters and dark matter, large construction, and that’s what I was interested in. And, we met before, I think. We met before.

De Swart:

Oh.

Bahcall:

But, once he was here, we talked quite a bit. And I think we wrote a paper together. I don’t remember. Maybe. Maybe we did write one or two. I don’t remember. But we talked a lot.

De Swart:

And did his disability have any effect on, on how serious people took, took him?

Bahcall:

It’s hard to know, you know.

De Swart:

Right.

Bahcall:

I think people respected him. He was just an observer – not an observer, but he just looked at the data. He was very old-fashioned. You know, he didn’t think about cosmology. I used it all later for cosmology, also. He just tried to understand the basic data, you know, the old-fashioned way, to look at the clusters, and the profile, and the velocity and measure each galaxy, you know. Very kind of the old-fashioned way. He was not looking into the cosmology, into the bigger questions. And, for what he did people took him seriously.

De Swart:

Yeah. So, because he actually has a lot of articles.

Bahcall:

A lot of articles.

De Swart:

On the virial discrepancy.

Bahcall:

Absolutely. Absolutely. Yes. Yes. Yeah.

De Swart:

So, that’s interesting. I didn’t know this. There’s actually very, you can find very little on Herbert Rood on the internet. There is…

Bahcall:

That’s probably because he, because of all the problems he had.

De Swart:

Yeah.

Bahcall:

He didn’t have a position. And, you know, it affects – he just enjoyed doing his research. But he would come to the talks. He would come to colloquia. He would sit and talk with people. Then he would get sick for a while and not come. You know? It was back and forth.

De Swart:

Yeah. But, because there’s one paper which you reference, but for example also is referenced in the article of Ostriker, and Peebles, and Yahil, which he wrote with Ivan King, I think?

Bahcall:

Yes. Yes. And that may be on the density profile?

De Swart:

Yeah.

Bahcall:

In the cluster?

De Swart:

Yeah.

Bahcall:

And, then you can combine it, maybe, to get the – I don’t remember that specific paper, but I mean I don’t remember the details of the original paper. And combine the density profile to understand the mass distribution there?

De Swart:

Yeah.

Bahcall:

A lot of that happened in the ‘70s. So, the ‘70s were enormously fruitful as far as the development of understanding of clusters, and groups, and galaxies, and profiles, and velocity profiles, and therefore dark matter, the comparison of the mass of the light, of putting all of that together.

De Swart:

Hmm. And Ivan King was he also working on clusters or was it just a…

Bahcall:

Ivan King was at Berkeley then and he was working mostly, he was working originally, I think, mostly on globular, on star clusters. And he fit what’s called the King Curve, and we used a lot of that too, which is just an isothermal. We just talked about it. The density distribution, it’s an isothermal. If you’re just out to the -2, it’s isothermic. But then, if it has a little curl in the center and then it cuts off a little bit, he fit it to that type of profile that have two parameters of core radius and truncation radius.

De Swart:

Ah, yeah.

Bahcall:

And was, at that time it was called the King Isothermal Curve, or King Curve, or something. So, you have some parameters. And I used it. So, that was popular at that time in the ‘60s and ‘70s.

De Swart:

Uh huh. And, you knew, did you know him?

Bahcall:

Ivan King I knew very well.

De Swart:

Ah.

Bahcall:

He is still alive. He’s quite old, but he’s still alive.

De Swart:

Okay.

Bahcall:

If you want to talk to him. He’s, I don’t think he’s in Berkeley anymore. He moved, maybe Washington, or Seattle, or someplace.

De Swart:

Okay.

Bahcall:

Yeah. He was a professor at Berkeley for many, many years. Also, very old-fashioned, classical astronomer.

De Swart:

So, what do you mean by “classical”? So, you mentioned that earlier.

Bahcall:

“Classical”… classical is like Zwicky. You know, he would sit in his office. You know, his office, I remember coming to his office. You could barely walk in. Just, paper piles all, all – you barely could move. And he would sit and look at all the plates, you know, photographic plates to measure things, and write everything. You know, the very classical astronomer. He would write everything down and do that. And did observations. Everything very precise. Everything very careful. And mostly, again, didn’t use it much for cosmology or for other things, but just to get the measurements done well, did it all himself, traced it very well, and so on. He was later involved with the Hubble Space Telescope, also in the beginning, which was about ’90. So, he was involved with that. And there he looked at the profiles in globular clusters, but he also did the same profiles for clusters of galaxies, and that’s when I was his paper.

De Swart:

Oh, yeah. Yeah. Yeah.

Bahcall:

So, I don’t know. He did work some on clusters of galaxies, as well as this globular star cluster thing. But he mostly, he did density profiles, and in globular clusters tried to understand the stellar ages. Did those types of things. And, cellular astronomy is a little more classic than (inaudible).

De Swart:

Oh, yeah. Yeah. So, you would say you were more of a modern astronomer, in that sense?

Bahcall:

Yeah. I think I was a more modern astronomer for the time.

De Swart:

Because you were doing…?

Bahcall:

Not doing stars. I still was using this thing to measure.

De Swart:

Yeah.

Bahcall:

You know, I was going outside, you know, into the extragalactic doing quasars, doing dark matter, doing galaxies, and using infrared cosmology. So, I was – at that time, at Caltech, again there were cosmologists but most of the others were just the old classical stellar astronomers, like – well, Ivan King was at Berkeley, but that type. It was, Jesse Greenstein was a good colleague and friend of ours, and others. Maarten Schmidt was also kind of more modern. You know, he got into the quasars and the high redshift. It just opened up a whole new window. So, got out of the stars.

De Swart:

So, so you would say that the, the generation, or the generation gap also played a role in the…

Bahcall:

I think so.

De Swart:

In how big cosmology was?

Bahcall:

I think so. I think it almost always does.

De Swart:

Yeah.

Bahcall:

You know, people are used to think the way they are trained to think. So, I know Jesse Greenstein saw that this quasar was a star, because, you know, he just knew stars.

De Swart:

Yeah. Yeah. Yeah. Yeah.

Bahcall:

And, Maarten Schmidt was younger at that time, and he said, “Well, that second redshift looks so well. Why is it such high velocity? It’s probably not a star.” You know. So, if you come not with a whole lot baggage of what you think you know, but with an open mind, you can make, sometimes you can make more out-of-the-box discoveries.

De Swart:

Yeah. So…

Bahcall:

I would still like to think that even I’m not the young generation now, I’m still trying to think outside the box. So, it’s not always the case.

De Swart:

Of course not.

Bahcall:

But, in general, there is some generation gap.

De Swart:

Well, yeah, because of the training, of course.

Bahcall:

Yeah.

De Swart:

Would be such. Yeah.

Bahcall:

Yeah.

De Swart:

But, of course, other things probably also played a role at the time?

Bahcall:

Well, yeah. The other things I remember, some people sometimes say, “If you…” I remember when we first did the cluster correlation function and that was a big discovery to find that it’s not the same as galaxies. It was ten times stronger. So, we found much larger cell structures in the universe, in the ‘80s, before the redshift survey. So, that was a big discovery. And, I remember somebody said, “Sometimes it’s good to have people who come either young or women who are not in the field, or people who are not brainwashed to think in a particular way, either from a different field, or sometimes just say a different gender, not many women in astronomy, so you come and you’re not brainwashed with all the things that people have done, and you think outside the box.” Somebody once said something very clever, said, “Sometimes you come and bring somebody from outside the field or women or, it really, it’s easier for them to make a bigger discovery, because there are not, they are not boxed into what, what they were trained to think.”

De Swart:

And you were both a woman and a nuclear physicist? (Laughs)

Bahcall:

Oh, that’s right. That’s right. That’s right. Yes. Yes. Yeah.

De Swart:

So, how was that, the being a woman part? Was it…

Bahcall:

I actually never felt, you know, it never even occurred to me that it’s anything to think, to even think about. I think it’s a cultural thing. Many more American women do think about it, frequently. I come from Israel and there we really, at that time, have not thought about it much. Women would serve in the Army. Women go fight. Women would do everything, work in the fields. It never even crossed my mind that, you know, if I want to do something does it make a difference if I’m a man or a woman?

De Swart:

Yeah. That’s interesting.

Bahcall:

It never really, it never really crossed my mind, until I came here, and a lot of young women started asking me, “How is it to be a woman in the field?” And I, you know…

De Swart:

It’s, “Hey, wait a second.”

Bahcall:

Yeah. First, I said, “What’s the difference?” I didn’t really – so, to me, I must say it had never, it never played a big role. I mean, I’m always trying to help women, trying to encourage women, try to make sure that there are more women coming, help them and so on. But, for me personally, work wise, I was the first woman to be a professor here in the department.

De Swart:

Oh, wow.

Bahcall:

Yeah. But see, even then it was not, it was not – it’s either me or another guy. So, they selected me, you know. So, to me it was not, it didn’t register as the first woman this or that.

De Swart:

And it’s always in hindsight that people, of course, care about something like this?

Bahcall:

Right. Yeah. I think it’s a matter of culture, and not training, but how you are raised and what you are used to.

De Swart:

Uhm-hmm. That’s interesting. Well…

Bahcall:

So, that never really, from my personal experience, did not play a huge role. Were there obstacles? Maybe there have been some. Maybe, you know, I could have this, or that, or that, but, you know, I never give it a second thought.

De Swart:

No. Well, that’s interesting.

Bahcall:

Vera Rubin, on the other hand, talks a lot about that, about her experience and how difficult it was, and how people didn’t take her seriously for a long time. I think people always took me seriously. So, you know, maybe I’m wrong, but I think people took me seriously. (Laughs) And so, I have no problem.

De Swart:

I think we have covered a lot already.

Bahcall:

All right.

De Swart:

So, maybe more like a general statement on the status of the problem of the problem of this whatever you want to call it, in the beginning maybe “missing mass,” at some point it turned into “dark matter.”

Bahcall:

Yeah.

De Swart:

But what would you say was the development of the, of the importance of the, of the problem? Or when, when did it get accepted?

Bahcall:

Well, I think it’s probably the most fundamental question in astrophysics, and cosmology, and in physics.

De Swart:

But when was it?

Bahcall:

That’s dark matter. What is it? You know.

De Swart:

Yeah. Yeah. Yeah.

Bahcall:

And we really don’t understand it.

De Swart:

Yeah.

Bahcall:

We can measure it really well, but we don’t understand exactly – we think it’s a new particle but we really, until we find it there will not be an answer. And somebody mentioned MOND today. And, some people, a few think, said, “There’s no problem of dark matter. You have to change Newton’s Law, you know.” By the way, was suggested before Milgrom. Some people frequently suggested. I don’t know names, but not “frequently.” Some people suggested, “Well, if you change Newton’s Law you don’t need dark matter.”

De Swart:

Okay. That’s interesting.

Bahcall:

So, nobody put together a scenario. And, you know, MOND is not a model. It’s just a scenario. There’s no GR (inaudible). There’s no real – but people did say, “Well, it’s all based assuming Newton’s Law. If Newton’s Law changed, then you don’t need dark matter.” That would, that all, that was said well before Milgrom. I always say it in my classes. I always say it in the lectures that I give. Because people should understand that there is a very basic fundamental assumption here for dark matter. Especially until we find the dark matter. Once we find it, fine. You don’t have to mention it again. But, until we find it, I don’t think it’s a hundred percent clear.

De Swart:

And, it seems that more and more…

Bahcall:

Maybe ninety-nine percent.

De Swart:

Yeah. Yeah. Yeah.

Bahcall:

But, you know, but it’s not a hundred percent.

De Swart:

Yeah. Yeah.

Bahcall:

We have to find this thing. It cannot be that we have it everywhere and, and we cannot detect it. So.

De Swart:

So, and when was it for you that, that dark matter turned into what you said, the most fundamental problem? When, when, when did this?

Bahcall:

I would say probably in the very early ‘70s.

De Swart:

Oh, really?

Bahcall:

Oh, yeah. I think so. I don’t remember my papers. I’ll have to look at them. But that’s when a lot of this discussion started coming together.

De Swart:

Oh, that’s interesting.

Bahcall:

As we talked about, the many different things. The x-rays from clusters was a big thing for me. I worked quite a bit on that. So, putting together the x-rays, the galaxy rotation curves, more velocities, and redshifts in clusters. So, all of that started coming together.

De Swart:

But that’s interesting because, for example, I think Jim Peebles would maybe say that it would be the beginning of the ‘80s, in the era of inflation and other cosmological models that were coming up.

Bahcall:

Well, but in the ‘80s the biggest question was, “What is omega matter?” And Jim Peebles and I talked. We’re good friends. We talk about it a lot and we are always like that. We are thinking very similarly. So, when everybody thought omega matter was one, we always kept talking about omega matter is low, based on various methods and so on. So, in the ‘80s, partly because of inflation, the biggest question was, “What was omega matter?” So, that’s really moving to the cosmology. “What is omega matter? Is it one? Is it .2? Is it . . .” you know. It was this big question of the cosmological parameter.

De Swart:

And was it…

Bahcall:

“Is it standard cold dark matter, open cold dark matter, Lambda CDM, Tilted CDM?” There were all these different things and there are, some of them are disposed with trial. But that was in the ‘90s.

De Swart:

But was this – so, you said – was, in whole of astronomy…

Bahcall:

In the ‘80s, but in the ‘70s – so, that was the ‘80s. It’s omega matter and moving towards the cosmology. In the ‘70s it was just the question of dark matter, not so much omega matter. There is dark matter in galaxies, in groups, in clusters. So, there’s the x-ray gas there. So, it was just this picture of the Peebles and Yahil and Ostriker paper of m/l (inaudible) increasing, all of that came in the ‘70s. So, that’s when I started paying much closer attention to it.

De Swart:

But the paper of Peebles and Yahil was on the mass density of the universe?

Bahcall:

That’s right. So, the mass density of the universe was also discussed then. But, in the ‘80s there was a big, big emphasis on determining – that was the main cosmological parameter. So, that was the main thing. It started in the ‘70s, but, but the ‘70s, the early ‘70s was when a lot of those that we just talked about, galaxies, and clusters, and started coming together, showing that there was dark matter. And then, if there’s dark matter, well, you ask, “Well, how much?” So, that’s natural.

De Swart:

And how did the focus on cosmology influence the field itself? Was it, was it splitting up or did all the astronomers went into more cosmological questions?

Bahcall:

Well, some people moved to cosmology after that, maybe in the ‘80s, especially after inflation was suggestion, both theoretically and observationally, to test it. So, more people clearly moved into – cosmology was a small field in the ‘30s, and ‘50s, and ‘60s with Sandage, and, you know, the people at Caltech. It was a big thing there. Sandage was one of the big fathers of cosmology. Anyway, and Jim Peebles, of course. But so I think in the ‘70s more people moved into trying to understand, maybe not so much the overall cosmology but really the rotation curves, and the groups, and the clusters, and dark matter, and what’s going on, and the gas in clusters, and what’s going on there. In the ‘80s, it was a push towards cosmology. And each of these decades more and more people moved in, both theoretically and observationally.

De Swart:

And was there…

Bahcall:

But the ‘90s—'80s and ‘90s—it became a big, big fight about the cosmology.

De Swart:

Yeah.

Bahcall:

ACDM, Lambda CDM, Open CDM, that was kind of late ‘80s, early ‘90s, and what, enormously pleased, and happy, and satisfied is that everything we showed in the ‘80s and the ‘90s about standard CDM being wrong, it showed that it’s either an open or Lambda CDM. It’s exactly correct with the same (inaudible) gave it.

De Swart:

So, in that sense, it’s worked really well?

Bahcall:

So, it’s really, you know, know, it’s omega matter .3, .2, .3. That’s what we’ve talked about.

De Swart:

And would you say that was a moment of crisis in which people were…

Bahcall:

Yes. There were, there were moments of really scientific – I don’t know “crisis,” but scientific confrontation or controversies about this SCDM, or Open CDM, or Titled CDM. And it’s, everybody was trying to understand what’s going on. Because I remember giving a lot of talks and there was a lot of papers maybe from that time, showing from different methods, from the mass-to-light the omega matter of .2. From the correlation function of clusters, in the ‘80s we got – and then in the ‘90s we fitted with cosmological simulation. We got it cannot possibly be omega matter one. It has to be small because you get this very large construction, which you cannot get if omega matter is high. So, the more matter you have the less structure you have. It’s gravity.

De Swart:

So, the crisis was more in getting the right cosmological model than, than the question of, “Is there more mass than we can see?”

Bahcall:

Well, those are connected.

De Swart:

Yeah. Okay.

Bahcall:

It’s the same question.

De Swart:

Yeah.

Bahcall:

So, either more dark matter – so, we claimed that by showing the mass-to-light, based on the cluster correlation function that we did in 1992 maybe was (inaudible) the cluster mass function, which people still use as one of the best cosmological tools now, and we suggested you can use, “How many clusters exist today at any given mass?” And that compared cosmological simulation you can see the omega matter one cannot, will give you ten times more clusters. So, we did a few different methods to show that omega – I can give you another paper that I have – that omega matter is small. And that was a biggest confrontation. And, when I would talk about it, some people would slowly start believing in it. I remember (inaudible) talk at MIT and (inaudible) said, “Well, I think it’s time to start thinking about open inflation,” you know, which I thought, “Gee. Wow.” I really was impressed with that. But then many people who said, “We cannot have, how can omega be .2 only? You know, it’s not natural. Not to any observational reason but for aesthetic reason? You know, it has to be one. How can it be .2?” Well, we were right, and I think I’m very happy about that. There is a paper. I don’t remember when we wrote it. Let’s see. Oh, that was ‘97. But it wasn’t any, like this, several of the same nature before. That was in ‘97 when we had (inaudible). This is a paper when I was elected to the National Academy.

De Swart:

Oh, wow.

Bahcall:

I gave, you know, a little talk there. And that’s the paper that I presented, “A Lightweight Universe”. And that was before the microwave background, before the supernova.

De Swart:

Oh, wow.

Bahcall:

So, you know, once the supernova came it became an issue for many people to buy a low omega matter universe, because with a supernova you can still get a total of one, and you have dark energy.

De Swart:

Yeah.

Bahcall:

But just to buy a low omega matter universe, and that’s one thing we concentrated on to determine, was difficult for many, not all but many people, to buy.

De Swart:

So, it’s interesting…

Bahcall:

Not Jim Peebles, not Joe Ostriker, not some people who are really smart, really could really see through. (Laughs) Look at the date. Look at what’s presented and makes sense. Yeah.

De Swart:

So, it’s interesting you say that these problems were the same problem. So, you would say then, when you, when they found this cosmological model people would also accept this notion of dark matter, the fact that…

Bahcall:

Yeah.

De Swart:

Okay.

Bahcall:

You mean, once the supernova thing came?

De Swart:

Yeah.

Bahcall:

Oh yeah.

De Swart:

Or even earlier?

Bahcall:

Well, oh, no. The fact that there was dark matter people have accepted already.

De Swart:

Before the cosmological disputes on the models?

Bahcall:

Oh, the fact that there was dark matter? Oh, yeah. That’s already accepted from the ‘70s.

De Swart:

Okay.

Bahcall:

That was, you know, both a combination of Zwicky and groups, and the rotation.

De Swart:

That’s interesting you mentioned….

Bahcall:

No. People accepted dark matter, except, of course, Milgrom, and MOND, you know, there are a few that still question it today. But, you know, 99.9% accepted it. And there was also, later on there was lensing. Probably, I don’t know when lensing came about, maybe early ‘90s there was lensing. So, you could see the gravitational lensing. And, once that was done that was pretty clear.

De Swart:

Yeah. Okay. But I was wondering if, if, how much impact these debates on cosmological models had on people accepting that there is extra mass, or no?

Bahcall:

No. No. No. So, that’s what I say. The ‘70s were different than the ‘80s and the ‘90s. Seventies it was just, “Oh, do we really have dark matter?” And the answer is “Yes.” And then, we can start thinking how much. But the main thing is to really – so, lots of rotation curve. Lots of velocities. Lots of virial theory. Lots of that. The ‘80s, there was more focus on how much total. In the ‘90s, and late ‘80s, and most of the ‘90s there was a big thing about the total cosmology. So, omega. So, it was not “Do we have dark matter or not?” but, “Is omega matter one or is it .2, .3?” That was the controversy. And it was very controversial.

De Swart:

Yeah.

Bahcall:

For, about more than a decade. For about a decade.

De Swart:

Yeah.

Bahcall:

Long time. Partly there were wrong observation, wrong analysis of observations with the big velocities people claimed observation. It was Dekel and Bernstein, and maybe Faber too. I don’t know. That claimed big velocities, used some code called (inaudible) that they wrote to convert from density fields to mass fields to velocity, and they claimed they see there were large velocities, and therefore you have to have a lot of matter, and therefore omega matter is one. That was after inflation. I think because inflation, they tried to get what they, thought they expected to get. That was very misleading, very unfortunate, and very bad for progress in the field, because they set the field back by several years. Until the – so, in many, many talks that I gave, I would show all of our methods about low omega matter and then somebody will say, “Well, what about (inaudible) and the high velocity? They get omega matter one.” I always said, “I think they are wrong.”

De Swart:

And, that was called (inaudible)?

Bahcall:

(inaudible). It’s Potential something. It’s converting from the density field to the potential field.

De Swart:

Oh. Okay. And that was Dekel, you said?

Bahcall:

It was Dekel and Bernstein.

De Swart:

Dekel’s with C-K, right?

Bahcall:

Dekel is – no, just K.

De Swart:

Okay.

Bahcall:

D-E-K-E-L. But he was wrong and he was misleading, and it was Joe Primack, and maybe even, I don’t know if Sandy Faber was involved or not, but they were very misleading. There were – I don’t know if the observations were wrong, but the (inaudible) code apparently, after years, several years, they found they had a mistake.

De Swart:

Oh, wow.

Bahcall:

But it was very bad for – sad because it kept it all up in the air. People who were not in the field always would question, “Well, they say one. Neta says omega matter is .2. Well, how do we . . .” And they understand it now. I didn’t, nobody understood the (inaudible). It was a black-box code. And it was clear – I remember a lot of discussions. Jim Peebles didn’t buy it at all. (Laughs) I didn’t buy it at all. And we tried to show that it didn’t seem reasonable. But they went around and gave lots of talks and that was not good for the field. Because, again, it’s not about whether there is dark matter or not, it’s just getting the right cosmology. Is omega matter one or low? And, of course, we won at the end, but I think it just shows that sometimes one group, who does a lousy job, can really mess up the field, hold back the field for a while, which is really bad. But, luckily, also we showed that using that method of peculiar (inaudible) motions is extremely sensitive. Because, you know, what they did is they measured the actual motion. So, for that we need to know what the redshift, that’s just the redshift, the redshift, you know, the full velocity, they have to subtract from that the actual distance. So, you have to measure distance indicator with supernova or with something else. That’s very absurd. Subtract the two and that gives you the actual peculiar motion of the Hubble diagram. And then, this peculiar motion tells you if you have big motion or small motion. The big motion you had high omega. But you’re subtracting two very big numbers. So, the uncertainty of the small number that you have is huge and they made a big thing about getting big numbers. We claimed that (inaudible) has to be very large, and a lot of systematics in there. That, you know, it just was not a reasonable way to go about it.

De Swart:

Uhm-hmm. And, so, do you remember the IAU on dark matter, which was given here in Princeton?

Bahcall:

Yes. Yes. Yes. That was like eighty-something.

De Swart:

‘85

Bahcall:

Yeah. Uhm-hmm. Yeah.

De Swart:

So, how would see, how would you say was the prospect then people were thinking about?

Bahcall:

I think that was when there was a lot, a lot of this discussion. That’s why we had this conference. I don’t remember now if it was a lot about the cosmology or it’s still about the just general dark matter and how much omega is, and so on. I think that’s what it was about. And I remember – oh, yeah. I now remember, Mark Davis was here, and he was part of that group. Very smart guy, but he was part of that group. And I remember I gave a talk, and again I talked about the low omega matter, a low, and I remember him saying, “Oh, we are going to hear again about the low omega matter,” because they wanted omega matter to be one. So, that’s what I mean. I was a period where it was intense, scientifically intense. Not unpleasant, but scientifically very intense to try and understand, “What’s the mass density of the universe? How much dark matter we have.” It was a very exciting time.

De Swart:

Yeah. So, if you have maybe another very vivid recollection then?

Bahcall:

Well, I just remember many questions or conversations, discussion at meetings after talks, when I would present everything. And some very good astrophysicists, very smart people would say, “Yes. It all looks very good. It all looks very right. It all looks very good. It all looks – but how can you, how can omega matter not…” I mean, that was a constant question. “How can omega matter not be one? It’s too much fine-tuning. How can it be like .2, .3?” And I remember my answer to that was, “Well, nobody knows, but what we need to do is go by the data. The data is what informs us what our universe looks like. Once we know what the data tells us about the universe, then we can go figure out how come it’s that thing.” We cannot go the other way. We cannot go and say, “Well, it’s fine-tuned. We don’t know; therefore we don’t believe it.” You have to go by the data. And I know when I give talks now I get a (inaudible) in one of those people there. And I, after my talk there I reminded him of that. He asked some question, and I reminded him of that, and I told him, “Well, we had this discussion many times after my talks. And, you were right, because, you know omega matter is not fine-tuned. That total is one. But I was certainly right that omega matter is low.” So, that’s, you cannot go just by the theory. You have to go by the data.

De Swart:

Who was that?

Bahcall:

That was relation of mine.

De Swart:

Oh, okay.

Bahcall:

Very nice guy. Very smart physicist. And he was right. I mean, it’s not fine – it is fine-tuned. It’s fine-tuned because why is omega matter .2?

De Swart:

Yeah.

Bahcall:

But the total is one.

De Swart:

Yeah.

Bahcall:

But I was certainly right on the omega matter. So, that’s why I always kept emphasizing, you just have to go, some – we don’t know why something is happening the way it is, we don’t yet understand. When we have to go first, we’ll find out, “What does the data tell us about the universe? How much matter is there?”

De Swart:

There was kind of a….

Bahcall:

And we cannot fudge it. You know, that’s what it is.

De Swart:

And that was kind of your philosophy of doing that?

Bahcall:

Yeah.

De Swart:

Yeah.

Bahcall:

Yeah. And, it still is, you know. You have to go by what the data is telling you and then you use it to compare with theory, with simulation, and use the data to inform you about what kind of models to think about.

De Swart:

Yeah. Okay. Thank you so much.

Bahcall:

Sure.

De Swart:

If there is anything which…

Bahcall:

Yeah. And I’m here, I’m here while you’re still here. I think I’m going to be away. Next week I’m at the Hubble Space Telescope Advisory Committee on Wednesday and Thursday, but otherwise I’m here this week and next week. So, if you have anymore questions or something, just stop by.

De Swart:

Yeah. Thank you so much. Maybe, well I’ll, actually I hope also to speak Jim and Jerry again.

Bahcall:

Good. Good.

De Swart:

Just to see if they…

Bahcall:

Have you talked to them already?

De Swart:

Yeah. Yeah. Yeah.

Bahcall:

You talked to Jerry?

De Swart:

Yeah.

Bahcall:

Okay.

De Swart:

And it was really…

Bahcall:

How did it go with Jerry?

De Swart:

Yeah. Very good. Very good, actually. Yeah.

Bahcall:

He will tell you that he discovered dark matter.

De Swart:

Well…

Bahcall:

A little bit.

De Swart:

A little bit, but it wasn’t that explicit. No, definitely not. But he did emphasize that he was working on rapidly-rotating stars.

Bahcall:

Right.

De Swart:

And, then went to look at rotating galaxies and see if they had the same equilibrium or non-equilibrium.

Bahcall:

Right. Yeah. Yeah. Another very famous person is a Jaan Einasto.

De Swart:

Uhm-hmm.

Bahcall:

So.

De Swart:

Yeah. So, he’s still in Estonia?

Bahcall:

He’s in Estonia.

De Swart:

Yeah.

Bahcall:

That’s right. It confused me. I know him well. He was just here recently.

De Swart:

Yeah. That’s a shame I missed him.

Bahcall:

Yeah. We had a very nice visit, a very nice time too. He’s one of those very classic, you know, talk about the classical, old-fashioned, but always thinking outside the box.

De Swart:

Hmm.

Bahcall:

Very old, classical Estonian. And he did a lot of his work very early on too.

De Swart:

Yeah. There are a lot of reviews of him on the history of that.

Bahcall:

Yes. Yes. So, yeah. So, anyway, I’m here if you want to have any more.

De Swart:

Yeah. Thank you so much.

Bahcall:

Just stop by. And that’s wonderful.

De Swart:

I’ll stop this.

[END]