Number 683, April 29, 2004 by
Phil Schewe and Ben Stein
Illuminating the Dark Ages
In the very early universe the so called "dark age" comes after the
time of the first atoms---a moment when suddenly neutral atoms, mostly
hydrogen, could form, allowing photons to stream freely, photons we
now see as the microwave background---but before the first stars formed.
But maybe this era needn't be so dark. Just as numerous finds of arts
and crafts from the European dark ages have helped to enlighten us
on what the sixth to the eleventh centuries were like, so too some
bits of light from the cosmic dark ages might illuminate that epoch.
Abraham Loeb and Matias Zaldarriaga of Harvard believe that the early,
cold, neutral hydrogen can be made to speak, as it were. These atoms,
in a redshift window of about 30 to 100, would be colder than the background
radiation. The atoms would absorb photons and cause a deficit in the
microwave background at cold hydrogen's characteristic wavelength of
21 centimeters. This absorption wavelength, in turn, would be stretched
out, courtesy of the universal expansion of the universe, to a wavelength
of 6-21 meters or so.
Because the cosmic hydrogen is not uniform, the level of absorption
varies across the sky and the microwave background would show anisotropies
at these long wavelengths. These anisotropies could be sought using
special radio interferometers. (Some efforts are already underway to
see this kind of light: see the LOFAR and SKA websites.)
Just as microwave telescopes mapping the early sky see minute temperature
variations, so the primordial hydrogen could also be mapped. This map
might well show the influence of dark matter through its influence
in shepherding early hydrogen.
Interest in this hydrogen has been expressed before, but the Harvard
proposal is the first to be specific about how to search for information
imprinted in the dark-age atom distribution. (Physical
Review Letters, upcoming article; contact Abraham Loeb, email@example.com
Magnesium-diboride superconductors can tolerate twice the usual amount
of magnetic field if you spike them with some carbon atoms. The main
reason superconducting wires are used as the windings in magnets is
not because they save energy, but because they can generate large magnetic
fields by carrying large current densities without the resistive heating
associated with ordinary copper wire, giving you a much more intense
field for the same amount of volume employed in your MRI machine.
MgB2 superconductors, which made their debut three years
ago (see Update
530), become superconducting at around 40 K, in a colder regime
than for the ceramic superconductors (which can be bathed in liquid
nitrogen), but much warmer than traditional metal superconductors (such
as niobium-tin) which must be cooled in liquid helium. Some consider
that the MgB2 materials (which can be chilled with refrigerators
without the use of expensive liquid helium) might be advantageous in
some applications where Nb3Sn is presently used. For this
to happen, the MgB2 materials need to be able to stand up
to high fields and high current densities.
At Iowa State, a new test of carbon-doped MgB2 shows that
the critical field can now be doubled, up to a value of 32.5 Tesla;
this is the field at which superconductivity in unadulterated MgB2 would
be undone. This is now higher than the best value for Nb3Sn.
The researchers (contact Paul Canfield, firstname.lastname@example.org, 515-294-6270)
would like MgB2 to tolerate even higher fields, and to enhance
the critical current too. (Wilke et al., Physical
Review Letters, upcoming article.)
What Kind of Fluid is a Quark-Gluon Plasma?
The hot soup of free quarks and gluons that existed in the very early
universe, and a state of matter that physicists have been trying to
re-create amid high-energy nuclear collisions, QGP is actually not
a superfluid, as the original version of Update
681 erroneously suggested.
According to University of Washington physicist Laurence Yaffe (206-543-3902,
email@example.com), QGP is actually a normal, conducting fluid.
It has viscosity, eliminating it from the list of superfluids. It is
somewhat electrically resistive, precluding it from being a superconductor.
Yaffe and coworkers recently performed calculations of several QGP
fluid properties from first principles (P. Arnold, G. D. Moore and
L. G. Yaffe, Journal of High Energy
Physics, 17 June 2003 and 14 February 2003).
Still, observations of high-density quark matter produced thus far
at Brookhaven's RHIC accelerator suggest that QGP might prove to be
the most ideal regular fluid observed in nature, according to Ohio
State nuclear theorist Ulrich Heinz (614-688-5363, firstname.lastname@example.org).
The viscosity of the RHIC matter appears to be exceedingly low, and
it redistributes its heat ("rethermalizes") extremely quickly. This
near-ideal regular fluid behavior should greatly facilitate comparisons
between theory and experimental observations of QGP, once its presence
is confirmed at RHIC.