Number 354, January 12, 1998 by Phillip F. Schewe and Ben Stein
COSMIC INFRARED BACKGROUND DISCOVERED. The Cosmic Microwave Background Explorer (COBE) collaboration, which six years ago reported the first evidence for structure in the microwave background (Update 77), has now finished a mapping of the whole sky at ten different infrared wavelengths, from 1 to 240 microns. After carefully subtracting the expected contributions from our own solar system and the Milky Way galaxy (understanding the foreground sources of infrared was itself a process that took years) what is left over is the cosmic infrared background, the cumulative IR radiation (amounting to one-half to two-thirds of the total light) coming from all the stars that have ever existed. Much of the light that reaches the detector has been scattered in transit by dust. The cosmic IR background appears uniform (no structure is apparent) and bears no information about when during the history of the cosmos the radiation was emitted. Nevertheless, the observations have helped to provide rough limits on the amount of star formation in the universe and confirms the suspicion that a lot of star birth has been obscured by dust. Michael Hauser, now at the Space Telescope Science Institute, delivered the main COBE report at last week's meeting of the American Astronomical Society (AAS) in Washington, DC.(Image at Physics News Graphics website.)
A 2.6 MILLION SOLAR MASS BLACK HOLE lurks at the center of the Milky Way. New measurements carried out with optical and radio telescopes have zeroed in on the heavy monster long known to exert a huge gravitational pull at the heart of our galaxy in the constellation Sagittarius. Andreas Eckart of the Max Planck Institute in Garching, Germany presented a film at the AAS meeting showing the proper motions (recorded over five years) of several stars within a few light days of the heavy object. The measured velocities of these stars, some as great as 1000 km/sec, lead to a mass estimate for the object of 2.6 (with an uncertainty of only 0.3) million solar masses. Considering that all of this mass must fit into dimensions of much less than a few light days across, Eckart asserted that the object could only be a black hole.
URANIUM PROSPECTING WITH NEUTRINOS. Up to 40% of the 40 terawatts of energy leaving the Earth's surface is believed to come ultimately from the radioactive decay of uranium-238 and thorium-232. Simple geological models predict where these isotopes are to be found: half in the crust beneath continents and half in the mantle. But geophysicists would like to map the deposits more accurately, especially since radiogenic heat has had a large role in determining the dynamics of our planet's interior. Now, scientists at Bell Labs, the Technical University of Munich, and Tohoku University (Japan) are proposing a scheme in which neutrinos would survey the Earth just as positrons reveal the presence of metabolic activity (parts of the brain lighting up, say) through the process of positron emission tomography. In this case radioactive deposits would announce themselves by the neutrinos they cast off. These neutrinos, which effortlessly plow through the Earth's bulk, would register in three surface detectors: Borexino, to come online in 1999 in Italy; Kamland in Japan, around the year 2001; and the proposed Geomanda detector at the South Pole. The numbers and energies of the neutrinos can be used not only to chart the density of the U and Th hoards, but also to differentiate between the two nuclides, thus providing a sort of global analytical chemistry of the Earth. (R.S. Raghavan et al., Physical Review Letters, 19 January 1998; contact Raju Raghavan, 908-582-4351, firstname.lastname@example.org; picture at Physics News Graphics website.)