UM Home | Search Advanced Search
PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 467, January 18, 2000 by Phillip F. Schewe and Ben Stein
THE X-RAY BACKGROUND, the glow of x rays seen in all directions in space, has now largely been resolved into emissions from discrete sources by the Chandra X-Ray Telescope, ending the notion that the x rays come from distant hot gas. Previously only about 20-30% of the x-ray background had been ascribed to point sources (by such telescopes as ASCA). Chandra was launched in July 1999 and put in an elliptical orbit. With its high angular resolution and acute sensitivity it could tell apart x- ray objects (many of them thought to be accretion disks around black holes) that before had been blurred into a continuous x-ray curtain. (Of course, now that the background has been resolved into points it ceases to be a background.) Richard Mushotzky of Goddard Space Flight Center reported these Chandra results at last week's meeting in Atlanta of the American Astronomical Society (AAS). Resolving the x-ray background was not all. Mushotzky added that the Chandra survey had revealed the existence of two categories of energetic galaxies that had been imaged only poorly or not at all by optical telescopes. He referred to one category as "veiled galactic nuclei," objects (with a redshift of about 1) bright in x rays but obscured by dust at optical wavelengths. The other category was "ultra-faint galaxies." One interpretation of these galaxies is that optical emission is suppressed owing to absorption over what could be a very long pathway to Earth. Mushotzky speculated that such high redshift (z greater than 5) galaxies could be the most distant, and hence earliest, objects ever identified. The XMM x-ray telescope, just launched, should provide complementary information in the form of high-precision spectra (from which redshifts are derived) of the distant objects.
OTHER CHANDRA RESULTS at the meeting included the mapping of a thousand x-ray stars in the Orion Nebula portion of our galaxy 1500 light years away, making this the highest density of x-ray sources yet recorded. Gordon Garmire of Penn State spoke about this finding as well as about the effort to find x-ray counterparts for objects cataloged in the Hubble Deep Field image made with visible light; some tentative matches were made. Meanwhile, Frederick Baganoff of MIT reported that Chandra's inspection of the center of the Milky Way revealed what might be the first recorded x-ray signal from the vicinity of the massive (2 million solar mass) black hole residing at or near the radio-bright object called Sagittarius A*. In x rays this object proved to be fainter than expected by a factor of 5. The supermassive black hole at the heart of our sister spiral galaxy, Andromeda, also is much cooler than expected. According to Stephen Murray from Harvard-Smithsonian, the measured temperature was only a few million K, compared to temperatures of tens of millions for much more modest x-ray stars in the same galaxy. None of this fits with theories of supermassive black holes. Finally, Claude Canizares of MIT summarized Chandra observations of supernova remnant E0102-72, located in the Small Magellanic Cloud. E0102-72 is the leftover from an explosion 1000 years ago of a huge star of 15-20 solar masses. A diffraction grating on the telescope was used to spread out incoming x rays into a spectrum which could be scanned for the presence of specific elements in the stellar debris. Canizares estimated that as much as 10 solar masses' worth of oxygen was present in the wreckage of the older star, enough to furnish thousands of solar systems like ours with the breathable element needed for much of life on Earth.
SOLITARY, WANDERING BLACK HOLES, unheralded by any bright accretion disk or rapidly orbiting stars or gas, have been detected through the process of gravitational microlensing. The Massive Compact Halo Object (MACHO) collaboration regularly views millions of stars in the direction of the dense bulge of our galaxy hoping to observe, every now and then, stars brightening courtesy of the lensing caused by the passage of some nonluminous object (hovering in the galaxy's halo) between us and the star. The brightening can last as short as two days or as long as 1000. Longer durations suggest either large or very slow lensing objects. David Bennett of Notre Dame reported at the AAS meeting on two such long-duration events in which the mass of the lens was calculated to be roughly 6 solar masses, too heavy to be a neutron star and more likely to be a black hole. Bennett speculates that the lone-wolf black holes form from supernova collapse and might be as common as neutron stars in the galaxy.
PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 479 April 13, 2000 by Phillip F. Schewe and Ben Stein
DARK ENERGY AND THE MICROWAVE BACKGROUND. The theory of general relativity introduced the notion that spacetime could be warped or curved by the presence of matter. Locally, stars or any object with mass will curve space, but the expansion of the universe itself may introduce a curvature of its own. This is how cosmologists summarize things: a static universe with no matter (if such a thing were possible) would have no curvature. If, however, the empty universe were expanding it would have negative overall curvature. Increase the mass density from zero and the curvature would be less negative. Add still more mass and you might reach a net zero curvature. The ratio of matter to the critical matter needed for zero curvature is called omega; the popular version of the big bang model, featuring a very rapid expansion in an early "inflation" phase, predicts that omega should equal 1 exactly. A new paper in Physical Review Letters by Scott Dodelson of Fermilab and Lloyd Knox of the University of Chicago (773-834-3287) provides the theoretical underpinning for the higher-precision mappings of the cosmic microwave background (CMB) reported over the past nine months. The paper was prepared just as the first of the observational results appeared last summer: a Princeton-Pennsylvania collaboration taking data from Cerro Toco in Chile. Their findings (preprint astro-ph/9906421) can be plotted as the size of the observed fluctuations in the CMB as a function of the angular size of the fluctuation region (actually astrophysicists usually transform the data so that it can be plotted against the size of angular moment, or "l"). These data and those of the "Boomerang" (preprint 9911444 and 9911445; also see Update 460) and "Viper" (preprint 9910503) groups sit right on top of a theoretical curve drawn by Dodelson and Knox corresponding to the case where omega equals 1 and the net curvature of the universe is zero. With the contribution of matter (luminous and dark) to the density of the universe expected to be about one-third the critical value (of omega=1), this presents a stronger-than-ever argument in favor of the existence of yet another form of energy, often called "dark energy," to provide the missing two-thirds of the energy needed to make omega=1. This dark energy would also provide the "negative pressure" or repulsiveness needed to make the expansion of the universe greater than in the past, a development suggested independently by studies of distant supernovas. (Dodelson and Knox, Physical Review Letters, 17 April; Select Article.)
PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 481 April 27, 2000 by Phillip F. Schewe and Ben Stein
BEST MAP YET OF THE COSMIC MICROWAVE BACKGROUND (CMB). The CMB is a redshifted picture of the universe at the moment photons and newly formed hydrogen atoms parted company roughly 300,000 years after the big bang. First detected in the 1960s, the CMB appeared to be utterly uniform until, eight years ago, the COBE satellite provided the first hint of slight temperature variations, on a coarse scale, with an angular resolution of about 7 degrees. Since then several detectors have obtained resolutions of better than 1 degree. Actually, the contribution to small-scale fluctuations in the CMB is customarily rendered in terms of multipoles (a sort of coefficient), denoted by the letter l. The contribution to the temperature fluctuations in the CMB for a multipole value of l comes from patches on the sky with an angular size of pi/l. COBE's CMB measurements extended to a multipole of only about 20, but a major new map, made using a detector mounted on a balloon blown all the way around the Antarctic continent, covers the multipole range from 50 to 600, thus probing CMB fluctuations with much finer angular detail, over about 3% of the sky. The 36-member, international "Boomerang" (Balloon Observations of Millimetric Extragalactic Radiation and Geomagnetics) collaboration, led by Andrew Lange of Caltech and Paolo de Bernardis of the University of Rome, confirms that a plot of CMB strength peaks at a multipole value of about 197 (corresponding to CMB patches about one degree in angular spread), very close to what theorists had predicted for a cosmology in which the universe's overall curvature is zero and the existence of cold dark matter is invoked. The absence of any noticeable subsidiary peaks (higher harmonics) in the data, however, was not in accord with theory. The shape of the observed pattern of temperature variations suggests that a disturbance very like a sound wave moving through air passed through the high-density primordial fluid and that the CMB map can be thought of as a sort of sonogram of the infant universe. (de Bernardis et al., Nature, 27 April 2000.)
PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 482 May 3, 2000 by Phillip F. Schewe and Ben Stein
MAGNETIC FIELDS ARE EVERYWHERE. The history of the universe is usually described in terms of the distribution of matter: first primordial knots, then clouds, galaxies, stars, and clusters. A parallel, and perhaps not unrelated, saga can be written for magnetic fields. Basically, Philipp Kronberg (416-978-4971) of the University of Toronto finds magnetic fields every place he has looked in the cosmos: within the Milky Way (where the fields are typically about 5 microgauss), in intergalactic areas within galaxy clusters (1-2 microgauss for the Coma cluster, 350 million light years away), and even outside clusters. The latter observations are brand new and were reported by Kronberg at the APS meeting (http://www.aps.org/meet/APR00/baps/vpr/layb7-02.html). Detecting weak magnetic fields outside clusters was difficult and required the use of new low-frequency receivers mounted on the Very Large Array (VLA) radio telescope. The radio range employed, around 75 MHZ, is normally problematic owing to scattering in the Earth's ionosphere, but new image processing techniques have allowed a sharp VLA "deep field" image to be formed. From the intensity of the radio glow, Kronberg deduced a magnetic field of about 10^-8 to 10^-7 gauss for a distant region outside any galaxy cluster, a place (near the "Great Wall") where fields had not been mapped before. Where did such fields come from? Kronberg suggests that huge shock waves, formed where two large streams of weakly magnetized gas come together, could amplify existing fields to much higher levels, as well as playing a part in the acceleration of cosmic rays. Angela Olinto (paper B7.1) of the University of Chicago (773-702-8206) discussed the idea of primordial magnetism, fields that might have existed at or shortly after the time of the big bang. Such fields, she speculated, might have come about through the development of some asymmetry (just as matter came to predominate over antimatter) in the infant universe. Early magnetism might then have influenced subsequent galaxy formation or even the distribution of matter now seen imprinted in the cosmic microwave background (CMB). She said that the surprising absence of subsidiary peaks in the CMB spectrum (see Update 481) might be attributable to magnetic effects. This hypothesis could be addressed, Olinto said, by the Planck satellite (launch date several years from now; see Update 342), dedicated to mapping the CMB with unprecedented precision.