Physics News Update, Number 479

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.)

STORAGE OF HARD X-RAYS IN A CRYSTAL RESONATOR, at least for as many as 14 back-and-forth cycles, has been achieved by physicists at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The resonator consists of a pair of vertical walls (each 70% reflective) 150 mm apart carved from a single crystal of silicon, thus ensuring the perfect alignment of rows of atoms in both walls.

X rays with an energy of 15.8 keV from the ESRF machine arrive in the resonator in bursts only 10^-10 seconds in duration and with a wavelength of only 7.8 x 10^-11 m. The x rays are retained by multiple Bragg scattering between the walls. Previously lower energy "soft" x rays had been stored in resonators but this is the first time for higher energy x rays. The new resonator also serves to sharpen the range of energy of x rays transmitted as compared to the range for the incoming x rays. Plenty of x-ray optics applications are expected. (Liss et al., Nature, 23 March 2000.)

THE LONGEST COMET TAIL ever recorded, more than 570 million km, has come to light in the form of a fluctuations in magnetic fields and in the amount and species of charged particles detected by the Ulysses spacecraft on 1 May 1996. A puzzle at first, the fluctuations are now attributed to Ulysses' inadvertent passage through the wake of Comet Hyakutake. The long tail is much greater than that of the previous record holder, the Great March Comet of 1843. If Hyakutake's tail had been visible on Earth at that point in the Comet's trajectory, it would have arced more than 80 degrees across. (Jones et al. and Gloeckler et al., Nature, 6 April 2000.)

TSUNAMIS IN LAKE TAHOE. One thinks of skiing and hiking in reference to this lake nestled between Sierra mountain peaks. Nevertheless, a system of faults near the lake might conspire to generate monster waves in the event of a magnitude-7 earthquake in the area. Simulations by researchers at the University of Nevada indicate that a tsunami and follow-up waves lasting perhaps for hours could produce wave heights as high as 10 m. (Ichinose et al., Geophysical Research Letters, 15 April 2000.)

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