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Number 460, December 7, 1999 by Phillip F. Schewe and Ben Stein
MEASUREMENTS OF THE COSMIC MICROWAVE BACKGROUND (CMB) provide new evidence that the expansion of the universe is accelerating. One of the greatest issues in cosmology is whether the current expansion will continue, reverse, or proceed at a diminishing rate. Supernova observations two years ago suggested that not only would the expansion not reverse but that it was in fact getting faster (Update 361). The new CMB mappings, carried out with telescopes on mountains and on balloons, reveal that the temperature of the microwave background varies in clumps with an angular size of about one degree on the sky, a result indicative of an overall "flat" geometry for the universe (New York Times, 26 November 1999). Another way of saying this is that the observed energy density of the universe is apparently equal to the critical density value of about 10-29 gm/cm3. But the amount of known matter (luminous and dark) is insufficient for producing a flat geometry, so additional energy, probably hiding in the universal vacuum, is needed. This energy, according to many theorists, would exert an effect equivalent to a repulsive form of gravity, thus working against the mutual gravitational attraction of galaxies. Much of the new work is available only in preprint form. For example, papers for one of the experiments, the "Boomerang" collaboration, which measures the CMB with a balloon-mounted detector, can be found on the Los Alamos server (Melchiorri et al., http://xxx.lanl.gov/abs/astro-ph/9911445.)
COOPERATIVE EVAPORATION, a process whereby droplets on a substrate do not evaporate independently but in a coordinated fashion, has been observed for the first time by physicists at the University of Konstanz (Claudia Schafle, claudia.schaefle@uni-konstanz.de). The researchers begin by laying down a periodic array of diethylene glycol drops 0.75 microns in radius and spaced by 2.5 microns (see figure at Physics News Graphics). (Condensing the droplets out of a supersaturated vapor onto a patterned grid of adsorption sites imposed on the surface with microcontact-printing was itself something of a feat). The Konstanz scientists found that some rows of droplets evaporated faster than other rows, leading to a sort of "superstructure." In other words, some drops would survive at the expense of the preferential evaporation of other drops in a methodical way. Previously scientists have considered how gas sensors comprised of liquid droplet arrays could be designed. The droplet size in such sensors can be made sensitive to environmental conditions by selective uptake of certain molecules. When monitoring the average droplet size by light scattering techniques, the concentration of the molecules can be determined. But for this to work the cooperative evaporation effect will have to be taken into effect. (Schafle et al., Physical Review Letters, 20 December 1999; Select Article.)
ATOM TRAP TRACE ANALYSIS, the search for tiny isotope fractions among atoms using a magneto-optic trap, may soon be preferable to accelerator mass spectrometry (in which atoms are heated, accelerated, and sent through a strong magnet, which sorts the atoms by mass) for certain radio-dating purposes. To demonstrate this idea, physicists at Argonne (Zheng-Tian Lu, 630-252-0583, lu@anl.gov) have detected traces of krypton-85 (with an abundance of only 10-11) and krypton-81 (abundance of 10-13) in an atom trap with an efficiency of 1 part in 107; accelerator mass spectrometry, which requires an accelerator, currently has a counting efficiency of a part in 105. Keeping track of Kr-85 atoms is important since they are produced chiefly in nuclear-fuel reprocessing plants, and (arising mostly since the 1950s) are used as a tracer of air and ocean currents. Kr-81, in contrast, is made in cosmic-ray showers in the upper atmosphere and (with a half life 40 times longer than C-14's) is preferable to carbon-dating for calibrating the antiquity of million-year-old samples of ice and ground water. (Chen et al., Science, 5 November 1999.)
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