Cosmologists Roll Out Flat Universe

Cosmologists Roll Out Flat Universe

by Simon Mitton

WHAT'S HOT IN PHYSICS...

Rank	Paper	Citations This Period Nov- Dec 00	Rank Last Period Sep- Oct 00
1	L. Randall, R. Sundrum, "An alternative to compactification," Phys. Rev. Lett., 83(23):4690-3, 6 December 1999. [Princeton U., NJ; MIT, Cambridge; Boston U., MA] *261PU	47	2
2	L. Randall, R. Sundrum, "Large mass from a small extra dimension," Phys. Rev. Lett., 83(17):3370-3, 25 October 1999. [Princeton U., NJ; MIT, Cambridge; Boston U., MA] *247VZ	46	1
3	S. Perlmutter, et al., "Measurements of W and LAMBDA from 42 high-redshift supernovae," Astrophys. J., 517(2):565-86, 1 June 1999. [10 institutions worldwide] *208VC	27	†
4	J.N. Bahcall, P.I. Krastev, A.Y. Smirnov, "Where do we stand with solar neutrino oscillations?" Phys. Rev. D, 58(9):6016, 1 November 1998. [Inst. Adv. Study, Princeton, NJ; U. Wisconsin, Madison; ICTP, Trieste, Italy] *134GV	26	5
5	N. Arkani-Hamed, S. Dimopoulos, G. Dvali, "Phenomenology, astrophysics, and cosmology of theories with submillimeter dimensions and TeV scale of quantum gravity," Phys. Rev. D, 59(8):6004, 15 April 1999. [Stanford U., CA; ICTP, Trieste, Italy] *186XE	25	6
6	C.C. Steidel, et al., "Lyman-break galaxies at z ł 4 and the evolution of the ultraviolet luminosity density at high redshift," Astrophys. J., 519(1):1-17, 1 July 1999. [Caltech, Palomar Observ., Pasadena; Space Telescope Sci. Inst., Baltimore, MD; Royal Greenwich Observ., Cambridge, U.K.] *211TH	21	†
7	P. de Bernardis, et al., "A flat Universe from high-resolution maps of the cosmic microwave background radiation," Nature, 404(6781):955-9, 27 April 2000. [17 institutions worldwide] *309HG	17	†
8	L. Baudis, et al., "Limits on the Majorana neutrino mass in the 0.1 eV range," Phys. Rev. Lett., 83(1):41-4, 5 July 1999. [Max Planck Inst. Nucl. Phys., Heidelberg, Germany; Russian Sci. Ctr., Moscow] *212VH	15	†
9	M. Uehara, et al., "Percolative phase separation underlies colossal magnetoresistance in mixed-valent manganites," Nature, 399(6736):560-3, 10 June 1999. [Rutgers U., Piscataway, NJ; Lucent Technol., Murray Hill, NJ; Aoyama Gakuin U., Tokyo, Japan; Tokyo Inst. Technol., Japan] *204RR	14	†
10	W. Voges, et al., "The ROSAT all-sky survey bright source catalogue," Astron. Astrophys., 349(2):389-405, September 1999. [Max Planck Inst. Extraterr. Phys., Garching, Germany] *240YB	14	†

SOURCE: ISI's Hot Papers Database. the full legend.

he geometry of the universe has always fascinated cosmologists. Circular symmetry entranced Greek philosophers, whose obsession was finally overthrown when Johannes Kepler showed that Mars moves on an elliptical orbit. Newton’s gravitational theory empowered mathematicians to unweave the geometry of planetary motions, and would lead to the discovery of Neptune. The introduction of four-dimensional geometry by Einstein, in the general theory of relativity, gave a mathematical explanation for the force of gravity. Einstein’s field equations permit a wide range of geometries, with curved universes, imploding universes, and even bouncing universes allowed within the solutions.

To discover what kind of universe we are living in it’s necessary to put some observational constraints on the theory, which the observational cosmologist does by looking at remote sources of radiation. The further photons have travelled through the universe to reach our telescopes, the more they tell us about its shape. Science Watch this period has two complementary papers on cosmic geometry, both of them new entrants to the Physics Top Ten. Paper #3 on observations of exploding stars in distant galaxies, together with #7 on microwave background radiation, shows mounting evidence for a flat universe.

In 1988 Saul Perlmutter started the Supernova Cosmology Project (SCP), with the primary goal of establishing the relationship between observed brightness and distance for Type Ia supernovae. Science Watch reported the results of a rival collaboration with similar goals in the last period (12 [1]:6, January/February 2001), where evidence that the universe is accelerating was garnered from just 10 supernovae. In #3 Perlmutter analyzes a larger dataset from 42 high-redshift supernovae and reaches essentially the same conclusions. The observational technique is extremely demanding because these supernovae (which erupt without warning) must be detected before they have reached their peak brightness, and then followed with spectrometry and photometry for two to three months. SCP uses several of the world’s largest telescopes to get data.

For each supernova the SCP measured the redshift, which is the easy part, and the effective peak magnitude, which is hard. The magnitude estimate needs correction for cosmological effects and the dimming of light by dust in our galaxy. The corrected data are then plotted on a Hubble Diagram (first used in 1929). At low redshift the magnitude-redshift relation is linear, and any deviation from this at high redshift carries information about the geometry of the universe. The conclusions in #3 include an age for the universe of 14.9 billion years, a compelling argument that the geometry is flat (follows Euclidean geometry), and evidence that the expansion is accelerating.

Newcomer #7 reports "Boomerang" observations of structure in the cosmic microwave background, the dying echo of the Big Bang that is almost, but not quite, isotropic. Tiny irregularities in the early universe imprinted the background in the form of small anisotropies of the temperature. These wrinkles in the background contain information about basic cosmological parameters. The Boomerang experiment is a microwave telescope taken to 38 km altitude by a balloon over Antarctica. In a 249-hour flight the telescope mapped a part of the sky that is free of thermal radiation from interstellar dust.

The resulting maps show variations at the level of 1 part in 100,000 in the cosmic background temperature of 2.73 K, and they give the angular scale of the minuscule fluctuations. What the astronomers are seeing in these ripples are sound waves caused by gravitational forces acting on plasma in the early universe. The power spectrum of temperature fluctuations peaks on an angular scale of about 1 degree which implies a flat universe, as suggested by the supernova data.

Papers #3 and #7 find that ordinary matter and dark matter together account for less than half the content of the universe. There is a ubiquitous dark energy as well, which is not subject to gravitational force, and which repels matter, so driving the acceleration of the universe. This fifth force of physics, known as quintessence, represents an energy intrinsic to the structure of spacetime.

Right now cosmologists are keen to get more evidence for quintessence, and a battery of new astronomical satellites are on the drawing board. Perlmutter’s supernova hunters are upping their game with a proposal for SNAP, the SuperNova Acceleration Probe, which will discover 2,000 supernovae a year. June 2001 should see the launch of the Microwave Anisotropy Probe, a mission to map the microwave ripples with even greater accuracy. These are exciting times for cosmologists, and more Hot Papers from them are in prospect.

Dr. Simon Mitton is Senior Fellow, St. Edmund’s College, University of Cambridge, U.K.