Physics Over the Moon with BOOMERANG’s Returns

Physics Over the Moon with BOOMERANG’s Returns

by
Simon Mitton

WHAT'S HOT IN PHYSICS
Rank	Paper	Citations This Period (Mar-Apr 03)	Rank Last Period (Jan-Feb 03)
1	Q.R. Ahmad, et al., "Direct evidence for neutrino flavor transformation from neutral-current interactions in the Sudbury Neutrino Observatory," Phys. Rev. Lett., 89(1): 1301, 1 July 2002. [17 institutions worldwide] *563YN	40	2
2	A. Bachtold, et al., "Logic circuits with carbon nanotube transistors," Science, 294(5545): 1317-20, 9 November 2001. [Delft U., Netherlands; Ecole Norm. Super., Paris, France] *491VF	38	†
3	R.R. Metsaev, "Type IIB Green-Schwartz superstring in plane wave Ramond-Ramond background," Nucl. Phys. B, 625:70-96, 18 March 2002. [Lebedev Phys. Inst., Moscow, Russia] *531CY	37	5
4	W.L. Freedman, et al., *"Final results from the Hubble Space Telescope* Key Project to measure the Hubble constant,"** Astrophys. J., 553(1): 47-72, 20 May 2001. [15 institutions worldwide] *442JA	36	†
5	Q.R. Ahmad, et al., "Measurement of the rate of ne, + d à p + p + é interactions produced by ⁸B solar neutrinos at the Sudbury Neutrino Observatory," Phys. Rev. Lett., 87(7): 1301, 13 August 2001. [15 institutions worldwide] *463LU	34	3
6	Q.R. Ahmad, et al., "Measurement of day and night neutrino energy spectra at SNO and constraints on neutrino mixing parameters," Phys. Rev. Lett., 89(1):1302, 1 July 2002. [17 institutions worldwide] *563YN	34	6
7	J. Kortus, et al., "Superconductivity of metallic boron in MgB₂," Phys. Rev. Lett., 86(20): 4656-9, 14 May 2001. [Max Planck Inst. Solid State Res., Stuttgart, Germany; Georgetown U., Washington, D.C.; Naval Res. Lab., Washington, D.C.; Iowa St. U., Ames] *431GM	33	7
8	A.Y. Liu, I.I. Mazin, J. Kortus, "Beyond Eliashberg superconductivity in MgB₂: Anharmonicity, two-photon scattering, and multiple gaps," Phys. Rev. Lett., 87(8): 7005, 20 August 2001. [Georgetown U., Washington, D.C.; Naval Res. Lab., Washington, D.C.; MPI Festkorperfor., Stuttgart, Germany] *465MJ	30	8
9	C.B. Netterfield, et al., "A measurement by BOOMERANG of multiple peaks in the angular power spectrum of the cosmic microwave background," Astrophys. J., 571(2): 604-14, 1 June 2002. [14 institutions worldwide] *556CB	29	†
10	S. Fukuda, et al., "Solar ⁸B and hep neutrino measurements from 1258 days of Super-Kamiokande data," Phys. Rev. Lett., 86(25): 5651-5, 18 June 2001. [27 institutions worldwide] *443ZG 28	28	10
SOURCE: ISI's Hot Papers Database. the full legend.

he hot physics story this period is the continuing fascination of the cosmic microwave background radiation (CMBR), and with what its properties can tell us about the early universe, as well as the properties of the universe today. Our Hot Paper #9, a newcomer, squeezes yet more information from cosmic background measurements made by a 12-day balloon flight in 1998-99.

When radio physicists discovered the CMBR nearly 40 years ago, they immediately foresaw that an accurate measurement of the spectrum might confirm that it is heat radiation left over from the Hot Big Bang of 14 billion years ago. Ten years ago results from the COBE satellite stunned the cosmology community: the spectrum exactly fitted the Planck thermal curve with T=2.78 K. But the real surprise lay in the tiny variations in temperature (~ 10^-5 K) across the sky. This anisotropy of the CMBR is the key to unlock secrets of the universe, because it shows that structure was present when the universe was only 300,000 years old. At the time of discovery, astronomers speculated that maybe the warmer regions condensed into the first galaxies, while the cooler regions became the intergalactic voids.

Photons in the CMBR scattered for the last time from free electrons in the hot plasma at a redshift of ~10^-3. They have traveled freely ever since, imprinted with the effects of their final acoustic scattering. Many processes can cause the temperature of the photons reaching us to be different in different directions. Untangling the underlying causes of these fluctuations is very demanding, but as #9 amply demonstrates, a searching analysis gives values for several of the fundamental cosmological parameters. The Hubble radius at the decoupling epoch subtends an angle of ~1°on our sky. This means that anisotropies at >1° probe density fluctuations that drill down to the primordial level, whereas the small angular scale variations tell us about the cosmological parameters that determine the matter-energy content of the universe.

In BOOMERANG’s first long-duration flight (257 hours of data), the balloon lofted cryogenic detectors to an altitude of 39 km. Paper #9, headed by Barth Netterfield (University of Toronto), selects high-resolution data from 1.8% of the sky. The resulting power spectra go out to much higher multipoles (600) than previously tried by the BOOMERANG collaboration. Therefore they are of higher precision, and work to smaller angular scales, than the "discovery" paper. The spectra contain multiple peaks and minima, as predicted by standard inflationary models with sound waves in the primordial plasma. An interpretation in conjunction with other cosmological data and models refines the values of seven cosmological parameters. An important achievement of #9 is the elimination of certain "alternative models" of the very early universe.

In one sense the values for parameters extracted by #9 are no surprise, because they agree with conclusions of earlier experiments. A further, even more detailed, analysis was released by the BOOMERANG team last December (J. Ruhl, et al., Astrophys. J., in press; astro-ph/0212229). The best fit is a 13.5 Gyr universe composed of 5% baryonic matter, 30% cold dark matter, and 65% dark energy.

Observational cosmology is now thriving as never before. There is a broad consensus on the age of the universe, the expansion rate, and the subdivision of mass-energy into baryonic matter, cold dark matter, and dark energy. Inflation is accepted as a given. In a sure sign that the subject is maturing, observational papers are now tending to reduce the error bars around a converging cluster of data, rather than suggesting that earlier papers are wrong. The numbers have reached the point where they are uncontentious.

Data from BOOMERANG and related CMBR experiments is being highly cited because it makes rich contributions to fundamental physics, not just cosmology. One impressive outcome is that the curvature of the universe is essentially zero, confirming the inflationary theory that is a keystone of the Hot Big Bang model. BOOMERANG data also probes neutrino physics, by severely limiting the possibility of a large cosmological lepton asymmetry—a requirement of some extensions to the standard model of particle physics.

Results from BOOMERANG 2 are now eagerly awaited. This mission, launched in January 2003, did not go too well. The balloon failed to reach its intended altitude, and subsequently lost altitude steadily, with the payload crash-landing on the Antarctic plateau. Fortunately the all-important measurements of CMBR polarization are intact, and presently being analyzed. No doubt further treats are in store for observational cosmologists.

Dr. Simon Mitton is a Director of Total Astronomy Ltd, Cambridge, U.K.

Science Watch^®, September/October 2003, Vol. 14, No. 5
Citing URL: http://www.sciencewatch.com/sept-oct2003/sw_sept-oct2003_page6.htm