Physics Top Ten: How the Exquisite Precision of Concordance Cosmology Challenges Theorists
The Physics Hot Ten currently has five papers on cosmological results from probes and telescopes in space (#1, #2, #3, #8, #10). The quest to understand the nature of the universe continues with two papers on searches for the Higgs boson (#4 and #7) and one of the hunt for dark-matter candidates (#6).
What’s Hot in Physics
|Rank||Paper||Citations This Period (Jul-Aug 12)||Rank Last Period (May-Jun 12)|
|1||E. Komatsu, et al., "Seven-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Cosmological interpretation," Astrophys. J. Suppl. Ser., 192(2): No. 18, February 2011. [14 U.S., U.K., and Canadian institutions]||215||1|
|2||D. Larson, et al., “Seven-year Wilkinson Microwave Probe (WMAP) observations: Power spectra and WMAP-derived parameters,” Astrophys. J. Suppl. Ser., 192(2): No. 16, February 2011. [14 U.S., U.K., and Canadian institutions]||46||2|
|3||E.L. Wright, et al., The Wide-Field Infrared Survey Explorer (WISE): Mission description and initial on-orbit performance,” Astronom. J., 140(6): 1868-81, December 2010.||41||9|
|4||ATLAS Collaboration (G. Aad, et al.), “Combined search for the Standard Model Higgs boson using up to 4.9fb-1 of pp collision data at root s=7 TeV with the ATLAS detector at the LHC,” Phys. Lett. B, 710(1): 49-66, 29 March 2012. [177 institutions worldwide]||41||+|
|5||C.R. Dean, et al., “Boron nitride substrates for high-quality graphene electronics,” Nature Nanotech., 5(10): 722-6, October 2010. [Columbia U., New York, NY; Sungkyunkwa U., Suwon, Korea; Natl. Inst. Materials Sci., Tsukuba, Japan]||38||8|
|6||XENON100 Collaboration (E. Aprile, et al.), “Dark matter results from 100 live days of XENON100 data,” Phys. Rev. Lett., 107(13): No. 131302, 22 September 2011. [13 institutions worldwide]||37||5|
|7||CMS Collaboration (S. Chatrchyan, et al.), “Combined results of searches for the standard model Higgs boson in pp collisions at root s=7 TeV,” Phys. Lett. B, 710(1): 26-48, 29 March 2012. [172 institutions worldwide]||35||+|
|8||G.L. Pilbratt, et al., Herschel Space Observatory. An ESA facility for far-infrared and submillimetre astronomy,” Astron. & Astrophys., 518: No L1, Jul-Aug 2010. [ESA facilities in Noordwijk, Netherlands; Madrid, Spain; and Darmstadt, Germany]||33||3|
|9||T2K Collaboration (K. Abe, et al.), “Indication of electron neutrino appearance from an accelerator-produced off-axis muon neutrino beam,” Phys. Rev. Lett., 107(4): No. 041801, 18 July 2011.||28||3|
|10||M.J. Griffin, et al., “The Herschel-Spire instrument and its in-flight performance,” Astron. & Astrophys., 518: L3, Jul-Aug 2010. [36 institutions worldwide]||25||+|
|SOURCE: Thomson Reuters Web of Science
NB. Only papers indexed by Thomson Reuters since September 2010 are tracked. The “+” sign indicates that the paper was not ranked in the Top Ten during the last period. In the event that two or more papers collected the same number of citations in the most recent bimonthly period, total citations to date determine the rankings
The Wilkinson Microwave Anisotropy Probe: Seven Year Results (WMAP7)
Paper #1, on the cosmological interpretation of data on the cosmic microwave background (CMB) radiation, has dominated the Physics Top Ten for almost two years, with citations continuing at 200 or more per bimonthly period. The Gruber Foundation awarded its Cosmology Prize 2012 to Charles L. Bennett (Johns Hopkins University) and the Wilkinson Microwave Anisotropy Probe science team that he led.
In summary, the WMAP findings on the cosmological parameters are these:
- the age of the universe is 13.75 billion years,
- the content of the universe is 4.6 percent baryonic matter, 22.7 percent dark matter, and 72.8 percent dark energy,
- there is now evidence that the very early universe did go through a period of stupendous inflation, just as theorists have suspected for decades,
- the geometry of the universe is flat.
The physics community’s intense interest in the WMAP7 arises because of the beautiful precision of its anisotropy map. The CMB is isotropic to 1 part on 100,000. Putting that another way, the mean temperature variations in the CMB are tens of microkelvin, superimposed on the average temperature of 2.725 kelvin. These little bumps and ripples are the signature of baryon acoustic oscillations (analogous to sound waves) in the early universe. WMAP can be imagined as a telescope for taking all-sky photographs of the pattern of oscillations 378,000 years after the Big Bang. These subtle variations show that the seed structure that would eventually evolve into galaxies and clusters of galaxies was present in the Big Bang.
Paper #2 is an analysis of the power spectrum of the oscillations, and this has contributed greatly to our understanding of cosmology. For example, the dark matter must be non-baryonic, and it interacts only weakly with atoms and radiation. Furthermore, the Hubble parameter, the uncertain value of which spooked cosmologists for half a century, is now known with a precision of 3 percent (71.0 ± 2.5 km s-1 Mpc-1) from the acoustic fluctuation data alone. This is a marked improvement on the notoriously challenging measurements of former years based on the velocity-distance scale of galaxies. WMAP data also place limits on the mass of the neutrino, and provide evidence for primordial helium, consistent with Big Bang theory predictions.
WMAP ceased science observations on August 20, 2010, and the final legacy report of the science team will shortly be released. Observations of the CMB will continue with the Planck spacecraft, a European Space Agency (ESA) mission launched in 2009.
Infrared Space Observatories
Another ESA facility, the Herschel Space Observatory, which was launched together with Planck, is the theme of papers #8 and #10. Herschel is the largest infrared telescope ever launched, with a 3.5m primary mirror. It is designed to observe the coldest objects in the universe: the launch included 2,000 liters of liquid helium which bubbles away at 1.4 K in a bid to keep its instruments as cold as possible. The on-board supply of helium will limit operations to about three years, ending in March 2013.
Paper #3 showcases the mission and the initial performance of NASA’s Wide-field Infrared Survey Explorer (WISE), which operated from January 2010 to February 2011. WISE has peered through the dense clouds of dust obscuring the disks of many active galaxies. Millions of supermassive black holes have been found, at the centers of galaxies cloaked in dust. It’s possible that WISE has uncovered a new active phase in the lives of extreme galaxies. Within our solar system, WISE scooped up thousands of asteroids, including a newly discovered specimen of “Trojan” asteroid (the designation for those that share an orbit with a larger body without colliding) gliding along the same orbit as the Earth.
The Higgs Boson
So, a great deal is being accomplished by space observatories, but meanwhile, crowds of physicists and technical staff with their feet on the ground are developing the search for the standard model Higgs boson at the Large Hadron Collider (LHC). Papers #4 and #7 from the collaborations responsible for the ATLAS detector and Compact Muon Solenoid (CMS) detector were published back to back in Physics Letters B on March 29, 2012. Both collaborations sought evidence of the Higgs boson in proton-proton collisions at 7 TeV.
Important null results are reported in both papers, by excluding boson mass ranges at the 95 percent confidence limit, and this information focuses attention on collision events in a narrow range of masses. These two papers caused great excitement because they found an excess of events consistent with the Higgs hypothesis at 124 – 126 GeV. But the margin of error, 3.5 standard deviations, was too small to claim discovery of a new particle, for which a significance of 5.0 is required.
These two papers prefigured the dramatic announcement at CERN, Geneva on July 4, 2012, that the ATLAS and CMS showed a five-sigma signal of a new particle with mass 125 – 126 GeV. The 45-year hunt for the origin of mass was over. In November 2012, CERN physicists stated confidently that the accumulating data is consistent with the Higgs boson. The Higgs particle shows no signs of exciting new physics beyond the Standard Model. That’s a blow for theorists working beyond the Standard Model in their quests for a better understanding of supersymmetry, dark matter, and dark energy.
Hunting for Dark Matter
Meanwhile, in Gran Sasso, Italy the direct search for dark matter continues, and that’s the subject of Hot Paper #6 on results from XENON100. The XENON dark matter program seeks evidence of postulated weakly interacting massive particles (WIMPs) scattering off a xenon target. This experiment is a huge challenge: the paper reports three candidate events only in 100 days of live data. The statistics are grim for those physicists who would like to find a bucket of dark matter to investigate. Nevertheless the experiment reaches new ground by setting the most stringent limits on dark matter interactions. This result places further constraints on searches for supersymmetric WIMP dark matter at LHC.
Dr. Simon Mitton is at the University of Cambridge. His latest popular book on cosmology, jointly authored with Jeremiah P. Ostriker, is Heart of Darkness: Unravelling the Mysteries of the Expanding Universe (Princeton University Press, 2013).
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