Research Front: Constraining Consensus Cosmology

July 2012

The history of astronomy is notable in the present century for the astonishing emergence of a new consensus that releases cosmologists from the wordy debates that have occupied philosophers for millennia. This revolution was sparked by the measurements of the cosmic microwave background (CMB) made by the Cosmic Background Explorer (COBE) in the 1990s. Its discoveries fuelled a huge expansion in observations of the CMB, notably by the Wilkinson Microwave Anisotropy Probe (WMAP), supported by ground-based and balloon-borne experiments.

When WMAP launched in 2001 astronomers were addressing questions that had been the focus of cosmology for half a century: How old is the universe? How fast is its expansion? What is its size, shape, and composition? And what is the origin of the large-scale structure?

Citation analysis shows that recent papers that are citing #1, #2, and #3 are seeking the answers to new questions: What is the nature of dark matter, and how has it sculpted large-scale structure?

Such questions were also the focus for the new field of supernova cosmology, in which remote stellar explosions are used by astronomers as standard candles to calibrate the distance scale of the universe. To establish the distance scale in the early universe, in the epoch before there were stars and galaxies, cosmologists use baryon acoustic oscillations (BAO), which are analogous to sound waves. They are observed via the CMB, and their length scale is completely independent of the supernova calibration.

AN EXPANDING RESEARCH FRONT

These three types of observation: CMB, BAO, and supernovae, have resulted in an observational database of high precision for theoretical cosmology. The emergence of precision cosmology, and the consensus it has spawned, is the subject of a Thomson Reuters Research Front. These Fronts, which represent discrete areas of research, are formed when members of a central “core” of foundational papers are frequently cited together by later papers. Citation analysis is used to identify the core of key papers as well as the citing reports; together, this grouping of papers constitutes a distinct node of research activity, whose relationship with other Fronts can also be gauged through citation links.

Current Research Front in Space Science
Selected core papers and key reports for Research Front on measuring the cosmological parameters, the cosmological constant, and dark energy, ranked by citations
Rank Paper Citations
1 D.N. Spergel, et al., “Three-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Implications for cosmology,” Astrophys. J. Supp. Ser., 170(2): 377-408, June 2007. [16 institutions worldwide] 3,145
2 E. Komatsu, et al., “Five-year Wilkinson Microwave Anisotropy Probe observations: Cosmological interpretation,” Astrophys J. Supp. Ser., 180(2): 330-76, February 2009. [16 institutions worldwide] 2,162
3 E. Komatsu, et al., “Seven-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Cosmological interpretation,” Astrophys. J. Supp. Ser., 192(2): No. 18, February 2011. [16 institutions worldwide] 1,066
4 J. Dunkley, et al., “Five-year Wilkinson Microwave Anisotropy Probe observations: Likelihoods and parameters from the WMAP data,” Astrophys. J. Supp. Ser., 180(2): 306-29, February 2009. [16 institutions worldwide] 851
5 A.G. Riess, et al., “New Hubble Space Telescope discoveries of Type Ia supernovae at Z>=1: Narrowing constraints on the early behavior of dark energy,” Astrophys. J., 659(1): 98-121, 10 April 2007. [9 U.S. institutions] 628
6 G. Hinshaw, et al., “Five-year Wilkinson Microwave Anisotropy Probe observations: Data processing, sky maps, and basic results,” Astrophys. J. Supp Ser., 180(2): 225-45, February 2009. [16 institutions worldwide] 604
7 M. Kowalski, et al., “Improved cosmological constraints from new, old, and combined supernova data sets,” Astrophys. J., 686(2): 749-78, 20 October 2008. [42 institutions worldwide] 504
8 W.M. Wood-Vasey, et al., “Observational constraints on the nature of dark energy: First cosmological results from the ESSENCE Supernova Survey,” Astrophys. J., 666(2): 694-715, 10 September 2007. [20 institutions worldwide] 433
9 W.J. Percival, et al., “Measuring the baryon acoustic oscillation scale using the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey,” Mon. Notic. Roy. Astron. Soc., 381(3): 1053-66, 1 November 2007. [6 U.S. and U.K. institutions] 346
10 W.J. Percival, et al., “Baryon acoustic oscillations in the Sloan Digital Skey Survey Data Release 7 galaxy sample,” Mon. Notic. Roy. Astron. Soc., 401(4): 2148-68, 1 February 2010. [25 institutions worldwide] 298
11 T.M. Davis, et al., “Scrutinizing exotic cosmological models using ESSENCE supernova data combing with other cosmological probes,” Astrophys. J., 666(2): 716-25, 10 September 2007. 274
12 M. Hicken, et al., “Improved dark energy constraints from ~ 100 new CfA supernova Type Ia light curves,” Astrophys. J., 700(2): 1097-1140, 1 August 2009. [7 institutions worldwide] 273

SOURCE: Thomson Reuters Web of Science

The accompanying table presents a listing in citation order of 12 papers, selected from this Front’s core of 33, plus one key paper (#1) that has been added because it was a precursor of the rapid growth of this field.

COSMIC MICROWAVE BACKGROUND DATA RELEASES

By far the most highly cited papers are those dealing with results from WMAP. They are responsible for five (#1, #2, #3, #4, and #6) of the first six papers in the tabulation. The project was directed by Charles L. Bennett of Johns Hopkins University, and the mission was a joint partnership between NASA and Princeton University. Observations ceased in October 2010 when the spacecraft was positioned in a secure graveyard orbit round the sun. Results have been released in stages: the most highly cited paper gives the results after three years, and it has been cited more than 3,100 times.

The three-year release reported in paper #1 confirmed that the observable universe conforms to a relatively simple cosmological model characterized by a flat geometry and containing cold dark matter and dark energy. This paper transformed the cosmology game because it really did narrow down the error bars on six fundamental parameters of cosmology, such as the matter density and baryon density. By collapsing parameter space (which is how cosmologists like to express the results), #1 ushered in a new era of precision cosmology.

The Research Front demonstrates clearly how later releases of WMAP data (#2 with upwards of 2,160 citations, and #3 with 1,066) have continued to drive the expansion of cosmology. Citation analysis shows that recent papers that are citing #1, #2, and #3 are seeking the answers to new questions: What is the nature of dark matter, and how has it sculpted large-scale structure?  What do we know about mysterious dark matter, and how certain are cosmologists that it really exists? Did cosmic inflation trigger the primordial fluctuations leading to galaxies?

The seven-year release (#3) takes knowledge of physical processes in the universe back to a redshift z = 1090, which is the epoch when matter and radiation decoupled. In a conclusion, the authors state that the “simple cosmological model” is now a remarkably good fit to data from the CMB, as well as the distance measurements from supernovae.

SUPERNOVA COSMOLOGY

The Research Front identifies a cluster of papers in supernova cosmology (#5, #7, #8, and #11). This is the field that resulted in the 1998 discovery of the acceleration of the expansion of the universe. These papers complement, and are independent from, the WMAP results. That’s why the new cosmology is a consensus.

Paper #5 is from the group led by Adam Reiss of John Hopkins University (who shared the 2011 Nobel Prize in Physics, as part of the trio that made the 1998 discovery). The report has been cited more than 620 times because the data from high redshift (z > 1) supernovae point to the presence of negative pressure, a signature of dark energy, and a “cosmic jerk” resulting in acceleration.

The Supernova Cosmology Project, based at Lawrence Berkeley National Laboratory, provides the group authorship for paper #7, now cited more than 500 times. This review considers data on 307 Type Ia supernovae, and analyzes the results in a self-consistent manner to put tight constraints on the cosmological parameters.

The high citation rate for #7 is boosted by an abundance of theoretical papers on the constraints that the observed values of the parameters have on models of the universe, and in particular, studies that further analyze the datasets from WMAP, supernova cosmology, and the Sloan Digital Sky Survey. A good example of this is a paper by Reiss and his associates on re-determining the Hubble constant, a rework of a very old problem. This paper (A.G. Riess, et al., Astrophys. J., 699[1]: 539-63, 2009) reduces the uncertainty in the Hubble constant to 4.8% (H0 = 74.2 ± 3.6 km s−1 Mpc−1). This improvement then feeds back to the WMAP and BAO data to refine the value of the dark energy parameter. The paper by Reiss et al. is typical of the major research effort underway to sift and merge the huge datasets now available on the structure and history of the universe.

In paper #8, the ESSENCE supernova survey deals with cosmic expansion in the last 5 billion years to determine whether dark energy is different from the cosmological constant proposed by Einstein in 1917. The conclusion is inconclusive: it is difficult to determine whether the underlying physics concerns particle physics or general relativity. Nevertheless, the paper opens a window on enquiries about the cosmological constant and the physics of vacuum energy, now hot areas in the application of general relativity to cosmology.

SOUNDING OUT BARYON OSCILLATIONS

One of the faster-moving recent papers in this Research Front is #10 on baryon acoustic oscillations (BAOs), now cited nearly 300 times. This examines the clustering properties of almost 900,000 galaxies. In simple terms, the analysis looks at the power spectrum of clustering indifferent “redshift slices” of the universe to see the influence of BAOs at different cosmic epochs. In this way a history of the emergence of cosmic structure can be created. The study of BAOs has become a major element currently in the search for the origin of structure in the early universe. The top journals citing this paper are Physical Review D (73 citations, 23% of total citations), Monthly Notices of the Royal Astronomical Society (65, 20%) and Journal of Cosmology and Astroparticle Physics (49, 15%).


Dr. Simon Mitton is Vice-President of the Royal Astronomical Society and is based at the University of Cambridge, U.K.

The data and citation records included in this report are from Thomson Reuters Web of ScienceTM. Web of ScienceTM is a registered trademark of Thomson Reuters. All rights reserved.