The Impact on Cosmology of the Wilkinson Microwave Anisotropy Probe

April 2013

Ever since the accidental discovery of the cosmic microwave background (CMB) in 1965, observations of this phenomenon have remained central to progress in cosmology. The Wilkinson Microwave Anisotropy Probe (WMAP), launched on June 30, 2001, provided full-sky microwave maps in five frequency bands, 23–94 GHz. The project was a partnership between Princeton University and the NASA Goddard Space Flight Center (GSFC).

In October 2010, NASA terminated the mission and placed the silent probe in a permanent orbit around the Sun. An editorial in Nature celebrated WMAP’s retirement with this encomium: “It turned cosmology from informed guesswork into a precision science, and brought our fuzzy understanding of the nature of the Universe into breathtaking focus” (467[7317]: 752, 14 October 2010).

Human Induced Pluripotent Stem CellWith the imminent publication of the nine-year summary of observations in the Astrophysical Journal Supplement Series (at this writing, still in press), cosmologists will have the final sky maps and cosmological results.

Four earlier data releases were accompanied by publications replete with essential detail on data processing and the elimination of systematic measurement errors, as well as groundbreaking analytical papers on the many implications for astronomy and cosmology. From the publication of the first results papers in 2003, ScienceWatch often reported that the findings of WMAP were producing many of the most highly cited papers in physics.

In December 2012 ScienceWatch featured the great challenges facing theorists as they address the highest-quality data from WMAP. Almost a century after the general theory of relativity some cosmologists are unsettled by the thought that the current concordance cosmology is not the only model that provides the best fit to the data.

In this latest update, ScienceWatch uses citation data compiled by Thomson Reuters to assess the impact of the WMAP mission on cosmology as a whole. The papers selected for analysis had the keywords “Wilkinson Microwave Anisotropy Probe” or “WMAP” in the title. The baseline time span for this database is (publication dates) January 1, 2003 to December 31, 2012. This analysis was created using the Web of Science® from Thomson Reuters. The resulting database contained 553 papers published in 53 journals, with 1,029 authors from 390 institutions, and based in 42 countries.

IMPACT OF WMAP'S FIRST YEAR

The central papers in this survey are those written by the WMAP science team, led by Charles L. Bennett (formerly of NASA GSFC, now in the Department of Physics & Astronomy, Johns Hopkins University, Baltimore, Maryland). Bennett and the team received the 2012 Cosmology Prize of the Gruber Foundation for this work.

Table 1 lists the 25 most highly cited WMAP papers, 18 of which are direct from the WMAP science team. It’s not at all surprising that #1, with more than 6,100 citations, is the first-year paper on the determination of the cosmological parameters: this paper alone transformed the cosmology game forever. For the first-year data release, the WMAP science team published 14 papers, and these had notched up 12,890 citations by the end of 2012.

Table 1
WMAP’s Most-Cited Papers, 2003-2012

Listed by citations

Rank Paper Citations
1 D.N. Spergel, et al., “First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Determination of cosmological parameters,” Astrophys. J. Suppl Ser., 148(1): 175-94, 2003. 6,120
2 D.N. Spergel, et al., “Three-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Implications for cosmology,” Astrophys. J. Suppl. Ser., 170(2): 377-408, 2007. 3,374
3 C.L. Bennett, et al., “First-year Wilkinson Microwave Anisotropy (WMAP) observations: Preliminary maps and basic results,” Astrophys. J. Suppl. Ser., 148(1): 1-27, 2003. 2,767
4 E. Komatsu, et al., “Five-year Wilkinson Microwave Anisotropy Probe observations: Cosmological interpretation,” Astrophys. J. Suppl. Ser., 180(2): 330-76, 2009. 2,438
5 E. Komatsu, et al., “Seven-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Cosmological interpretation,” Astrophys. J. Suppl. Ser., 192(2): No. 18, 2011. 2,112
6 M. Tegmark, et al., “Cosmological parameters from SDSS and WMAP,” Phys. Rev. D, 69(10): No. 103501, 2004. 1,571
7 J. Dunkley, et al., “Five-year Wilkinson Microwave Anisotropy Probe observations: Likelihoods and parameters from the WMAP data,” Astrophysical J. Suppl. Ser., 180(2): 306-29, 2009. 937
8 A. Kogut, et al., “First-year Wilkinson Microwave Anisotropy Probe observations: Temperature-polarization correlation,” Astrophys. J. Suppl. Ser., 148(1): 161-73, 2003. 755
9 H.V. Peiris, et al., “First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Implications for inflation,” Astrophys. J. Suppl. Ser., 148(1): 213-31, 2003. 753
10 G. Hinshaw, et al., “Five-year Wilkinson Microwave Anisotropy Probe observations: Data processing, sky maps, and basic results,” Astrophys. J. Suppl. Ser., 180(2): 225-45, 2009. 687
11 C.L. Bennett, et al., “First-year Wilkinson Microwave Anisotropy (WMAP) observations: Foreground emission,” Astrophys. J. Suppl. Ser., 148(1): 97-117, 2003. 548
12 G. Hinshaw, et al., “Three-year Wilkinson Microwave Anisotropy Probe (WMAP(1)) observations: Temperature analysis,” Astrophys. J. Suppl. Ser., 170(2): 288-334, 2007. 522
13 G. Hinshaw, et al., “First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: The angular power spectrum,” Astrophys. J. Suppl. Ser., 148(1): 135-59, 2003. 520
14 D. Larson, et al., “Seven-year Wilkinson Microwave Anisotropy Probe (WMAP) observation: Power spectra and WMAP-derived parameters,” Astrophys., J. Suppl. Ser., 192(2): No. 16, 2011. 464
15 L. Page, et al., “Three-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Polarization analysis,” Astrophys. J. Suppl. Ser., 1170(2): 335-76, 2007. 436
16 E. Komatsu, et al., “First-year Wilkinson Microwave Anisotropy,  (WMAP) observations: Tests of Gaussianity,” Astrophys. J. Suppl. Ser., 148(1): 119-34, 2003. 425
17 L. Verde, et al., “First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Parameter estimation methodology,” Astrophys. J. Suppl. Ser., 148(1): 195-211, 2003. 381
18 N. Jarosik, et al., “Seven-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Sky maps, systematic errors, and basic results,” Astrophys. J. Suppl. Ser., 192(2): 1-15, 2011. 316
19 J. Ellis, et al., “Supersymmetric dark matter in light of WMAP,” Phys. Lett. B, 565(1-4): 176-82, 2003. 290
20 A. de Oliveira-Costa, et al., “Significance of the largest scale CMB fluctuations in WMAP,” Phys. Rev. D, 69(6): No. 063516, 2004. 287
21 M. Tegmark, et al., “High resolution foreground cleaned CMB map from WMAP,” Phys. Rev. D, 68(12): No. 123523, 2003. 258
22 P. Vielva, et al., “Detection of non-gaussianity in the Wilkinson Microwave Anisotropy Probe first-year data using spherical wavelets,” Astrophys. J., 609(1): 22-34, 2004. 251
23 M. Viel, et al., “Constraining warm dark matter candidates including sterile neutrinos and light gravitinos with WMAP and the Lyman-alpha forest, Phys. Rev. D, 71(6): No. 063534, 2005. 233
24 A. Coc, et al., “Updated big bang nucleosynthesis compared with Wilkinson Microwave Anisotropy observations and the abundance of light elements,” Astrophys. J., 600(2): 544-53, 2004. 225
25 M.R. Nolta, et al., “Five-year Wilkinson Microwave Anisotropy Probe observations: Angule power spectra,” Astrophys. J. Suppl. Ser., 180(2): 296-305, 2009. 219
SOURCE: Thomson Reuters Web of Knowledge

Collective Citations to First-Year WMAP reports, by year, 2003-2012Graph 1 charts the citation rate of these papers by year. 2005 marked the peak with nearly 2,500 citations. The rate of decline is surprisingly slow, given that by 2006 cosmologists were switching to the third-year data, and from 2009 to the five-year data. The graph illustrates that the initial data release kick-started a seismic shift in cosmology. Our quantitative knowledge of the nature of the universe improved enormously.

 

WMAP’S NEXT TWO RELEASES

By improving the error statistics, the releases at three, five, and seven years have greatly increased confidence in the data.

The three-year WMAP results led to five papers from the science team. Paper #2 in Table 1 improved the values of the cosmological parameters to such an extent that consensus cosmology took off. Henceforth there has been widespread agreement on what values for matter density, the age of the universe, and so on, should be plugged into models of the universe, and agreement on what needs to be explained. The three-year report analyzed the temperature anisotropy data (#12) and the polarization anisotropy data (#15). The latter opened a window on physical conditions in the early universe. To date, the quintet of three-year papers has logged roughly 4,480 citations.

For the five-year results, Bennett’s team crafted eight professional papers, three of which are highly cited (#4, #7, #10). The total citation score exceeds 4,500, which is interesting because these papers have gained slightly more citations than the older three-year papers. This is a reflection of the continuing gains in accuracy by later papers: the five-year cluster by Bennett and his colleagues gave updates on data processing and improved sky maps.

HOW WMAP BOXED IN COSMOLOGY

The big story in the seven-year reports is about ΛCDM cosmology—that is, models of the universe with a cosmological constant (lambda, or dark energy) plus cold dark matter. The most highly cited of the six reports in that collection is ranked #5 in Table 1. It placed very tight constraints on the six parameters that rule ΛCDM models.

With the nine-year data, observational cosmology has achieved even tighter lock-down for the values of those parameters. The nine-year data gives t0 = 13.772 ± 0.059 Gyr for the age of the universe, and H0 = 69.32 ± 0.80 km s−1 Mpc−1, for the Hubble constant.

Overall, the WMAP mission has compressed the cosmological parameter volume by a factor of 68,000 for the standard six-parameter ΛCDM model, based on WMAP data alone. When cosmological data from other observations are combined with WMAP, the cosmological parameter volume shrinks even further. The lasting impact of WMAP is to have founded a new field: precision cosmology with its consensus model of the universe.

AUTHORS

A Web of Science® analysis of the authorship of the 553 papers is quite remarkable. There are just 22 authors who have published 20 papers or more, all of whom participated at some stage in the WMAP science team. A dozen authors participated on 30 or more papers; these names are featured in Table 2. Topping the list with 36 papers are David N. Spergel (Dept. Astrophysics, Princeton University, and first author of paper #1), and Edward L. Wright (Department of  Physics and Astronomy, University of California,  Los Angeles). Team leader Bennett contributed to 34 papers.

But below the WMAP team in the author rankings there are 75 authors with five or more papers to their name, 325 with two papers or more, and 1,000 authors with at least one paper. So the number of careers that have been benefited from having open access to WMAP data is considerable.

Table 2
WMAP Reports: Prolific Authors, 2003-2012

(Listed by number of papers)

Name Papers
David N. Spergel
Princeton University
36
Edward L. Wright
UCLA
36
Alan Kogut
NASA
35
Anthony J. Banday
CESR, France
34
Charles L. Bennett
Johns Hopkins University
34
Mark Halpern
University of British Columbia
34
Gary Hinshaw
NASA
34
Lyman Page
Princeton University
34
Edward Wollack
NASA
34
Norman Jarosik
Princeton University
33
Michele Lymon
Columbia University
32
Greg S. Tucker
Brown University
32
SOURCE: Thomson Reuters Web of Science

INSTITUTIONS

Where do these researchers work? Table 3 lists the top ten institutions when ranked by papers in this Web of Science® sample. There would have been something seriously amiss had Princeton and NASA not taken positions #1, #2, respectively. But Germany’s Max Planck Society is on the podium at #3, and the University of Cambridge (tied with Brown University at #8) is another European institution in the top ten. Both Max Planck and Cambridge have a long tradition of exceptional work in cosmology.

Outside the top ten, there are a further 80 universities and institutions that produced five or more papers, which once again illustrates the effectiveness of free distribution of astronomical data.

Table 3
WMAP Reports: Prolific Institutions, 2003-2012

(Listed by number of papers)

Rank Paper Citations
1 Princeton University 57
2 NASA 47
3 Max Planck Society 46
4 Caltech (including Jet Propulsion Lab) 44
University of Chicago 44
5 University of British Columbia 38
6 University of Oslo 36
7 University of Oxford 35
8 Brown University 32
University of Cambridge 32
9 University of Warsaw Observatory 27
10 University of Toronto 26
SOURCE: Thomson Reuters Web of Science

JOURNALS

Table 4 shows how the WMAP sample is distributed across the journals. The Astrophysical Journal family has three entries here (#3, #5, and ##9), giving a total count 134 papers, amounting to 24.2% of the sample. Just seven papers separate Physical Review D (116 papers, 21.0%) and Monthly Notices of the Royal Astronomical Society (109 papers, 19.7%).

Table 4
WMAP Reports: Most Prolific Journals, 2003-2012

(Listed by number of papers)

Rank Journal Papers
1 Physical Review D 116
2 Monthly Notices of the Royal Astronomical Society 109
3 Astrophysical Journal 93
4 Journal of Cosmology and Astroparticle Physics 49
5 Astrophysical Journal Supplement Series 31
6 Astronomy Astrophysics 23
7 Physics Letters B 17
8 International Journal of Modern Physics D 13
New Astronomy Reviews 13
9 Astrophysical Journal Letters 10
10 Physical Review Letters 8
SOURCE: Thomson Reuters Web of Science

OPEN DATA RELEASE

These statistics demonstrate the extraordinary extent to which the WMAP policy of making all the data freely available to the online community has paid off handsomely by crowd-sourcing the data analysis. The analysis shows a truly global sharing of the fruits of the mission has taken place. This enables even the smallest institutions to participate, by networking and pooling resources with the largest centers.

WMAP is not the first space mission to have adopted the open release of data, but it is surely the highest-impact mission to have done so. Other astronomy observatories that have taken this line included the 2dF Galaxy Redshift Survey at the Anglo-Australian Observatory (data publicly released 2003), and the Sloan Digital Sky Survey (SDSS), which made its data available to the general public in annual increments from 2000 to 2008.

The release of astronomical data in an early and ongoing fashion is of course a normal requirement of funding authorities: they must demonstrate fairness and openness in the diffusion of results. When individual universities and small national observatories could conduct research entirely within their own facilities, which was true until the 1970s, survey data tended to be released only when the home team of theorists had thoroughly analyzed the findings. For example, with radio astronomy surveys in the 1950s and 1960s, rivalry about cosmological implications became intense, with observational data a closely guarded secret. There has been a behavioral change among astronomers that has taken a generation or more, driven by the need to share enormous datasets throughout the global community.

Paper #6 in Table 1, with 1,571 citations, is by Max Tegmark (Department of Physics, MIT) who with his colleagues developed data-analysis tools based on information theory to process the data from SDSS and WMAP. It was a seminal paper because it gave independent validation to the parameter measurements of the two missions, both of which have been transformative for understanding the structure of the universe on the grandest scale.


Dr. Simon Mitton is Vice-President (2012–2014) of the Royal Astronomical Society, and co-author with Jeremiah P. Ostriker of Heart of Darkness: Unravelling the Mysteries of the Invisible Universe (Princeton, 2013).

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.