Standard Cosmological Model Survives New
WMAP Scrutiny
by Simon Mitton
Physics Top Ten
Papers
Rank
Papers
Citations This Period
(Sep-Oct 07)
Rank Last Period (Jul-Aug
07)
1
D.N. Spergel, et al.,
"Three-year Wilkinson Microwave Anisotropy
Probe (WMAP) observations: Implications for
cosmology,"Astrophys J. Suppl. Ser.,
170(2): 377-408, June 2007. [13 U.S. and Canadian
institutions] *178TD
107
†
2
K.S. Novoselov, et al., "Two-dimensional
gas of massless Dirac fermions in
graphene,"Nature, 438(7065): 197-200, 10 November 2005. [U.
Manchester, U.K.; Inst. Microelect. Tech., Chernogolovka,
Russia; Radboud U., Nijmegen, Netherlands] *982BV
102
1
3
Y.-B. Zhang, et al., "Experimental
observation of the quantum
Hall effect and Berry’s phase
in graphene,"Nature, 438(7065):
201-4, 10 November 2005. [Columbia U., New York, NY]
*982BV
95
2
4
M.F. Skrutskie, et al., "The Two Micron
All Sky Survey (2MASS),"Astronom. J.,
131(2): 1163-83, February 2006. [11 U.S. institutions]
*010RX
61
4
5
P. Astier, et al., "The Supernova Legacy
Survey: measurement of Om, O?, and
w from the first year data set,"Astron. & Astrophys., 447(1): 31-48, February
2006. [18 institutions worldwide] *007GS
53
3
6
D.J. Eisenstein, et al., "Detection of the
baryon acoustic peak in the large-scale correlation
function of SDSS luminous red galaxies,"Astrophys. J., 633(2): 560-74, 10 November 2005.
[29 institutions worldwide] *983NK
39
8
7
N.M.R. Peres, F. Guinea, A.H. Castro Neto,
"Electronic properties of disordered
two-dimensional carbon,"Phys. Rev. B,
73(12): 125411, March 2006. [Boston U., MA; U. Minho,
Braga, Portugal; ICMM, Madrid, Spain] *028DP
37
†
8
W. Ma, et al., "Thermally stable,
efficient polymer
solar cells with nanoscale control
of the interpenetrating network morphology,"Adv. Funct. Materials, 15(10): 1617-22, October
2005. [U. Calif., Santa Barbara] *976VL
35
5
9
J.K. Adelman-McCarthy, et al., "The Fourth
Data Release of the Sloan Digital Sky Survey,"Astrophys. J. Suppl. Ser., 162(1): 38-48, January
2006. [61 institutions worldwide] *009RS
34
†
10
G. Li, et al., "High-efficiency solution
processable polymer photovoltaic cells by self-organization
of polymer blends,"Nature Materials,
4(11): 864-8, November 2005. [U. Calif., Los Angeles; Natl.
Renewable Energy Lab., Golden, CO] *979GS
The top paper for this period deals with the implications for cosmology
from the first three years of observations with the Wilkinson Microwave
Anisotropy Probe (WMAP), which soared aloft on June 30, 2001. WMAP audits
the cosmic microwave background (CMB) radiation to give
us a balance sheet of the assets in the universe.
The results in paper #1 mark the halfway stage of this six-year mission,
which has already led to a dramatic improvement in the measurement of six
key cosmological parameters, as well as providing watertight evidence in
favor of a simple cosmological model. The high citation rate reflects the
fact that almost every paper in cosmology these days has to cite #1 in its
introductory section. The report is now the frame of reference for all
cosmological investigations.
The concordance model of big-bang cosmology is a stellar achievement. It
stands on four pillars of wisdom. Its key observational successes are its
ability to account for: the accelerating expansion of the universe, the
cosmic microwave background radiation, the creation of the light elements
in the early universe, and the origin of large-scale structure in the
universe.
Two distinguishing features of the model include a so-called lambda (L )
term that represents the dark energy causing an acceleration of the
expansion, and the presence of cold dark matter (CDM) that is
non-baryonic and is dark because it cannot emit photons. These two
contributors to the heart of darkness that is our universe account for
almost 96% of its mass-energy.
Cosmologists happily play around in parameter space. Their standard model
has six fundamental parameters that can only be determined by observation.
Three of these parameters are easy to understand: the density of matter,
the density of baryons, and the rate of expansion (the Hubble constant).
But a further three are less intuitive. They concern properties of the
cosmic microwave background, which is truly a fossil from the moment the
universe became transparent to its own radiation. And just like
palaeontologists, observational cosmologists are able to track down the
environmental conditions at the time of the fossil’s formation. Hot
Paper #1 dramatically shrinks the volume of parameter space that describes
our universe.
WMAP scores on large-scale structure. By measuring the polarization in the
CMB it is possible to look at the amplitude of the fluctuations of density
in the universe that produced the first galaxies. That is a real
breakthrough in our understanding of the origin of structure.
However, WMAP is not the only show in town. The Sloan Digital Sky Survey
and the 2dF Galaxy Redshift Survey have mapped the large-scale distribution
of matter and galaxies in exquisite detail. The cosmological information
from these sources is in many respects orthogonal to WMAP so the
concordance model is not heavily dependent on the assumptions of a single
mission.
Paper #1 demonstrates that the standard model has survived another rigorous
set of tests. Errors in the data are reduced threefold. The data are so
constraining that there is little wriggle room for the basic L CDM model.
By combining WMAP with other measurements it is possible to place
significant limits on the geometry of the universe, which is nearly flat
(almost no spatial curvature), the nature of dark energy, and a mechanism
to generate the primordial ripples that ultimately became gravitational
tidal waves producing galaxies.
It is now abundantly clear that this new cosmology requires new physics.
Dark matter and dark energy nowhere feature in the
palette used so creatively by particle physicists for whom the next big
story might be detecting the Higgs boson at the Large Hadron Collider.
But the cosmologists have work in progress as well. WMAP data are not a
good fit for present ideas on structure formation in the universe.
What will WMAP do in the second half of its mission? Further observations
will test the inflationary paradigm that holds that the flatness of the
universe is due to an exponential expansion driven by energy in the vacuum.
Better still, WMAP still has the capacity to probe directly the physics of
the first moments of the big bang.
Dr. Simon Mitton is a Fellow of St. Edmund’s College,
Cambridge, U.K.