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The Hot Papers in physics are yet again dominated by
cosmology, with two papers, #1 and #4, from the Wilkinson
Microwave Anisotropy Probe (WMAP), and #6 on the
latest data release from the Sloan Digital Sky Survey. Paper
#4, the fifth in the annual series, published February 2009,
has soared into the Top Ten just weeks after publication.
Clearly that would not happen if the paper merely updated its
forebears with slightly better data on the cosmic microwave
background (CMB). So, what's new this time?
WMAP is of central importance to observational cosmology
because its mission is to place stringent limits on
six parameters that codify a cold-dark-matter universe with a
cosmological constant lambda, that takes account of
dark
energy. This LCDM model is the foundation of concordance
cosmology, the all-party consensus. So, to a certain extent,
observational cosmology could have become a somewhat pointless
quest for another place of decimals on just six numbers. But
that's not how the game is being played, as #4 makes abundantly
clear.
Concordance cosmology has provided stable answers to such
questions as the age of the universe, its expansion rate, its
composition, and the origin of structure. WMAP
contributed to this by measuring the statistical properties of
temperature fluctuations in the CMB. The consensus is that the
spatial geometry of the universe is flat, and it contains
atoms, dark matter, and dark energy.
But, cosmologists ask themselves, is the universe really flat,
and what are the equations of state for dark energy and dark
matter? Then there is the vexed question of cosmic inflation:
what is the correct model for that? Paper #4 is just one of
seven WMAP five-year papers now providing intellectual
turning points for these cosmological conundrums.
Inflation refers to an event 10-36 seconds after the
Big Bang: the observable universe originated in a tiny causally
connected region that suddenly ballooned from almost nothing to
immensity. Because inflation smoothed out inhomogeneities and
curvature, WMAP has by now become a formidable tool
for discriminating between rival models of the inflationary
universe: #4 reports there are already more than 100 candidates
in the literature, and competition between them no doubt
accounts for the continuing high citation rates of
WMAP papers.
So, in this quinquennial report, the WMAP team led by
Eiichiro Komatsu (Dept. of Astronomy, University of Texas at
Austin) takes a close look at curvature in the universe. This
fascinating area connects fundamental physics to observational
cosmology via the predictions of the numerous models that were
developed to make predictions testable by WMAP. The
main conclusion of a highly technical analysis is a tight
restriction on curvature, with complete flatness an allowable
solution. A robust prediction of inflation is that the early
universe has primordial gravitational waves, arising from the
same generating mechanism as the primordial density
fluctuations that explain the presence of structure (clusters
of galaxies) in the universe. The WMAP data are
sensitive to the presence of primordial gravitational waves,
and when combined with results from supernova cosmology, the
outcome disfavors a number of popular inflation models.
In terms of understanding dark energy, by placing tight limits
on its equation of state, WMAP is making real
progress. The observational issue is that the measured cosmic
distances of Type Ia supernovae cannot fit the data unless
accelerated expansion is allowed in the universe of today. This
uptick in the expansion rate is presumed caused by an
additional energy component that acts like negative pressure.
This is not a new idea, but physicists have struggled for the
better part of a decade to get to grips with the puzzle, only
to find a growing circle of ignorance. The latest WMAP
analysis suggests that dark energy is consistent with vacuum
energy.
There is news about neutrinos. The limit on the total mass of
massive neutrinos is given as < 0.67 eV. The result is
significant because the neutrino mass is of fundamental
importance in physics. WMAP has provided an upper
limit from a cosmological point of view, which complements the
laboratory results from atmospheric and solar neutrinos.
Furthermore, WMAP confirms that the number of neutrino
species in the early universe was three, just like
today.
Dr. Simon Mitton is a Fellow of St. Edmund’s
College, Cambridge, U.K.
KEYWORDS: WILKINSON MICROWAVE ANISOTROPY PROBE, WMAP, FIFTH
DATA RELEASE, CONCORDANCE COSMOLOGY, EIICHIRO KOMATSU, DARK
ENERGY, COLD DARK MATTER.