Cosmology Soars to New
by Simon Mitton
Physics Top Ten
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Half of the papers in the Physics Top Ten, #1, #2,
#3, #4, and #7, capture one of the greatest events in the
entire history of cosmology. In the past ten years,
attempts to understand the nature of the universe have
progressed from a handful of numbers, some with significant
error bars, to a truly precision science that now places
strong boundary conditions on cosmological models:
parameter space for modelling freely is getting much
smaller, and the misfits are being thrown out. How has this
been achieved? Two keys have unlocked a vast vault of
First, NASA’s Cosmic Background Explorer (COBE),
launched in 1989, quickly produced a spectacular result:
the cosmic background (CMB) has a thermal temperature of
2.725 K. This result confirmed the "hot big bang" paradigm.
Subsequently, COBE data revealed exquisite structure in the
CMB, structure that had been imprinted shortly after the
big bang. For these discoveries, John C. Mather and George
Smoot won the Nobel Prize in Physics in 2006. In a public
statement, the Nobel Committee said "the COBE project can
also be regarded as the starting point for cosmology as a
The development of supernova cosmology provided the second
key some ten years ago when it became clear that Type 1a
supernovae could be used as accurate standard candles to
map the universe out to a redshift z = 1. The application
of these supernovae to observational cosmology led a
surprise: the expansion of the universe is accelerating.
With these two key stages, the so-called Lambda-Cold
Dark Matter (?CDM) model of the
universe took off, thanks to its ability to account for
the accelerating universe (the lambda term) and the
large-scale structure properties associated with the
Hot Paper #1 showcases the third-year results from the
Wilkinson Microwave Anisotropy Probe. Science
Watch has already given full coverage of the
first-year data which fits the ?CDM model. In this scheme
the universe is flat, homogeneous, and isotropic; composed
of baryonic matter, radiation, and dark matter; and has a
cosmological constant ? driving the acceleration.
Considered as dark energy, the ? term weights in with 73%
of the energy density of the universe
What the related paper #2 is all about is the extent to
which three years of data acquisition, reduction, and
refinement have significantly improved knowledge of the
cosmological constants. Opportunity is taken in #1 to
relate the three-year results from those for the Sloan
Digital Sky Survey (SDSS, subject of #3 and #7),
small-scale CMB data from ground-based groups, and much
larger samples of high-z supernovae (subject of #4). Errors
on the WMAP data are now 3 times smaller, and yet the ?CDM
model continues to thrive. There is little room for
significant changes to the basic model.
Paper #4, from Adam Riess (Johns Hopkins University,
Baltimore) and colleagues, reveals remarkable news from the
Hubble Space Telescope concerning 21 z = 1 Type 1a
supernovae. These beacons push the look-back time to 10 Gyr
(gigayears), and provide a factor of 2 improvement to
constrain the equation of state for dark energy.
That’s important because the CMB results have left
astronomers in the dark about how dark energy should be
factored in beyond z = 1.8. Rapidly evolving dark energy is
ruled out, and negative pressure, a defining characteristic
of dark energy, appears to be present at z = 1, which is
before the epoch of acceleration. In fact, Riess et
al. now strengthen their claim that a cosmic jerk,
some 5 Gyr ago, snapped the universe out of deceleration
under gravity, and propelled into the present inflationary
phase. Although Riess first made these claims in 2003,
there was always a nagging doubt about their authenticity.
The latest result pushes the frontier of exploration a
further 5 Gyr beyond the tipping point at 5 Gyr.
Finally, #7 announces the fifth data release of the SDSS,
which now encompasses spectra and photometric data
for106 galaxies and 105 quasars. The
high citation rate reflects the huge value of this database
for cosmological studies: more than 1,000 papers have cited
the successive SDSS releases.
Dr. Simon Mitton is a Fellow of St. Edmund’s
College, Cambridge, U.K.
Keywords: cosmology, COBE, Cosmic Microwave Background
Explorer, cosmic microwave background, CMB, supernovae,
Adam Riess, Wilkinson Microwave Anisotropy Probe, Sloan
Digital Sky Survey, SDSS.
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