
Results provide fresh support for Einstein’s cosmological
constant
Analysis of data from the 10-meter South Pole Telescope is
providing new support for the most widely accepted explanation of
dark energy, the source of the mysterious force that is responsible
for the accelerating expansion of the universe. The data strongly
support Albert Einstein’s cosmological constant – the leading model
for dark energy.
The results also are beginning to hone in on the masses of the
neutrinos, the most abundant particles in the universe, which until
recently were thought to be without mass. A series of papers
detailing the SPT findings have been submitted to the Astrophysical
Journal.
“The results released to date are just the beginning of what
we’ll be able to accomplish with the South Pole Telescope – the
present analyses are based on only 100 of the over 500 galaxy
clusters we’ve detected so far. We can expect much tighter
constraints on dark energy and the neutrino masses with the full
data set,” said McGill University physics professor Gil Holder.
McGill Prof. Matt Dobbs, postdoctoral scientist Keith
Vanderlinde, and graduate student Tijmen de Haan recently returned
from the geographic South Pole after having installed on the
telescope a new detector readout system, developed and built at
McGill, the only Canadian university partner in the project. This
electronics system, together with new detector technology, will
allow the telescope to search for signatures produced a fraction of
a second after the big bang, and refine the measurements of matter
and neutrino properties.
The most widely accepted property of dark energy is that it
leads to a pervasive force acting everywhere and at all times in
the universe. This force could be the result of space having
energy, even when it is free of matter and radiation. This
energy of empty space, called the cosmological constant, was
originally hypothesized by Einstein in order to explain why the
Universe was static and not collapsing; he later considered this to
be one of his greatest blunders after learning that the universe is
not static, but expanding.
In the late 1990s, astronomers discovered that the expansion of
the universe appeared to be accelerating according to
cosmic-distance measurements based on the relatively uniform
brightness of exploding stars. Gravity should have been slowing the
expansion, which followed the big bang.
Einstein introduced the cosmological constant into his theory of
general relativity to accommodate a stationary universe, the
dominant idea of his day. But his constant fits nicely into the
context of an accelerating universe, now supported by countless
astronomical observations. Others hypothesize that gravity could
operate differently on the largest scales of the universe. In
either case, the astronomical measurements are pointing to new
physics that has yet to be understood.
The SPT was specifically designed to tackle the dark energy
mystery. The 10-meter telescope operates at millimeter wavelengths
to make high-resolution images of the cosmic microwave background
(CMB), the light left over from the big bang. Scientists use the
CMB in their search for distant, massive galaxy clusters that can
be used to pinpoint the mass of the neutrino and the properties of
dark energy.
“The CMB is literally an image of the universe when it was only
400,000 years old, from a time before the first planets, stars and
galaxies formed in the universe,” said Bradford Benson, a
postdoctoral scientist at the University of Chicago’s Kavli
Institute for Cosmological Physics. “The CMB has travelled across
the entire observable universe, for almost 14 billion years, and
during its journey is imprinted with information regarding both the
content and evolution of the universe.” Benson presented the SPT
collaboration’s latest findings, Sunday, April 1, at the American
Physical Society meeting in Atlanta.
As the CMB passes through galaxy clusters, the clusters
effectively leave “shadows” that allow astronomers to identify the
most massive clusters in the universe, nearly independent of their
distance. McGill graduate student Tijmen de Haan was one of the
lead authors on the paper (http://arxiv.org/abs/1112.5435)
submitted to the Astrophysical Journal analysing galaxy clusters
with a combination of SPT data and images recorded by x-ray
satellites. de Haan explains, “These measurements reveal how many
clusters formed throughout the history of the universe. These are
the largest gravitationally collapsed objects in the universe.
Their growth rate is sensitive to the mass of the neutrinos and the
influence dark energy has on the growth of cosmic structures. They
reveal the constituent building blocks of the universe.”
The existence of neutrinos was proposed in 1930. They were first
detected 25 years later, but their exact mass remains unknown. If
they are too massive they would significantly affect the formation
of galaxies and galaxy clusters.
The SPT team has now placed tight limits on the neutrino masses,
yielding a value that approaches predictions stemming from particle
physics measurements.
The SPT survey is also being used to make maps of the
distribution of matter in the universe by measuring subtle shifts
in apparent position on the sky of the cosmic microwave background
with unprecedented accuracy. McGill graduate student Alex van
Engelen, who was the lead author on a recent paper (http://arxiv.org/abs/1202.0546)
submitted to the Astrophysical Journal presenting the most precise
measurement of this effect to date, explains that “the shifts are
caused by the gravitational force from these mass fluctuations,
which are primarily made of dark matter.”
The South Pole Telescope collaboration is led by the University
of Chicago and includes research groups at Argonne National
Laboratory, Cardiff University, Case Western Reserve University,
Harvard University, Ludwig-Maximilians-Universität, McGill
University, Smithsonian Astrophysical Observatory, University of
California at Berkeley, University of California at Davis,
University of Colorado at Boulder, University of Michigan, as well
as individual scientists at several other institutions.
McGill researchers participating in the South Pole Telescope
collaboration include faculty members Matt Dobbs and Gil Holder;
postdoctoral scientists Amy Bender and Keith Vanderlinde; and
graduate students Tijmen de Haan, Jon Dudley, Alex van Engelen, and
James Kennedy.
The SPT is funded primarily by the United States National
Science Foundation’s Office of Polar Programs. Partial support is
also provided by the NSF-funded Physics Frontier Center of the
KICP, the Kavli Foundation and the Gordon and Betty Moore
Foundation. The Canadian team receives support from Natural
Sciences and Engineering Research Council, Canadian Institute for
Advanced Research, and Canada Research Chairs program.