Lasers
and the Universe
CMB was first discovered by Penzias
& Wilson in 1965 at another New Jersey location:
AT&T Bell Labs in Murray Hill. Penzias &
Wilson received a Nobel prize for their CMB discovery,
although it took scientists at Princeton to identify
the CMB as the light from the Big Bang explosion.
The radiation comes to us from approximately
380,000 years after the Big Bang (the surface
of last scattering). At this point, the previously
opaque universe cooled down enough to allow protons
and electrons to form atoms and thus became clear.
It is the tiny fluctuations in the CMB, however,
that are of importance. In 1992, NASA’s
COBE satellite detected these ripples. Since it
was over 13 billion years ago when the universe
was age 380,000, a map of the CMB can be compared
to a baby picture of an 80 year-old person taken
on the first day of life. Later the COBE results
were verified by a balloon experiment called FIRS.
TOP: Baby Picture of Universe from
COBE spacecraft, 1992, 380,000 years
after the Big Bang
BOTTOM: Baby Picture of Universe
from WMAP spacecraft
showing much
improved resolution. ACT will be even
better |
.
NASA’s WMAP, the Wilkinson Microwave AnisotropyProbe
subsequently obtained a much higher resolution
image. But ACT will have better resolution than
all these earlier experiments.
Recent experiments
have shown these photons of light to be slightly
polarized due to the nature of the plasma fog
that characterized the early universe. A fascinating
thing about polarization in the CMB is it gives
us information about the velocity of the plasma
at the surface of last scattering. The patterns
of this motion tell us about the kind of oscillations
that the plasma was undergoing --- namely, large
scale sound waves. The wavelength of the sound
wave could not exceed the size of the universe
at the time of the oscillation. Thus, the frequency
spectrum of these sound waves reveals information
about the expansion of the universe. Also, certain
patterns in the polarization could indicate the
presence of gravity waves in the early universe.
Another goal of ACT is to determine when dark
energy began interfering with galaxy-cluster formation
and thus became an important force in the evolution
of the universe. Around five years ago, observations
of the CMB began to suggest that matter and radiation
(light) could not account for the geometry, or
warp, of space-time, which drives universal dynamics.
What we now suspect to be at play is dark energy,
described in its simplest form by Einstein's Cosmological
Constant as causing the universe to expand indefinitely.
More complicated theories suggest that the dark
energy can evolve, perhaps one day reversing the
accelerated expansion and causing the universe
to collapse in upon itself.
ACT will detect nearby
galaxy clusters as well as galaxy clusters from
the age when clusters only began to form (and
all the clusters in between). Because the dark
energy bends spacetime and therefore moves galaxies
either together or apart, the clustering of galaxies
is a process that would be sensitive to the character
of the dark energy. By detecting the number of
formed clusters at each epoch of the universe,
ACT will make unprecedented measurements in order
to probe the nature of the dark energy.
One of
the collaborators for the ACT project is NASA
Goddard, three hours away from Princeton. They
have an identical laser to Princeton’s,
which they used previously to supply laser machined
silicon detectors to Princeton.
Besides NASA Goddard
and Princeton University, there are thirteen other
universities and institutions collaborating on
ACT. The 6 meter single dish telescope is presently
being assembled by AMEC Dynamic Structures in
Vancouver, B.C.. The Atacama Desert was chosen
for its aridity (it is the driest desert, with
its low humidity matched only by the South Pole).
The apparatus will be located on the western face
of Cerra Toco Mountain at an elevation of 5200
m.
How does the detector identify
such minute fluctuations in the CMB? The fluctuations
ACT is intending to “see” are at a
temperature of 10-30 microKelvin (µK) .
One might well ask how this can be done when the
detector temperature is 300 milliKelvin (mK)?
The answer is “by integration” which
takes a lot of time. To get the silicon detector
so cold necessitates serious cryogenic cooling—utilizing
first Helium 4then “pumped” Helium.
The cryogenic dewar with the silicon detector
inside is to be located at the focal plane of
the microwave telescope. The detector is also
known as a radiometer or polarimeter because it
is to detect polarized microwave radiation.
READ
MORE:
More information
on ACT can be found at: http://www.hep.upenn.edu/act/act.html
More information on CMB, anisotropies,
or surface of last scattering can be found at: http://lambda.gsfc.nasa.gov
and http://www.astro.ubc.ca/people/scott/faq_intermediate.html
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