In 1965, Penzias and Wilson of Bell Laboratories made the startling discovery
of an all pervasive background radiation from the Universe. Since then several
further missions, COBE and WMAP, have probed deeper into the properties and
meaning of the CMBR. WMAP, in particular, has shaped the current picture of
our Universe leading to the startling result that around 72 % of the Universe
is composed of Dark Energy and that
although the rest of the Universe is composed of matter, more than 80 % of
the matter is called Dark Matter and is
composed of, as yet, some unknown particle. Only 4.6 % of the Universe is
composed of Normal Matter.
The CMBR has properties:
The radiation was
of very low temperature,
T ~ 2.725 Kelvin or -455 Farenheit (or -270 C) !!!, and came from everywhere
The spectrum of the radiation
was well-described by a
blackbody spectrum.
The radiation was
isotropic, i.e., it had very close to the same
temperature all across the sky -- temperature differences of
< 0.004 % on angular scales of 7 degrees
(excluding a well-known 0.12 %
variation known as the
dipole anisotropy and
finer, lower amplitude temperature variations).
Interpretation
The CMBR is thought to be the relic of the Big Bang.
In the past when the Universe was much smaller, it was also much
hotter.
Well, blackbody radiation typically comes from objects which are hot
and dense (and opaque), e.g., the coils on your electric stove, ... .
Since the speed of light is finite ===> when we look at distant objects
we see the objects as they appeared in the past. If an
object is at a distance of D, then the light we receive today left the
object a time roughly (D/c) in the past.
After the Epoch of Recombination the Universe was transparent to
radiation and so we can see essentially throughout the Universe.
However,
before the Epoch of Recombination, the Universe was opaque to optical
radiation and we cannot see into this time.
When we look at the CMBR, we are seeing the Universe as it was at
the time of Recombination or when the Universe was 377,000
years old (z~1,000) and had a temperature of around 3,000 Kelvin.
Isotropy and Homogeneity of the CMBR
The CMBR is the same to within 0.004 % on scales < 7 degrees
(excluding the dipole anisotropy).
This has severe implications which we will address in the discussion of
the Cosmological Principle.
The dipole anisotropy
arises due to the motion of the Milky Way galaxy through the Universe.
The Local Group of galaxies which contains the Milky Way is moving
toward Hydra with a speed of around 500 kilometers per second.
The low-amplitude, small-scale fluctuations seen in the CMBR are both
good and may turn out to be rather vexing at the same time.
We know that on certain scales the Universe is inhomogeneous; there
are observed structures in the Universe like galaxies, clusters of galaxies,
clusters of clusters of galaxies, voids, ... .
These structures represent fluctuations in the material content of the
Universe. For example, the average density of the Universe is less than
1 hydrogen atom per 100,000 cubic centimeters. The average density in our
galaxy is something like 1 hydrogen atom per cubic centimeter. That is,
galaxies are something like > 100,000 times
denser than the average chunk of the
Universe.
Presumably such structures were formed by the mutual gravitational
attraction of the particles which make up the galaxies. A similar argument
can be made for clusters of galaxies, clusters of clusters of galaxies,
voids, ...
The upshot is that we can figure out roughly how long the formation
process should take, and how large fluctuations need to be around
the Epoch of Recombination when the CMBR was formed.
The current size of structures are consistent with the
measured smoothness of the CMBR. If we start to measure larger
structures then the sizes of the fluctuations observed in the CMBR will
not be consistent with a simple picture of gravity producing the
inhomogeneities observed in the Universe.
