Cosmic Microwave Background Radiation Penzias and Wilson (see right) of Bell Laboratories announced the startling discovery in 1965 that the Universe was bathed in a low intensity background radiation that appeared to come from everywhere using the telescope shown to the left and right.

General Properties of the CMBR

• The radiation has very low temperature (T ~ 2.7 Kelvins or -455 Farenheit !!!).
• The spectrum of the radiation is well-described by a blackbody spectrum.
• The radiation is isotropic, i.e., it is very close to the same temperature all across the sky -- temperature differences of < 0.004 % (~ 0.001 Kelvin) on angular scales of 7 degrees (excluding a well-known 0.12 % variation known as the dipole anisotropy).
• The temperature over the sky, although very smooth does exhibit structure. The WMAP (Wilkinson Microwave Anisotropy Probe) experiment finds the temperature map and structure measure and the Boomerang experiment finds the temperature map and structure measure. The WMAP results offer strong evidence that the Universe is flat, is one in which dark energy forms 73 % of the Universe (which brings up the Cosmological Constant, Einstein's greatest blunder?), that dark matter is ~23 % of the Universe, and only a few % is normal baryonic matter, and that an inflationary epoch must have occurred (an epoch in which the Universe increased in size by a factor of 10²² over 10^-30 seconds!).

• 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.
  • As the Universe expanded, it cooled down until today, the current temperature of the radiation is only 2.7 Kelvin! When the CMBR formed, the temperature of the Universe was ~3,000 Kelvin.

• What are we looking at when we look at the CMBR? We see the Universe when it first became transparent to light after what is known as the Epoch of Recombination.

  • Well, blackbody radiation typically comes from objects which are hot and dense (and opaque), e.g., the coils on your electric stove, ... . At early times when the Universe was hot, hydrogen atoms did not exist in the Universe (there were only individual protons and electrons) and the Universe was opaque to the passage of light, and we could only see objects very close to us (local objects).
  • At the formation of the CMBR, the Universe had cooled enough for hydrogren atoms to form. At this time, the Universe became transparent to light, and we were then able to see throughout the Universe (to see to large distances). The time when hydrogen atoms form is known as the Epoch of Recombination.
  • When we look at the CMBR, we see the Universe as it was just after the Epoch of Recombination.
    • Because 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.
    • When we look at the CMBR, produced around the Epoch of Recombination, because the light has had ~13 or so billion years to travel to us (the Universe is ~ 13.7 billion years old), we are probing the Universe at large distances and hence, large redshift, z ~ 1,000, when we study the CMBR. Redshift z ~ 1,000 corresponds to a time 379,000 years after the start of the expansion of the Universe.

  
  Isotropy

• The CMBR is the same to within 0.004 % (to within ~ 0.001 Kelvin) on scales < 7 degrees (excluding the dipole anisotropy).
• This has severe implications:

  • Diametrically opposed regions on the sky require longer than the age of the Universe to communicate with each other. This means that they are not in causal contact. As a result there is no way for these two regions to have mixed together to reach a common temperature, as say as systems would which are in thermal equilibrium; or there is no way for these two regions to have synchronized their watches so that they would know that they should start expanding at the same time.

    This is the HORIZON problem.

  • 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 part of the Unvierse in front of us appears hotter (the ight from in front of us is blueshifted) than the part of the Unvierse behind us (the light from behind us redshifted).
  • The low-amplitude, small-scale fluctuations seen in the CMBR are good and very important.
    • 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 fluctuations of the CMBR and offer strong constraints on properties of the early expansion of the Universe!