Cosmic Microwave Background Radiation

In 1965, Penzias and Wilson of Bell Laboratories made the startling discovery of an all pervasive background radiation from the Universe.

• The radiation was of very low temperature (T ~ 2.7 Kelvins or -455 Farenheit !!!) 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.

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.
  • As the Universe expanded, it cooled down until today, the current temperature of the radiation is only 2.7 Kelvin!
• What are we looking at when we look at the CMBR?
  • 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 between 500,000 to 1,000,000 years old and had a temperature of around 3,000 Kelvin.

  Isotropy

  The CMBR is the same to within 0.004 % on scales < 7 degrees (excluding the dipole anisotropy).

  This has severe implications:

11 b l.y. <------------------- US --------------------> 11 b l.y.

  CMBR                                                    CMBR

• 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 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 barely 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.