The cosmic microwave background (CMB) radiation is one of the three sets of evidence for the Big Bang. First observed rather accidentally by the radio engineers Arno Penzias and Robert Wilson, the CMB is a diffuse, uniform background of microwave colored radiation that comes from all directions in the sky. Astronomers have sought progressively more detailed observations of the CMB, using satellites and balloons to get above the bulk of the Earth's microwave hampering atmosphere. At the time of this writing, the European Planck mission is in the process of gaining a new data set. This is the third in a series of satellites that began with COBE continued by WMAP.

The microwave background radiation has two striking features. First, the spectrum is thermal, falling with great accuracy along a curve that defines a temperature of 2.726 K. Note that we can quote the temperature to four significant figures and an accuracy of a few thousandths of a degree — it is the most accurately determined number in cosmology. The cosmic background radiation has the most perfect thermal spectrum observed anywhere in nature. Since astronomers believe that the radiation dates from about 300,000 years after the big bang, this means that the universe was in thermal equilibrium at that time. Second, the radiation is almost totally isotropic, meaning that it has the same intensity in all directions. If we represent the intensity of the microwave radiation by the surface of a pond 100 meters across, then the biggest ripples are only a few millimeters high. These small fluctuations are generated by minute differences in density in the early universe; differences that lead to the eventual formation of structures and voids.

When we observe the CMB, we have to correct it for our own motion, including; the Sun's motion around the center of the galaxy, the galaxy's motion in the Local Group, and the Local Group's motion toward the Virgo cluster. The background is slightly warmer, or blueshifted, in the direction we are moving (toward the constellation Leo) and slightly cooler, or redshifted, in the opposite direction (toward the constellation Aquarius).

The CMB shines on us from all directions. This does not mean we are at the center of the Universe, rather it means the CMB was released from every point in space. If we travelled to the other edge of visible universe, we'd still see a CMB, but the photons we'd be seeing would have originated from a different place in space. Here is a way to think of this. If you swim under water at night, your headlight will illuminate a set sphere of material around your head, and as you swim the volume of water you can see will always be the same size sphere, but the sphere you see will change. With the CMB, as we move we see photons that originated from different places (although massive, and impossible movement would be required to see an actual change), but we always seem to be at the center of our own sphere of CMB.

The CMB permeates all of space, and the energy it generates is something that has to be considered when scientists consider how the universe will end, and how long it takes black holes to evaporate. In every breath you take, you inhale 100,000 or so of these ancient photons. The photons are so feeble that there is no health hazard posed by this radiation; the power is only 10^-5 Watts, or a ten millionth of the luminous intensity of a light bulb, but it is everywhere. To see this radiation for yourself, tune your TV between stations. About 1% of the noise on the screen is due to interactions with the cosmic background radiation. Evidence of the Big Bang is all around us.

The discovery of the cosmic microwave background radiation is a striking confirmation of the big bang model. The temperature matches that expected for redshifted radiation emitted by a hot gas soon after the universe began. We can see directly back to 400,000 years after the big bang, when the universe was a few percent of its current age. The uniformity is a direct verification of the cosmological principle, an indication that at least the early universe was homogeneous and isotropic. Most important of all, the cosmic microwave background radiation is evidence that the universe has evolved; it is a fossil that tells of a hot, dense, and featureless universe out of which galaxies, stars, planets, and people were forged.

Author: Chris Impey
Co-Author: Pamela Gay
Multimedia Aggregator: Erik Brogt
Last modified: Monday, August 30, 2021, 11:33 AM