The Cosmic Microwave Background Radiation
Perhaps the most conclusive (and certainly among the most carefully examined) piece of evidence for the Big Bang is the existence of an isotropic radiation bath that permeates the entire Universe known as the "cosmic microwave background" (CMB). The word "isotropic" means the same in all directions; the degree of anisotropy of the CMB is about one part in a thousand. In 1965, two young radio astronomers, Arno Penzias and Robert Wilson, almost accidentally discovered the CMB using a small, well-calibrated horn antenna. It was soon determined that the radiation was diffuse, emanated unifromly from all directions in the sky, and had a temperature of approximately 2.7 Kelvin (ie 2.7 degrees above absolute zero). Initially, they could find no satisfactory explanation for their observations, and considered the possibility that their signal may have been due to some undetermined systematic noise. They even considered the possibility that it was due to "a white dielectric substance" (ie pigeon droppings) in their horn!However, it soon came to their attention through Robert Dicke and Jim Peebles of Princeton that this background radiation had in fact been predicted years earlier by George Gamow as a relic of the evolution of the early Universe. This background of microwaves was in fact the cooled remnant of the primeval fireball - an echo of the Big Bang.
If the universe was once very hot and dense, the photons and baryons would have formed a plasma, ie a gas of ionized matter coupled to the radiation through the constant scattering of photons off ions and electrons. As the universe expanded and cooled there came a point when the radiation (photons) decoupled from the matter - this happened about a few hundred thousand years after the Big Bang. That radiation cooled and is now at 2.7 Kelvin. The fact that the spectrum (see figure) of the radiation is almost exactly that of a "black body" (a physicists way of describing a perfect radiator) implies that it could not have had its origin through any prosaic means. This has led to the death of the steady state theory for example. In fact the CMB spectrum is a black body to better than 1% accuracy over more than a factor of 1000 in wavelength. This is a much more accurate black body than any we can make in the laboratory!
By the early 1970's it became clear that the CMB sky is hotter in one direction and cooler in the opposite direction, with the temperature difference being a few mK (or about 0.1% of the overall temperature). The pattern of this temperature variation on the sky is known as a "dipole", and is exactly what is expected if we are moving through the background radiation at high speed in the direction of the hot part. The inference is that our entire local group of galaxies is moving in a particular direction at about 600 km/s. In the direction we are moving the wavelengths of the radiation are squashed together (a blue-shift), making the sky appear hotter there, while in the opposite direction the wavelengths are stretched out (redshift), making the sky appear colder there. When this dipole pattern, due to our motion, is removed, the CMB sky appears incredibly isotropic. Further investigations, including more recent ones by the COBE satellite (eg Smoot et. al.), confirmed the virtual isotropy of the CMB to better than one part in ten-thousand.
A map of the sky at microwave frequencies, showing that the CMB is almost completely the same in all directions.
Given this level of isotropy, together with the accurate black-body spectrum, any attempt to interpret the origin of the CMB as due to present astrophysical phenomena (i.e. stars, dust, radio galaxies, etc.) is no longer credible. Therefore, the only satisfactory explanation for the existence of the CMB lies in the physics of the early Universe.
The Cosmological Dark Ages
The age of the universe is around 10 to 20 billion years. The early Universe was so hot and dense that it was like the conditions within a particle accelerator or nuclear reactor. As the Universe expanded it cooled, so that the average energy of its constituent particles decreased with time. All of the high energy particle and nuclear physics was over in the first 3 minutes (see the book of that name, written by Steven Weinberg in 1977). By that time all of the main constituents of the Universe had formed, including the light elements and the radiation.It is generally believed that little of note happened for the next 300,000 years or so. This period is sometimes referred to as the "Dark Ages" of the Universe. One way to learn about physical processes which might have occurred at these times is to search for minor deviations from a black-body in the spectrum of the CMB. An injection of energy, through for example a decaying exotic particle, could distort the spectrum a little away from the characteristic blackbody shape. So far no such distortions have been found, so we have no reason to believe that anything particularly exciting happened during this time.
The important thing which happened at about 300,000 years after the Big Bang is that the Universe became cool enough for the atoms to become neutral. Before that time all of the protons and electrons existed as free ions moving around in a plasma. Every time that a proton snatched an electron it would be zapped by a photon with high enough energy to rip them apart again. Only after about a few hundred thousand years was the average temperature low enough that the protons could hold onto their electrons to form neutral hydrogen atoms. This period is referred to as the epoch of "recombination" (in general when atoms become neutral after being ionized we talk of them recombining -- here in fact the ions and electrons are combining for the first time, so it should perhaps be called "combination"!).
When the Universe was ionized, the matter was constantly interacting with the radiation, ie photons were continually being scattered by ions and electrons. Looking back at the CMB we see the surface of "last scattering", when the photons last significantly interacted with the matter. At earlier times the universe is opaque, and so we don't see back further than the epoch of recombination. Between last scattering and today the universe is almost totally transparent. So when we look at the CMB we are seeing, in each direction, out to when the radiation last scattered. This means we are effectively seeing back in time to a few hundred thousand years after the Big Bang.
After the Universe recombined, the stars, galaxies and clusters of galaxies started to form. We know little in detail about this process, largely because it is a very complex physical process. One of the biggest uncertainties is understanding the "seeds" from which the galaxies and other structures grew. Everything that we see with optical telescopes (or telescopes in any other wavelength range) tells us about objects which have existed in the last 10 billion years or so. It becomes more and more difficult to probe conditions in the Universe at earlier times.
Detailed observations of the CMB provide exactly the sort of information required to attack most of the major cosmological puzzles of our day. By looking for small ripples in the temperature of the microwave sky we can learn about the seed fluctuations as they existed 300,000 years after the Big Bang, and well before galaxies had started to form. We can also learn what the Universe as a whole was like back then: whether it was open or closed; what the dominant form of dark matter is; and how the Universe has been expanding since that time. Through careful examination of the Cosmic Microwave Background we can probe the cosmological Dark Ages.
Temperature Fluctuation
While the CMB is predicted to be very smooth, the lack of features cannot be perfect. At some level one expects to see irregularities, or anisotropies, in the temperature of the radiation.These temperature fluctuations are the imprints of very small irregularities which through time have grown to become the galaxies and clusters of galaxies which we see today.