It's a really big subject and I'm only an interested amateur myself. I'll try to address a few points.
The cosmic microwave background (CMBR) was discovered about 50 years ago. Two astronomers, Penzias and Wilson if I recall correctly, won the Nobel prize for its discovery, but I believe that various other astronomers had also previously noticed it. It's an extremely low level background of microwave radiation that has a black body spectrum, the spectrum radiation by a body that is in thermal equilibrium. It is at a temperature of about 2.7 degrees Kelvin. What is so unusual about the CMBR is not only its black body spectrum but its extremely isotropic nature. In other words, its uniformity in all directions of the sky. I think that measurements of the CMBR have found that it is uniform in all directions to about 1 part in 100,000.
The big bang theory argues that this CMBR was produced by the big bang itself, which essentialy created our universe. Big bang theorists have argued that no other known mechanism other than the big bang itself could produce such an isotropic background. This is probably true. Various other mechanisms have been proposed by various astronomers, but none has so far really panned out. For example, Eric Lerner, a "plasma cosmologist", proposed that the CMBR was produced by electron scattering of the intergalactic plasma. But this has, I believe, been shown to not be a feasible source of the CMBR. The problem with the big bang explanation is, there is no way to empirically test out whether the big bang itself would produce such a background. It's not like we can go into the laboratory, initiate a big bang, and see what happens. So the assumption that a big bang could produce the CMBR cannot be empirically verified; it can only be deduced from known physics.
The cosmological red shift was first observed by the American astronomer Edwin Hubble. He discovered that the spectrographs of distant galaxies were shifted to the red end; that is, their frequencies were shifted down relative to where they should be based on the material that they contain. He further discovered that the amount of the red shift was linearly proportional to the distance of the galaxy from Earth. The proportionality constant has long since been named the Hubble constant.
The most common explanation for the red shift is that it is based on the Doppler effect. When an object recedes from an observer, its frequency is red shifted proportionally to its speed of recession; when it moves toward an observer, its frequency is correspondingly blue shifted to higher values. So, if the red shift is caused by the Doppler effect, this means that distant galaxies are receding from us, and that their rate of recession is proportional to their distance from us. This scenario is consistent with the big bang, in which the universe is expanding due to the expansion of space time itself, causing the recession speed of each object relative to every other object to be proportional to its distance, as observed.
So the CMBR and the Hubble red shift are the two principle pieces of observational evidence used to validate the big bang.
Like the CMBR, various alternative proposals have been made to explain the red shift. So far, none has really panned out either. But various static universe models have proposed different mechanisms which could cause light to redden as it travels through the universe, the reddening proportional to the distance it has traveled. For example, various "tired light" explanations have been proposed. If such were the true explanation, it would mean that the red shift was not Doppler at all, and that the universe is therefore not expanding but rather static.
If you're interested in exploring this further, there's a great web site with a bulletin board where visitors can ask any questions they wish or start their own topics. There are several professional astronomers who frequent the board, most of whom are big bang supporters, by the way. The web site is:
www.badastronomy.com/phpBB/index.php
This site is hosted by "the bad astronomer", Phil Platt. He has written a book on the subject of bad astronomy. His purpose is to educate the public about astronomy as a field and to debunk bad astronomy, whenever and whereever it occurs. There is a section for dissident cosmological theories, called "Against the mainstream", in which people discuss various non mainstream alternatives to the standard big bang theory, which is now the mainstream cosmological theory. |
