A conceptual understanding of the big bang will suffice to explain the CMBR. According to the big bang theory, the early universe was "optically thick", meaning light couldn't travel very far without being absorbed. You can see that this would have to be so because a young universe would be very small and therefore very dense. Dense objects (like your hand or a table or the sun) are opaque to light and easily absorb. This is how the universe was.

Anyway, the universe was expanding. As it expanded, it got less and less dense and, eventually, it stopped being opaque to light. This point in time is often referred to as "the surface of last scattering", implying that the photons have not been scattered or absorbed since that time. The photons left from this period have a characteristic temperature (about 2.73 K, as given by a blackbody law) and make up the CMB. It's kind of like a photograph of the early universe. I believe that George Gamow predicted it before it was discovered. Many astronomers think he should have won the nobel prize.

• Several predicted the CMBR... - bruce - October 31, 2002 - 20:23 UTC
  • This is always the case - cosmotiger - October 31, 2002 - 20:53 UTC