Hmm. After reading thru the previous two educators responses, I find it difficult to explicate the theory in a more cogent manner.
Or, to say it differently, for most readers, little needs to be added without resorting to Hawking, et. al. We know that:
- The universe apparently had a beginning. Astrophysicists have dated it to between 13 and 14 billion years ago. Sources for this vary, but think of nuclear decay, for one.
- The Big Bang theory implies something similar, but not the same as, an explosion: given this, the universe should be expanding and continue to expand. Measurements of the velocities of galaxies and galactic groups support this.
- Anything similar to an explosion should produce heat. There is a what appears to be a cosmic background radiation that is about 3 degrees above absolute zero.
Recent discoveries point to problems with the theory, but have yet to discredit it completely.
Big Bang's Gravitational Wave Goes Bust, Due to Dust