4.2 Evidence for a big bang
Having interpreted the redshift as indicating a recessional speed proportional to distance, one may extrapolate into the future to predict how the positions of the galaxies will evolve with time. One can also run the sequence backwards, so to speak, to discuss what their positions were in the past. Clearly, at former times the galaxies were closer to each other.
But not only that. Because of the proportional relationship between speed and distance (Equation 6), at a certain time in the past, all the matter of the Universe must have been together at a point of extraordinarily high density. It was from this condition that it subsequently expanded giving the matter of the Universe its present-day distribution. This is our first indication that the history of the Universe featured an explosive event.
This has become known as the big bang. It is believed to have marked the beginning of the Universe. (Actually the phrase ‘big bang’ is used in two ways: (i) to denote the instant at which cosmic expansion begins; and (ii) to refer to that instant plus the sequence of events immediately following. It is usually clear from the context which of these meanings is intended.)
It is possible to deduce much about the nature of the big bang and how long ago it took place – in other words, how old the Universe is. But we need to be sure that there really was a big bang. What we seek is evidence that is independent of the observation of moving galaxies. The remainder of this course is devoted to describing just such confirmatory observations. Not only do they add to our confidence that the Universe did indeed have a definite beginning, they also inform us that the beginning was exceedingly violent – the big bang was hot. This indication assumes great importance when we seek to get some understanding of the varying types of process that must have been taking place during the initial stages of expansion – during the first years, minutes, even fractions of a second after the instant of the big bang.