What is right about the big bang theory?

Answer 1

First, let's set some expectations. The Big Bang Theory is a theory of how the Universe came to be what it is today, given the observational fact that, except for some local gravitational binding, all of the galaxies in the Universe are traveling away from each other at a speed proportional to the separation distance. In particular, the Big Bang Theory is not, to paraphrase Alan Guth, a theory of what banged, how it banged, or what caused it to bang.

The galaxies are moving apart; therefore, at an earlier time, they were closer together. If you go back far enough in time, all of the galaxies were collected together in the same place. Taken to the logical conclusion, at some point in the past all of the matter in the Universe was condensed to a single superdense point.

Now, starting from that point, how did the Universe get to be what it is today? It is apparently obvious that this superdense point of matter expanded, and that expansion is still continuing. The smoking-gun confirmation of this initial superdense (and superhot) state is the existence of the cosmic microwave background radiation.

It turned out that this model of simple, direct expansion from a point was inadequate to explain certain observations. Some years ago, Alan Guth showed that it was likely that the Universe had undergone a short period of superfast expansion now called "inflation". During this short interval, the Universe expanded far faster than the speed of light. It was subsequently realized that inflation solved some of the then-existing observational difficulties (see the "horizon problem" and the "flatness problem").

Examination of the properties of the cosmic background radiation, and subsequent attempts to model the formation of galaxies in the very early Universe, led to the realization that it took too long to form structures out of the hot, uniform gas implied by the background radiation. Dark matter provides the additional gravitational attraction necessary to begin building galaxies. Thus, the Big Bang essentially requires dark matter, which was originally postulated as the solution to observational problems that had nothing to do with the Big Bang.

So, what's "right" about the Big Bang Theory is that it fits observations of the large-scale structure and early development of the Universe. Any theory that might come along with intent to replace it must explain the structure and early history of the Universe better than the Big Bang Theory currently does.