cosmology - Does red shift evidence necessarily imply that the universe started from a singularity?

We are taught that the universe began as a singularity - an infinitely small and infinitely dense point. At the beginning of time there was a 'Big Bang' or, more accurately, 'Inflation'.

The main evidence for this is the observation of the red shift of all of the galaxies. This shows us that as time increases, the universe becomes bigger. A logical outcome of this is that going back in time, the universe shrinks. This is then extrapolated back to the beginning of time, where the universe was infinitely small.

However, the only thing that I can see that we know for sure is that the universe used to be smaller. This does not necessarily imply that it used to be infinitely small. How do we know, for example, that the universe doesn't oscillate and that we are in a time where the universe is expanding (and accelerating) and will eventually contract again?

What other evidence is there that suggests we started from an singularity?

Answer

There are 3 observations that support the big bang theory, i.e. origin of the universe in a singularity:

1. The redshift of galaxies, as you already mentioned.


2. The cosmic background radiation.


3. The amounts of different nuclei in the universe, notably the preponderance of light elements like hydrogen and helium.

Each of these alone would probably not be sufficient to support the big bang theory. The redshift of galaxies could be explained by some other theory, some have been suggested by Hoyle and Narlikar in the past. Probably the other two phenomena could be explained independently as well, but it is the conjunction that fits so well with the big bang hypothesis.

Does that settle the matter once and for all? Short answer is no. Since these 3 observations have been made and confirmed, more detailed observations have been added to the mix and this has complicated the story for the big bang model. But that would take us into a longer post. The current model which is the most widely accepted is the so-called Lambda-CDM model.

As for the problem of the universe starting in a real singularity, instead of a very dense state, this is still an open problem related to a yet to be invented (or completed) theory of quantum gravity. Our current understanding of singularities in General Relativity is going back to the Penrose-Hawking singularity theorems. They are of the kind "Here be dragons!" in that they delineate the conditions for singularities to form and point where our knowledge ends. More can not be done, because a singularity is basically a failure of the theory.