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Many people think that according to big bang cosmology, first there was empty space, then there was an explosion in the middle of the emptiness, and now all the galaxies are flying away from that original point in space.

If they discuss this with a physicist, they will be told no, in the big bang theory, space is filled with matter from the beginning, it's space itself that is expanding, and so on. See e.g. this Phys.SE post. And then, if the conversation gets that far, all those unexpected details will be explained as making sense in general relativity.

This is fine as a description of what physicists actually think. But it doesn't address what's wrong with the "naive big bang idea" as a theory in itself.

So I have two questions:

  1. What is the most sophisticated realization of the naive big bang idea, as a cosmological theory?

  2. What are the best arguments against the viability of the naive big bang idea?

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Mitchell Porter
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1 Answers1

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Actually there is a geometry that describes something like the naive idea of the Big Bang. But it's a bit of a cheat because it's really just a piece of the usual expanding universe metric.

The first metric suggested to describe a collapsing star was the Oppenheimer-Snyder metric, which describes a spherical ball of dust collapsing under its own gravity. Inside the ball the metric is the same FLRW metric that describes an expanding universe (with time reversed) and outside the ball the metric is the Schwarzschild metric. The geometry is adjusted so that the metrics match at the surface of the ball.

The Oppenheimer-Snyder metric describes a collapsing ball of dust, but if you run it in reverse it would describe a ball of dust expanding outwards from a singularity. This would look just like the naive idea of the Big Bang happening at a point. Although the metric describes only dust, I don't think there would be any problem enhancing it to include radiation and dark energy as we observe in our universe.

However this is not a realistic model for the universe because it's highly contrived. Apart from having some deity on hand to set up the two metrics and patch them together it's hard to see any mechanism by which a geometry like this could arise. You would also need the radius of the ball at the current time to be greater than 46 billion light years otherwise we'd see its edge.

There is a certain intellectual amusement in working out how the naive idea of a Big Bang could be described, but I trust the contrived nature of the proposed geometry makes it clear just how silly the idea is.

John Rennie
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