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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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    My impression is that the standard history of the universe as seen here https://en.wikipedia.org/wiki/Big_Bang#Overview is not longer based on the naive BB model, i.e the mathematical singularity at the beginning of the universe. The naive BB is supposed to be an envelope which at the moment uses an effective quantization of gravity to avoid the singularity at the beginning, and introduces of dark energy to take care of the observation of continuing expansion. Unless this is what you call "naive BB" – anna v Feb 05 '16 at 08:22
  • The concept that all comes from nothing once is derived from religion. Today, minds are open with ideas like multiverse, repetitive bounces , etc. Copernic is the king –  Feb 05 '16 at 13:16
  • There are no sophisticated realizations of false ideas. What's wrong with the naive big bang ideas is that it doesn't reproduce the measurements. – CuriousOne Feb 05 '16 at 14:45
  • @CuriousOne Sophistication is relative. The minimum I was after is a quantitative model based on laws or principles. The average "naive big bang theorist" is just a layperson thinking intuitively. – Mitchell Porter Feb 05 '16 at 20:02
  • @MitchelPorter: I am sorry, but science is not accountable for false thinking patterns, otherwise science in the US would be mostly working on the problem "How God did it! Amen!". That's just not a useful way of looking at what science does. – CuriousOne Feb 05 '16 at 21:24
  • @CuriousOne That's not a reasonable comparison. An explosion in the conventional sense is a type of physical process for which there are models. It is completely reasonable to wonder whether the expanding universe was produced by an explosion. And I have noticed that exposition regarding this issue does not say "why primordial explosion is a physically wrong idea", it says "why big bang theory isn't a primordial explosion theory". – Mitchell Porter Feb 05 '16 at 22:07
  • @MitchellPorter: Where is the explosion in the correct theory? I can't see it. The matter density stays homogeneous at all times. If anything, it's very hard to explain why the universe is as homogenous as it is... an explosion model makes that impossible because it starts out inhomogeneous. – CuriousOne Feb 06 '16 at 01:06
  • Probably not what you are looking for, but int Raychaudhuri's book "Theoretical Cosmology" there is a section on Newtonian cosmology, which describes something of the sort in your question. And strangly leads to the Friedmann's equations! – MBN Feb 07 '16 at 09:18
  • @igael Or maybe religion settled on it because it fit with the natural philosophers of the day? In any case, I don't see why "it all came from nothing" is intrinsically more metaphysically absurd than "It was all there since eternity"...? – Thriveth Feb 07 '16 at 11:10
  • @Thriveth : for my own , I prefer "we don't know but there are theories of what was maybe possible". The Big bang , as it was populated , was the theory from the beginning. Now , it is a theory of the universe past , not necessarily of all the past –  Feb 07 '16 at 14:13

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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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  • Why do we have to use general relativity to prop up a false explanation when we know how to apply it correctly? Am I misunderstanding something? – CuriousOne Feb 05 '16 at 14:44
  • Why it will be time independent??? Are you considering empty interior of the black hole (no matter except at singularities)? – OON Feb 05 '16 at 15:48
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    @John Rennie Schwarzschild metric corresponds to zero energy-momentum tensor everywhere except singularities. If you get rid of all matter in FLRW you'll get Minkowski spacetime that also is time-independent. So, what do you want to say? – OON Feb 05 '16 at 15:56
  • @John Rennie My point is that comparing purely Schwarzshild solution with our universe makes absolutely no sense because our universe has some matter. If you really add matter originating at white hole singularity and then collapsing at black hole singularity you can't use textbook Schwarzschild metric. – OON Feb 05 '16 at 16:10
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    @John Rennie And at low density the universe is approximately Minkowski. You need to honestly take into account the influence of the matter on the metric to compare the black hole interior "cosmology" with FLRW. – OON Feb 05 '16 at 16:18
  • Frankly it's still not that good. The need to match solutions is just a consequence of the sharp boundary between the universal bubble and the Void. Smooth it and the matching will be done automatically. So the only really contrived thing is why the matter is not unifomly distributed in the whole universe but concentrated in that ball which is however uniform. Anyway, not prefering "contrived" is not a solid argument against, it's only a selection rule when we can't distinguish the scenarios empirically. The latter is what I would concentrate on. Perhaps myself tonight or this weekend. – OON Feb 05 '16 at 19:14