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I have read to Sean Carroll that he says that the Big Bang model is correct, but the Big Bang event is incorrect, so what is the difference? And everyone knows that the Big Bang model is linked to the Big Bang event, which is the singularity, so how can there be a Big Bang without the singularity?

The Big Bang “model” (hot expanding universe) is true; the BB “event” (early singularity) is just conjecture.

(The Big Bang singularity isn't even sensible conjecture. There are no singularities in quantum mechanics. Could be a first moment of time.)

https://twitter.com/seanmcarroll/status/734865105792303104

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    $\begingroup$ It would help to post a link to where Sean Carroll said that so that we can see it in context $\endgroup$
    – Dale
    May 4 at 13:18
  • $\begingroup$ twitter.com twitter.com › status Sean Carroll on Twitter: "(The Big Bang singularity isn't even ... $\endgroup$
    – مروان حسين
    May 4 at 13:34
  • $\begingroup$ Sorry I was unclear. You should edit the question to include a direct link to where Sean Carroll said that so we can see it in context $\endgroup$
    – Dale
    May 4 at 13:54
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    $\begingroup$ @MichaelSeifert I haven't read Sean Carrol's books, so I can't tell you how accurate they are. One would hope that his skills as a textbook author are better than his skills as a popular science writer. I will leave it at that. $\endgroup$
    – FlatterMann
    May 4 at 18:44
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    $\begingroup$ @مروانحسين “the universe began after it did not exist” - The word “after” in your statement implies that time (and so also space) existed before the universe began. Something like, “there was an empty space and time was running, but nothing was there, so nothing was happening; and then, after all that, the universe appeared out of nowhere”. This picture is completely wrong. The concept of the universe includes space and time. When we say, the universe started at the Big Bang, we mean that time and space started as well. “Before” requires time. Without time, “before” has no meaning. $\endgroup$
    – safesphere
    May 15 at 2:18

3 Answers 3

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It is hard to tell with such a poor reference. Twitter is not a scientific publication so statements there are not held to the usual scientific standards.

The Big Bang model is a classical (not quantum) model where the universe is modeled as being approximately isotropic and homogenous at the largest scales, and it was hotter and more dense earlier than it is now, and it has expanded and cooled as time progressed. There is ample observational evidence supporting this model.

If we extrapolate this model backwards in time, well past the point where we have observational data, then we get a singularity (what Carroll is calling the Big Bang "event"). Because this requires extrapolation beyond the point where we have observational data, it is reasonable to call this a conjecture.

There can be a hot dense universe that expands and cools without a singularity if our classical models break down sometime beyond the limit of our observational data. The details will require a quantum theory of gravity.

Since we know that in ordinary circumstances our classical models break down at small scales, many scientists (apparently including Sean Carroll) expect that this will happen for the Big Bang model as well. The result could be a solution to some as yet unknown cosmological equations that matches the Big Bang model at later times but diverges from it for the very dense early universe.

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    – Buzz ♦
    May 5 at 17:12
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In general, Twitter or anyone making any statement in a non-peer-reviewed journal should be regarded as being somewhat weak. At that, the Big Bang model is quite well understood in reference to physical conditions that are attributed to our cosmology. As far as I understood, the "event" is the singularity, and this as well is quite well-defined in terms of the physical conditions leading to it in a time-reversed cosmology. However, what precisely happened at the singularity is a very complex question that I can safely say no one has the answer to. However, even if a quantum theory of gravity comes around, I doubt it would resolve all singularities. Maybe singularities in some particular cosmologies, but perhaps not in general for any cosmology satisfying the Penrose (or Hawking-Penrose) theorem. The Big Bang as a model and its phenomenological or thermodynamic aspects are rather well understood, but the singularity in itself has not been fully understood. But, even the question here is vague; perhaps there is a more precise question, I would be glad to point something out if there is.

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    – Community Bot
    May 4 at 18:41
  • $\begingroup$ Technically a flat FLRW solution doesn't really have a singularity. It's always flat. The density diverges for cosmological time going towards zero, but that's about it. Why does it do that? Because we don't have an actual equation of state in general relativity for spacetime. The theory lacks any sense of physical scale. One could even argue that the theory lack anything resembling actual physics. Of course it will show divergences on both ends. $\endgroup$
    – FlatterMann
    May 4 at 18:51
  • $\begingroup$ This means that the Big Bang was the beginning of the universe, right? $\endgroup$
    – مروان حسين
    May 4 at 19:13
  • $\begingroup$ @مروانحسين "This means that the Big Bang was the beginning of the universe, right?" All we can say with certainty is that it's the beginning of time as we know it. What was there "before" is educated speculation. Quite possibly, it's like asking what is north of the north pole. $\endgroup$
    – RonJohn
    May 5 at 12:03
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Stephen Hawking had this idea once, that using imaginary time we can build a model of spacetime which is infinite but without boundaries -- that is without a Bing Bang singularity.

See https://www.hawking.org.uk/in-words/lectures/the-beginning-of-time and a related StackExchange discussion at Imaginary time concept in S.Hawking's No Boundary proposal, extra-time dimensions and the Big Bang

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