No. The Big Bang Theory predicts an infinite future (and heat death) for the universe but says nothing about what might have come before inflation, so an eternal universe is possible although I believe it is considered unlikely. It would require some kind of fundamental state change, since the universe before the singularity would have to have been different in some significant way than the current universe, otherwise the singularity would not have happened (we see no singularity in the future of the current universe). I'm not widely read in this but I have never encountered a theory of an eternal universe that seemed to be anything other than pop-sci blather.
I believe the historical idea of an eternal universe is one that is static, unchanging, and has no beginning and no end. The big bang theory absolutely rules this specific type of eternal universe out, as it says that at every scale the universe is neither static nor unchanging but has a very dynamic existence. At its largest scales it expands and at smaller scales it experiences huge changes from the formation and evolution of galaxies, stars, planets, and other bodies. What the BBT doesn't say is whether or not the universe has a beginning or an end. The fact that the BBT predicts a singularity doesn't mean that it has a beginning, it means that the theory can no longer be used at that point. Another one must be found. As for an ending, there isn't one predicted by the BBT, but who's to say our current understanding of physics is sufficient to predict what will happen hundreds of billions of years from now?
It means that before approx 14bn years ago, nothing of our present observable Universe existed.
This does not rule out the existence of some kind of Universe beyond the observable.
However if something does exist beyond that it probably never can be observable, thus is not something which science can address.
There are all manners of metaphysical speculations, but they really don't explain anything.
Back when I was about 2nd year in high school, my English/Spanish/History teacher knew of my interest in science, and he gave me a paperback book by Fred Hoyle on the Steady-State Universe. He said, "This is an old theory that has now been proven wrong, but it is still an interesting read". I really enjoyed the book, and it motivated me to go on and learn more about the BBT and more modern understandings.
Hawking and Penrose showed that given certain assumption the big bang is associated with a spacetime singularity which marks the beginning of the universe. However these assumptions are no longer considered realistic and so its quite possible that the universe existed eternally into the past. there is no reason to either confirm nor deny that possibility. What most cosmologists agree is necessary is quantum theory of gravity to be able to probe this further. There are no theories of quantum gravity that have passed experimental verification. But there are some that theorists think have a lot going for them. When applied to the big bang these theories seem to suggest the universe existed before the big bang. Perhaps eternally into the past. I think these are the best bets we have at the moment, but they are not more than that. good bets but not verified experimentally. It is not impossible that we will be able to probe this experimentally and there are suggestions for how to do this. but it hasn't been done yet. So we dont know.
The universe will expand forever into the future assuming dark energy is a cosmological constant. There is always the possibility that dark energy is not a constant in which case the future of the universe is more uncertain.
Ok, so I have read now that the current model of the universe has no boundary. That is an assumption.
But, what I am trying to understand is this: Big Bang says that the the Big Bang itself created space.
We know that matter was compacted to a tiny point.
My question is, how can that tiny compaction of matter have no boundary???
When this tiny point expands, then surely it expands WITH a boundary, right?
Where? Please give a reference.
To be precise, our best current model of the universe has the universe being spatially infinite.
No, it's a conclusion based on evidence.
No, it doesn't.
No, we know that our observable universe, right after the Big Bang, occupied much less spatial volume than it does now. But our observable universe is not the entire universe.
Here is the reference for space been created by the Big Bang: https://www.space.com/52-the-expanding-universe-from-the-big-bang-to-today.html
"The universe did not expand into space, as space did not exist before the universe, according to NASA Instead, it is better to think of the Big Bang as the simultaneous appearance of space everywhere in the universe. "
That is a pop-sci presentation authored by a science reporter. It falls well short of the Physics Forums standards. However, the language used is closer to the mark than most popularizations.
The first line from your link -
"The universe was born with the Big Bang as an unimaginably hot, dense point."
Replace "universe" with "observable universe" and imagine that the hot, dense stuff is of infinite extent. The "point" being discussed is the tiny amount of the dense stuff that we can today see as our observable universe, but that point was not all that existed - the young universe was very dense, but it wasn't more or less bounded than today's universe. We have no model or theory to suggest that physical things can transition from finite extent to infinite extent or vice versa, so the thinking is that if the universe is infinite in extent today, then it always was, even if it was was much more dense in the past than it is today.
Its hard to describe with words something that is of infinite extent expanding and becoming less dense. I am told it is easier to describe with mathematics than words, or at least it is possible to be more precise about what one is talking about with math than words.
I suggest you don't read too much into someones claim that it is better to think the big bang as the simultaneous appearance of space everywhere in the universe. Whether it is really better to think of things that way or not is a very judgement driven conclusion - reasonable people can disagree. In most discussions I have read, participants have trouble agreeing on what descriptions like that really mean or are trying to get across.
As @jbriggs444 has pointed out, this is not a valid reference. Please consult a cosmology textbook.
How best to proceed when words become ambiguous? The short answer is mathematics. Mathematics accommodates infinities [both countable and uncountable] with relative ease whereas ambivalence towards the principles of cause and effect is a Gordian Knot for logic.
What evidence is there to show that the the current model of the universe is specially infinite??
Is it observable evidence??
How can we 'observe' that something is 'infinite'??? I don't understand this.
Post #23 for some digestible discussion on that.
You mean, what evidence is there to show that our best current model of the universe, which says that it is spatially infinite, is correct? The fact that the universe, according to our best current measurement, is spatially flat, and that we see no evidence of non-trivial topology (e.g., we don't see multiple images of the same distant object in different directions). Spatially flat + no non-trivial topology = spatially infinite.
[Edit: corrected to "no non-trivial topology" above.]
@PeterDonis, I think you mean "no non-trivial topology" there.
Yes. Post corrected.
I think people are being too heavy handed with claims that data support an infinite universe. It equally supports a closed universe with a radius of curvature much larger than the Hubble scale.
I wonder what William of Occam would make of this dilemma.
Good point. What does Bayesian model selection have to say? Do you have a good prescription for a prior on the model space?
The prior that appears to be implicitly adopted by our best current model is that spatial flatness is more likely, other things being equal. But I don't think anyone has explicitly thought that out or advanced an argument for such a prior.
Of course part of the problem is that it's not even clear what, exactly, the model space is or how to parameterize it.
The assumption referenced by @Varsha Verma is the assumptions of homogeneity and isotropy. Together (with simple topology) they also imply the absence of a boundary, regardless of whether the universe is finite or infinite. So the lack of a boundary is indeed an assumption, but as you say the spatial infinite-ness is a conclusion based on the evidence.
Actually, I think they imply the absence of a boundary even with a non-simple topology. A flat 3-torus, for example, still has no boundary.
Here is a picture of a torus. I can see boundaries here. Especially the 'hole' in the middle is a boundary right.
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