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The Question:

According to Prof. Brian Cox in the first 30 seconds of this YouTube video from 4 years ago, we do not know whether the universe had a beginning. Is it still the case that we do not know?

Thoughts:

I know about the Big Bang, that, strictly speaking, it is not the point at which the universe "began", if it did so at all.

I hear people claim a past-infinite universe is logically impossible. However, from my experience as a mathematician, I am aware of finite situations (let's call them) with no start; case in point: $(0,1):=\{x\in\Bbb R\mid 0<x<1\}$ is a finite interval with no smallest element.

My Background:

Aside from Classical Mechanics at A Level back in 2009, I have no formal education in Physics beyond GCSE. I read Simon Singh's, "Big Bang" and Brian Cox et al.'s, "Why does $E=mc^2$?" around 2013. I have forgotten most of the details.

I'm studying for a PhD in Group Theory.

Shaun
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    How could we? What experiment or observation could tell us? – John Doty Mar 14 '24 at 15:28
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    I don't know, @JohnDoty. That's hardly a satisfying answer for me to reach on my own as a layperson. What arguments are there that no experiments nor observations could tell us? – Shaun Mar 14 '24 at 15:31
  • Not the "I don't know", but the "How could we know?" - just to clarify . . . – Shaun Mar 14 '24 at 15:34
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    You can make a mathematical model that extrapolates our observations back in time, but you'll reach a point where it cannot be compared with observation. How do you then verify that the extrapolation continues to be a valid model? – John Doty Mar 14 '24 at 15:54
  • Possible duplicate: https://physics.stackexchange.com/q/5150/2451 , https://physics.stackexchange.com/q/2355/2451 and links therein. – Qmechanic Mar 14 '24 at 17:42
  • Since it is from 2011 and addresses only the Big Bang (yet not the beginning of the universe), that is not a duplicate, @Qmechanic; please reopen. I would like to know arguments about whether the universe began, not just the Big Bang. – Shaun Mar 14 '24 at 18:03

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We do in fact have good observational evidence for the big bang. And by running high-energy physics experiments in particle colliders, we can reproduce much of the sequence of events which took place then- and compare the result with observations on the real world out there. We thus consider the big bang to represent the beginning of the universe as a convenient approximation.

The real question is this. We can take the particle experiments and extrapolate them to higher energies, representing earlier time slices in the big bang. But there comes a point where we know those extrapolations will necessarily go wrong because those models break down and stop working- and our accelerators can't reach those energies, ruling out experiments.

Then what we look for are mathematical models of the unknown physics, stuff them into the existing models, and run the models forward in time to lower energies. Then we look at what the model says we should see and compare them to observations.

So far, we got essentially nothing- that is, no detectable low-energy consequences of ultra-high energy physical processes that we think might have occurred at the earliest time slice of the big bang. But we haven't given up- we just keep looking.

niels nielsen
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    Thank you. I knew about your first paragraph here. I suppose I could have explained myself better. – Shaun Mar 14 '24 at 16:57