I How long did inflation last in an eternal inflating universe?

[valenumr]

Having read this thread, I'm still unclear regarding use of the word infinite. I have been reading Max Tegmark who mentions Eternal Inflation as being infinite, but if it had a finite past this can't be possible. Also, what does spatially infinite mean? If space is growing from a point then it can't be infinite, surely? In my simple view you can only talk about something being infinite if it started that way.

If, at some point time was going backwards, presumably with respect to how we see it, as there is no absolute arrow of time, and it's infinite, doesn't that create a paradox whereby how does something change or start if there is an infinite amount of time preceding it?

Im going to go outside of my expertise by far, but my understanding is that the universe was infinite before the "big bang", and that happened everywhere. Like everything expanded from any point of view. Not at some singular point. My understanding is there was not some singular point that blew up into our universe. But everything everywhere expanded into what we see today. It's a bit weird to wrap ones head around, but not really.

 

[valenumr]

Yes. Why wouldn't it be?

I mean, if I understand the concept correctly, it isn't something that we observe in our little bubble of a much larger universe. So it seems non-copernican. I suppose a better question is whether there is any observable evidence of eternal expansion edit: inflation.

 

[Ibix]

I have been reading Max Tegmark who mentions Eternal Inflation as being infinite, but if it had a finite past this can't be possible. Also, what does spatially infinite mean? If space is growing from a point then it can't be infinite, surely? In my simple view you can only talk about something being infinite if it started that way.

If the universe is spatially infinite it always was, indeed. "Space growing from a point" is one of those myths that you see quite a lot - probably arising from confusion between the universe and the observable universe. The latter is a finite region of the infinite former.

 

[Ibix]

I mean, if I understand the concept correctly, it isn't something that we observe in our little bubble of a much larger universe. So it seems non-copernican. I suppose a better question is whether there is any observable evidence of eternal expansion.

I think the point is that each bubble is spatially infinite, but is only a portion of the inflating spacetime. You can do this in a curved spacetime in such a way that the bubbles don't overlap.

 

[PeterDonis]

if I understand the concept correctly, it isn't something that we observe in our little bubble of a much larger universe.

Yes, we do; if inflation is correct, then there is a region of spacetime in our causal past that was inflating, and since it's in our causal past, we can in principle observe it now.

I suppose a better question is whether there is any observable evidence of eternal expansion edit: inflation.

There is indirect evidence that inflation proponents say is evidence for inflation. I think it's fair to say that it is still an open question whether there are other possible explanations for that evidence that do not involve inflation.

As for "eternal" inflation, that's actually the simplest inflation model you can have (because it doesn't require you to explain why inflation only happened in the causal past of our particular universe). So evidence that inflation happened in our causal past is also evidence for eternal inflation if you apply Occam's razor.

 

[kimbyd]

Thinking about this again, perhaps it isn't so much that eternal inflation describes an infinite universe as it is that the model itself is infinite by construction.

The way eternal inflation works is that as the universe inflates, each individual patch will tend to a smaller value of the potential energy, leading to slower inflation over time. However, the specific field value will fluctuate a little bit, so that some regions will end up inflating faster instead. Eternal inflation happens when the small fraction of faster-inflating regions inflate enough faster that their volume grows faster than the rest. Thus any given volume of inflating universe will tend to have parts which keep inflating forever.

The key words there are "tend to". This isn't a guarantee. For a given inflating volume, there is some probability that it will eventually stop inflating altogether. The inflation only becomes truly eternal if you allow that volume to be infinite.

This whole discussion says nothing whatsoever about whether our universe is infinite. It's just logic within the model for why they would use an infinite universe as part of it: it makes it actually eternal rather than just probably eternal. This means that we probably shouldn't take too seriously the infiniteness of that model: it's not a prediction in any sense, just a simplification.

The real consequence is that because the eternal inflation model is infinite, we have to contend with the consequences of said infinity. I believe one of those consequences is the measure problem, which makes it basically impossible to calculate relative probabilities (you can still say whether or not something has some probability of occurring, but not whether it is more or less likely than some other thing).