Independent Inflationary Regions in an Infinite Universe?

[mister i]

TL;DR

In an infinite universe, could different inflationary processes have occurred in different regions, each giving rise to its own local universe?

In an infinite universe, could different inflationary processes have occurred in different regions, each giving rise to its own local universe? (differents observed universes), independent and not causally connected to the others? I have read the Multiverse entry on Wikipedia, but this possibility does not seem to be mentioned there.

 

[PeterDonis]

For reference, the Wikipedia page you mention is here:

https://en.wikipedia.org/wiki/Multiverse

 

[PeterDonis]

And from that Wikipedia page (thanks to @renormalize for the quote):

Proponents and skeptics
Modern proponents of one or more of the multiverse hypotheses include Lee Smolin, Don Page, Brian Greene, Max Tegmark, Alan Guth, Andrei Linde, Michio Kaku, David Deutsch, Leonard Susskind, Alexander Vilenkin, Yasunori Nomura, Raj Pathria, Laura Mersini-Houghton, Neil deGrasse Tyson, Sean Carroll and Stephen Hawking.
Scientists who are generally skeptical of the concept of a multiverse or popular multiverse hypotheses include Sabine Hossenfelder, David Gross, Paul Steinhardt, Anna Ijjas, Abraham Loeb, David Spergel, Neil Turok, Viatcheslav Mukhanov, Michael S. Turner, Roger Penrose, George Ellis, Joe Silk, Carlo Rovelli, Adam Frank, Marcelo Gleiser, Jim Baggott and Paul Davies.

So there does not seem to be any consensus on the question you're asking.

 

[javisot]

TL;DR: In an infinite universe, could different inflationary processes have occurred in different regions, each giving rise to its own local universe?

In an infinite universe, could different inflationary processes have occurred in different regions, each giving rise to its own local universe? (differents observed universes), independent and not causally connected to the others? I have read the Multiverse entry on Wikipedia, but this possibility does not seem to be mentioned there.

"A Level II multiverse, defined by Max Tegmark, consists of distinct, distant "bubble" universes formed through chaotic eternal inflation, where space expands forever. " (google)

 

[PeterDonis]

"A Level II multiverse, defined by Max Tegmark, consists of distinct, distant "bubble" universes formed through chaotic eternal inflation, where space expands forever. " (google)

Sure, but the OP's not asking how to define a multiverse. He's asking whether inflationary theory actually says a multiverse exists. And, as has already been posted, there is no consensus on that. Of course Tegmark thinks it does: he's listed as one of the proponents in the Wikipedia article that was quoted. But "Tegmark says so" doesn't guarantee that it's true.

 

[renormalize]

In an infinite universe, could different inflationary processes have occurred in different regions, each giving rise to its own local universe? (differents observed universes), independent and not causally connected to the others? I have read the Multiverse entry on Wikipedia, but this possibility does not seem to be mentioned there.

Yes, the possibility is mentioned in the Wikipedia article, namely as the "inflationary multiverse", in which multiple "bubble universes" arise due to Eternal Inflation.

 

[Jaime Rudas]

Yes, the possibility is mentioned in the Wikipedia article, namely as the "inflationary multiverse", in which multiple "bubble universes" arise due to Eternal Inflation.

As a supplement, this Andrei Linde's paper contains a good description of the eternal inflation hypothesis.

 

[javisot]

Sure, but the OP's not asking how to define a multiverse. He's asking whether inflationary theory actually says a multiverse exists. And, as has already been posted, there is no consensus on that. Of course Tegmark thinks it does: he's listed as one of the proponents in the Wikipedia article that was quoted. But "Tegmark says so" doesn't guarantee that it's true.

I agree that all of this is theoretical and possibly has no relation to reality, and that the informal proposal of bubble universes might not even be theoretically true.



What is the hierarchy of sizes?

Multiverse-universe > bubble universe > observable universe

I understand that the concepts of "observable universe" and "bubble universe" refer to different things, right?

 

[Jaime Rudas]

Sure, but the OP's not asking how to define a multiverse. He's asking whether inflationary theory actually says a multiverse exists.

The OP's not asking whether inflationary theory actually says a multiverse exists. He's asking whether different inflationary processes could have occurred in different regions, each giving rise to its own local universe. This is similar to the eternal inflation hypothesis that Tegmark includes in his classification.

 

[PeterDonis]

The OP's not asking whether inflationary theory actually says a multiverse exists. He's asking whether different inflationary processes could have occurred in different regions, each giving rise to its own local universe.

That's how Tegmark defines a Level II multiverse. It's true that he defines other levels as well. But the OP appears to be asking about that one. Asking whether the processes could have occurred that give rise to a Level II multiverse is the same as asking whether a Level II multiverse exists. The answer is that nobody knows for sure.

This is similar to the eternal inflation hypothesis that Tegmark includes in his classification.

Yes, which he says gives rise to a Level II multiverse.

 

[PeterDonis]

I understand that the concepts of "observable universe" and "bubble universe" refer to different things, right?

Yes.

 

[Jaime Rudas]

Asking whether the processes could have occurred that give rise to a Level II multiverse is the same as asking whether a Level II multiverse exists.

It's not the same to ask whether something could have happened as to ask whether it actually did happen. Moreover, the question seems to be aimed more at finding out whether this particular type of multiverse is among those described in the Wikipedia article.

 

[PeterDonis]

It's not the same to ask whether something could have happened as to ask whether it actually did happen.

It is when you're talking about multiverse hypotheses. That's the whole point of them: that every possible universe that could happen, does happen. The question is, is that actually true?

 

[javisot]

Personally, I think the OP's question is answered by referencing Tegmark's Level 2 multiverse. On the other hand, Peter's point is appropriate, since the "bubble universe" proposal is far less formalized than it seems.

 

[mister i]

How do you know? Why don't we let the OP decide what he's asking, and whether the posts in this thread are useful to him? We've each expressed our opinions, but it's up to him.

My question was: In an infinite universe, could different inflationary processes have occurred in different regions, each giving rise to its own local universe?

In the Wikipedia article on the Multiverse, Level II effectively answers part of the question, although its title is Level II: Universes with different physical constants, and it discusses eternal inflation (which is somewhat beyond what I had in mind).

In any case, it seems clear that the answer is yes: in an infinite universe, different inflationary processes could have occurred in different regions.

If you allow me, within my limited knowledge, I would go a bit further and say that in an infinite universe, even before any inflation began, the universe already had an infinite volume. Therefore, it seems likely that there were more than just a single small homogeneous region. The Wikipedia article on cosmic inflation (theory) says: “The inflation solution starts with a tiny universe in thermal equilibrium, then expands…”

Of course, it may also be that the universe is not infinite but slightly curved and finite, in which case this discussion would largely disappear.

 

[javisot]

In any case, it seems clear that the answer is yes: in an infinite universe, different inflationary processes could have occurred in different regions.

Strictly speaking, this is not proven theoretically, much less experimentally.

 

[mister i]

Strictly speaking, this is not proven theoretically, much less experimentally.

Of course, I added “could”, but has cosmic inflation actually been experimentally confirmed?

 

[Jaime Rudas]

Peter's point is appropriate, since the "bubble universe" proposal is far less formalized than it seems.

Far less formalized in what sense? I ask because it doesn't seem to me that inflation with a multiverse is less formalized than, for example, inflation without a multiverse.

 

[javisot]

Far less formalized in what sense? I ask because it doesn't seem to me that inflation with a multiverse is less formalized than, for example, inflation without a multiverse.

The idea that hasn't been formalized is that if you have an infinite universe with eternal inflation, you "necessarily" have a level 2 multiverse. That "necessarily" isn't clear. The idea of bubble universes is on par with conformal cyclic cosmology, in that there's no specific formulation and many unanswered questions.

 

[javisot]

Of course, I added “could”, but has cosmic inflation actually been experimentally confirmed?

No, the fact that inflation fits the pattern doesn't mean it's experimentally confirmed. To confirm it experimentally, we would need other types of tests, for example, detecting B-modes in the polarization of the CMB

 

[Jaime Rudas]

The idea that hasn't been formalized is that if you have an infinite universe with eternal inflation, you "necessarily" have a level 2 multiverse. That "necessarily" isn't clear.

Do you have any references that support the idea that eternal inflation doesn't necessarily imply a multiverse?

 

[PeterDonis]

My question was: In an infinite universe, could different inflationary processes have occurred in different regions, each giving rise to its own local universe?

And the answer to that is, we don't know, because there is no consensus among the relevant scientific community, and we have no evidence either way.

That covers both possible meanings of your "could"--whether you mean "does the theory say this happens" or "do we have evidence that this happens".

 

[PeterDonis]

eternal inflation (which is somewhat beyond what I had in mind).

I'm not sure what you think the difference is between what you described and eternal inflation.

it seems clear that the answer is yes: in an infinite universe, different inflationary processes could have occurred in different regions.

No, it's not clear that the answer is yes--there are physicists on both sides of the question.

in an infinite universe, even before any inflation began

We don't know that there was such a time. In at least some eternal inflation models, there was never a time before inflation began.

the universe already had an infinite volume.

In any model with a spatially infinite universe, yes, it will have been spatially infinite at all times in the model.

Therefore, it seems likely that there were more than just a single small homogeneous region.

This doesn't follow. Indeed, our best current model of our universe does not have this property.

The Wikipedia article on cosmic inflation (theory) says: “The inflation solution starts with a tiny universe in thermal equilibrium, then expands…”

This is a good example of why Wikipedia is not a good reference to use directly. Note that the statement you give here contradicts what you said earlier, that a spatially infinite universe is always spatially infinite.

it may also be that the universe is not infinite but slightly curved and finite, in which case this discussion would largely disappear.

No, it wouldn't, because even if our universe is "slightly curved and finite", that doesn't rule out the possibility that it is one of an infinite number of "bubble universes" in an eternal inflation multiverse.

 

[PeterDonis]

has cosmic inflation actually been experimentally confirmed?

That's one of the things there isn't consensus on. Some physicists think the evidence we have confirms it, others disagree.

 

[javisot]

Do you have any references that support the idea that eternal inflation doesn't necessarily imply a multiverse?

For example, a reference from one of the greatest physicists, https://arxiv.org/abs/1707.07702 (S.W. Hawking)

"Based on this we conjecture that the exit from eternal inflation does not produce an infinite fractal-like multiverse, but is finite and reasonably smooth."

 

[Jaime Rudas]

For example, a reference from one of the greatest physicists, https://arxiv.org/abs/1707.07702 (S.W. Hawking)

"Based on this we conjecture that exit from eternal inflation does not produce an infinite fractal-like multiverse, but is finite and reasonably smooth."

Yes, it's true that Hawking conjectures that the exit from eternal inflation doesn't necessarily produce a multiverse, but I don't think that justifies claiming that the multiverse proposal is "much less formalized than it seems."