I Do "bubble universes" in eternal inflation have their own spacetime?

[Suekdccia]

TL;DR Summary

In the context of eternal inflation models, can "bubble universes" be described by their own metric, different from the one at the "background"?

In the context of the model of eternal inflation, if an inflating "pocket universe" disconnects from an the background spacetime, does it mean that the baby universe itself can have its own spacetime?

can they be described by a different spacetime metric than the background?

if the original spacetime was asymptotically flat, can the new "universes" be described by an asymptptically flat spacetime as well? (Ref: https://arxiv.org/abs/0706.1233)

 

[Vanadium 50]

How could you tell experimentally? And if you can't, how is this science?

 

[Suekdccia]

How could you tell experimentally? And if you can't, how is this science?

Good question, however this should be asked to people working in these models in theoretical physics. I'm just an amateur asking questions!

 

[Vanadium 50]

If we're the wrong people to be asking, why are you asking us?

 

[Suekdccia]

If we're the wrong people to be asking, why are you asking us?

I have a specific question about the theoretical framework of eternal inflation. Indeed a different and important question would be how could this be verified experimentally, but that was not my inquiry. I was asking that question in this forum hoping to find someone working in cosmological inflation. If you are not one of those, then you might indeed be the wrong person to answer it

 

[Demystifier]

How could you tell experimentally? And if you can't, how is this science?

The question by OP is to be understood as a question about mathematics of a physical model. Mathematical questions, of course, cannot be answered experimentally, yet such a question is not only a question for mathematicians, but for theoretical physicists as well.

The answer to the question by OP is yes, the disconnected universe in these models is disconnected in the topological sense, so its metric has nothing to do with the metric of the parent universe.

 

[martinbn]

The answer to the question by OP is yes, the disconnected universe in these models is disconnected in the topological sense, so its metric has nothing to do with the metric of the parent universe.

Hm, are you sure?

 

[Vanadium 50]

You can argue it's not physics, it's math - and therefore not science.

I have my own theory of gravity - it agrees with GR in all respects save one: inside a black hole there are herds of prancing invisible pink unicorns, Is this science? If not, why not, and how does this differ from the proposed model?

 

[DaveC426913]

I think what the OP is hoping for is some plausible inference we might be able to make from what we do know.

For example, do we think that time existed before the BB? If not, when did the arrow of time start? If it started after the supposed conditions required for bubbbe universes, then theres no reason they would have compatible times. If time came first, then we might infer that they inherited the same time.

I realize this is clumsy and non-technical; it is only half-formed, and I don't have a lot of time to polish it.

 

[Demystifier]

I have my own theory of gravity - it agrees with GR in all respects save one: inside a black hole there are herds of prancing invisible pink unicorns, Is this science? If not, why not, and how does this differ from the proposed model?

The difference is that pink unicorns are ad hoc, while parallel universes are obtained by a natural extrapolation of a theory tested in one regime into another regime where it is not (and perhaps cannot) be tested.

 

[martinbn]

What is meant by "the inflating pocket universe disconects from the background spacetime"? Can someone (the OP) provide a reference for that?

 

[martinbn]

The difference is that pink unicorns are ad hoc, while parallel universes are obtained by a natural extrapolation of a theory tested in one regime into another regime where it is not (and perhaps cannot) be tested.

But GR doesnt have a background metric or a background spacetime. It is not a matter of a natural extrapolation. It is an unclear use of non standard terminology.

 

[Demystifier]

But GR doesnt have a background metric or a background spacetime. It is not a matter of a natural extrapolation. It is an unclear use of non standard terminology.

I was talking about topologically disconnected spacetimes, not about background metric. Two topologically disconnected universes, each with its own metric, can be viewed as one solution of the Einstein equations.

 

[martinbn]

I was talking about topologically disconnected spacetimes, not about background metric. Two topologically disconnected universes, each with its own metric, can be viewed as one solution of the Einstein equations.

You said that the OP question falls into extrapolations of well established theories. But he talks about background metrics and spacetimes, so it is not just an extrapolation on his part. He uses non-standard and unclear terminology. He needs to clarify.

Why do you think that in the eternal inflation case you will have topologically disconnected spacetimes?

 

[Vanadium 50]

The difference is that pink unicorns are ad hoc,

Like Old Quantum Theory?

That's not a scientific argument of the truth or falsehood of a proposition, It lies somewhere between philosophy and taste.

 

[Demystifier]

You said that the OP question falls into extrapolations of well established theories. But he talks about background metrics and spacetimes, so it is not just an extrapolation on his part. He uses non-standard and unclear terminology. He needs to clarify.

I agree.

Why do you think that in the eternal inflation case you will have topologically disconnected spacetimes?

In general you will not. But the paper (posted by OP) considers the baby universe scenario which, in my understanding, means topologically disconnected spacetime. See Fig.1d in the paper.

 

[Demystifier]

That's not a scientific argument of the truth or falsehood of a proposition, It lies somewhere between philosophy and taste.

This applies to this very sentence as well. Whenever you say something like "this is science and that is not", such a statement is not itself scientific. There is no strict demarcation between science and non-science, and any attempt to define such a demarcation is non-scientific by its own definition. If you think it's obvious what is and what isn't science, see also https://en.wikipedia.org/wiki/Demarcation_problem

 

[mitchell porter]

if the original spacetime was asymptotically flat, can the new "universes" be described by an asymptptically flat spacetime as well?

You might suppose that a baby universe which formed by pinching off a finite volume from the parent universe, would necessarily be compact rather than asymptotically anything (since the latter implies infinite spacelike extent).

However, if you look at figure 2 (page 5) of https://arxiv.org/abs/0712.0571, you will see how an expanding compact region could have an alternative coordinatization that is spatially infinite. In the (x,t) coordinates at the lower left, the nucleated bubble appears as a finite line segment in the x direction, that is expanding to the left and the right. (Try to bear in mind that the single x coordinate is schematic, and this "line segment" really stands for a volume of three spatial dimensions.)

But then consider the coordinate system in which the horizontal hyperbolas in the upper half of the diagram, describe equal-time surfaces for a different time coordinate, "tau". In this coordinatization, the nucleated bubble is described as a set of spacelike hyperboloids of infinite extent. The hyperboloid starts out sharply kinked (approximating the V shape) but gets smoother and flatter with time.

It's a neat conception but I'm not sure how robust it is. The "V" is not just a mathematical light-cone, it's supposed to be a domain wall separating the inflationary and non-inflationary domains, and presumably it has a finite thickness and other physical features, which might mean that the (x,tau) coordinate system breaks down when you're close to the domain wall.

Also, the picture seems to me a product of semiclassical thinking, in which a unique quantum tunneling event occurs at the base of the V, but otherwise the two domains are modelled classically (i.e. by unquantized GR), whereas a more consistent or fundamental picture would model the whole diagram quantum mechanically. But maybe no one knows how to do that rigorously, they just have handwaving as to how different tunneling events would correspond to different, decoherent histories in some quantum histories formalism?

 

[martinbn]

In general you will not. But the paper (posted by OP) considers the baby universe scenario which, in my understanding, means topologically disconnected spacetime. See Fig.1d in the paper.

I don't think that is what thy mean. It is hard to tell because they don't give much detail nor justification. But I think they mean causally disconnected regions not topologically disconnected.

 

[Vanadium 50]

Its science if it can be tested experimentally. Seems straightforward.

 

[Demystifier]

Its science if it can be tested experimentally. Seems straightforward.

Is theoretical analysis of the black hole interior behind the horizon - science?

 

[Demystifier]

I don't think that is what thy mean. It is hard to tell because they don't give much detail nor justification. But I think they mean causally disconnected regions not topologically disconnected.

The paper, indeed, does not explain, but I have seen other papers where baby universe meant topological disconnection. But it refers to 3-dimensional spaces, not 4-dimensional spacetimes. The 4-dimensional spacetime is of course connected, in the past before the baby universe splits from the parent.

 

[PeterDonis]

if the original spacetime was asymptotically flat

No spacetime in any inflation model is asymptotically flat.

 

[Demystifier]

Its science if it can be tested experimentally. Seems straightforward.

Would you say that any claim that can be tested experimentally is a scientific claim? For example, how about the claim that pigs can fly, it can be tested experimentally, but is that claim scientific?

 

[Hornbein]

Would you say that any claim that can be tested experimentally is a scientific claim? For example, how about the claim that pigs can fly, it can be tested experimentally, but is that claim scientific?

Hmm, this is off topic but I can't resist. I would say the proposition "pigs can fly" would be answered by a demand to demonstrate an instance of such. Should this prove successful then one could say it was a scientific claim. The proposition "pigs can't fly" is not falsifiable by any experiment so this is not a scientific claim. One might provide compelling arguments against this but that's another matter.