Bubble universes and physical constants

[JuneSpring25]

TL;DR

A question about bubble universes and physical constants

Hello, couple of questions about bubble universes and the physical constants. I understand in an eternal inflation scenario universes bubble off the original during the inflation phase. Firstly, according to this theory, does the new universe creation only happen at an early inflation phase of a universe - before the phase we are in within our current observable universe (or bubble)?

Secondly, I understand that the physical constants would be different in different bubbles. Would the Planck units be different or would these be the same since the universes begin in the same space time?

Also if the eternal inflation theory continually creates new universes from one universe, why don't we eventually run out of matter for new bubble universes?

Thanks for any answers!

 

[mathman]

Answers to your questions are educated guesswork.

 

[PeterDonis]

according to this theory, does the new universe creation only happen at an early inflation phase of a universe

If you mean, could our universe bubble off another one according to this theory, no. New universe creation in eternal inflation models (at least as I understand it) only happens in the inflating portion of the spacetime; once a new universe is "bubbled off", it is no longer inflating.

I understand that the physical constants would be different in different bubbles.

I think this is a speculative property; it doesn't necessarily have to be the case in an eternal inflation model, although many such models seem to include it.

if the eternal inflation theory continually creates new universes from one universe

It doesn't, at least not the way you are using the term "universe", namely, to mean one of the "bubbles" after it gets formed, like our universe. The eternally inflating region of the overall spacetime is not a "universe" in this sense.

why don't we eventually run out of matter for new bubble universes?

The matter that is in a universe like ours does not come from the eternally inflating region as matter. Part of the process of a new universe "bubbling off" from the eternally inflating region is that "matter" and "radiation" (the quantum fields in our Standard Model of particle physics, quarks, leptons, gauge bosons) are "created" from energy that is transferred from the inflaton field (the field that drives eternal inflation). The eternally inflating region contains no matter or radiation; all of its energy density is stored in the inflaton field.

 

[PeterDonis]

Answers to your questions are educated guesswork.

In terms of what eternal inflation models say, no, most of them are not: the models give well-defined answers to most of them. Whether those models will actually turn out to be correct is a different question.

 

[JuneSpring25]

Thanks, I appreciate the answers, that's really helpful. Would the Planck constants be different in these different bubbles? Do the Planck constants exist at the early inflation stage?

 

[PeterDonis]

Would the Planck constants be different in these different bubbles?

Depends on the model.

Do the Planck constants exist at the early inflation stage?

If you mean, is there anything corresponding to "Planck's constant" in the eternally inflating region, again that depends on the model.

 

[LastScattered1090]

Also if the eternal inflation theory continually creates new universes from one universe, why don't we eventually run out of matter for new bubble universes?

The matter that is in a universe like ours does not come from the eternally inflating region as matter. Part of the process of a new universe "bubbling off" from the eternally inflating region is that "matter" and "radiation" (the quantum fields in our Standard Model of particle physics, quarks, leptons, gauge bosons) are "created" from energy that is transferred from the inflaton field (the field that drives eternal inflation). The eternally inflating region contains no matter or radiation; all of its energy density is stored in the inflaton field.

@PeterDonis I don't know much about eternal inflation, but if it's anything like slow-roll, then isn't the energy density of the inflaton field roughly constant (only very slowly changing) with time? If so, then couldn't the OP's question be reformulated as something like, "why is the total energy associated with the scalar field able to increase without bound, as space expands in the eternally-inflating region?" I realize the answer could be just, "because that's how physics works, according to these models." But I thought I'd ask if this was another valid way of looking at it.

 

[PeterDonis]

isn't the energy density of the inflaton field roughly constant (only very slowly changing) with time?

In the region of spacetime that is inflating (which in eternal inflation models is everywhere that isn't inside a "bubble" that has turned into an ordinary universe like ours), yes. But when a "bubble" forms (a region that turns into an ordinary universe like ours), it is because the energy density of the inflaton field decreases rapidly, and that energy density gets transferred to (at least in our universe) the Standard Model fields. Specific inflation models differ on exactly what causes that transition.

couldn't the OP's question be reformulated as something like, "why is the total energy associated with the scalar field able to increase without bound, as space expands in the eternally-inflating region?"

The "why don't we eventually run out of matter" part could, I suppose. But the answer is still basically the one I gave in post #3. The short version is that there is no global "conservation of energy" in GR, and in particular there isn't in the spacetimes being considered in inflation models.

 

[LastScattered1090]

In the region of spacetime that is inflating (which in eternal inflation models is everywhere that isn't inside a "bubble" that has turned into an ordinary universe like ours), yes. But when a "bubble" forms (a region that turns into an ordinary universe like ours), it is because the energy density of the inflaton field decreases rapidly, and that energy density gets transferred to (at least in our universe) the Standard Model fields. Specific inflation models differ on exactly what causes that transition.The "why don't we eventually run out of matter" part could, I suppose. But the answer is still basically the one I gave in post #3. The short version is that there is no global "conservation of energy" in GR, and in particular there isn't in the spacetimes being considered in inflation models.

That's more or less what I expected but helpful to hear, thanks. Thanks also for the clarification about what happens to the energy density of the inflaton field as part of "reheating."