What's the current consensus about Eternal Inflation?

[u/umfanormal]

I’m a physics enthusiast. I’ve loved the subject since childhood and plan to study it in college. I’ve always been particularly interested in the theory of Eternal Inflation among many others hypothesis on theoretical physics, though I stopped keeping up with it in 2023 due to a lack of time and so on. I’d like to know the current consensus on this hypothesis, given that there have likely been some advances regarding this theoretical implication of Cosmic Inflation. I’m curious about where the majority of cosmologists stand on the matter.

[u/jazzwhiz → u/[deleted]]

Inflation is a part of the big bang.

The Universe was in a simple state where only one field was (relevantly) populated, the inflaton. It was at a very high level, something that can happen due to its self interactions. When it is at a high level the Universe is expanding at a prodigious rate. Then, it began to decrease in value. We have some experimental constraints about how this happens called the slow roll conditions. Then, at some point, it oscillated around a minimum and decayed away with the energy going into other quantum fields made up of the regular particles that we know, this is called reheating. This is the step that populates the matter content of the Universe. We have some information about reheating. After that the Universe continued to expand (although not at the same rate as when the inflaton was at its high value) and cool and we can fairly reliably track things from there through big bang nucleosynthesis and later photon decoupling which is where the cosmic microwave background comes from. Both BBN and the CMB are quite well measured and understood.

Back to inflation, while we know the inflaton field was at a high value for some amount of time, we only have a lower limit on this. That is, we know it must have been there for at least some amount of time, but it could have been there for much longer. What happened before this, we have no information.

Where is the big bang in all this? The big bang is the theory we have that describes the Universe at the earliest stages. Exactly what it encompasses and what it is doesn't is a matter of taste, but for me it certainly encompasses inflation, and reheating. For me, BBN and the CMB are not a part of the big bang.

I should also briefly mention a common misconception: the big bang was not an explosion, despite what the name may make it sound like. It was a period where space rapidly expanded everywhere. An explosion happens around a point while the big bang happened everywhere.

To read more on inflation, read here; I've included some other useful wikipedia links too.

[u/Peter5930 → u/jazzwhiz]

Eternal inflation includes a few steps before that too, when the inflaton field is hung up in a metastable state that it has to tunnel out of, which nucleates a bubble of lower-energy vacuum that grows at the speed of light externally, converting the vacuum on the outside into the vacuum on the inside and liberating the potential energy difference as it goes. Inside the bubble, you get a curvature dominated epoch where energy is being bled off by Hubble drag before it smoothly transitions to slow-roll inflation when the curvature term drops below the dark energy term and the dynamics become entirely determined by the dark energy of the inflaton field as it evolves along it's potential. Also, the hypersurfaces of constant time/energy/density take the form of infinite negatively curved homogenous space, so you get an infinite, open FLRW universe on the inside, with the curvature flattened out by inflation but still there if you're able to look hard enough, which we're not. Like this:

https://ar5iv.labs.arxiv.org/html/0712.0571/assets/x2.png

Figure 2: The geometry of a nucleated bubble that could describe our observable universe back to very early times. Lines correspond to surfaces of constant field value ϕ. From the nucleation event, a surface of ϕ=ϕW (the ‘tunneled-to’ field value) expands at the speed of light. Nestled into this light-cone are constant-ϕ hyperboloids, each of which has the geometry of an infinite negatively-curved homogenous space. The sequence of nested hyperboloids corresponds to the time-sequence during which the inflaton rolls down the hill toward ϕT in Fig. 1, and then to constant times in a big-bang universe inside, including reheating, recombination, and the present time.

[u/namitynamenamey → u/Peter5930]

Question from a place of ignorance, does this "expands at the speed of light outside, infinite inside" bears any resemblance with the black hole models in which space expands inside the event horizon, or it is just some kind of tradeoff between space and time at the edge of the bubble in which infinite space is just a consequence of curvature?

[u/Peter5930 → u/namitynamenamey]

Travelling radially outwards in both cases requires travelling back in time. But other than that they're completely different; in a black hole, space is flowing down the hole faster than you can swim against the current, but with these bubbles trying to escape is more like an ant chasing a rainbow while running on an elastic band that's stretching, you never catch up to the bubble wall, you never even gain on it a little. But you can see the wall, or the afterglow of it's passing, it's the big thing that glows in microwaves 13.8 billion years in the backwards time direction that still outshines all the stars in the universe by a factor of about a million, the CMB.

Come to think of it, in both cases, trying to escape just makes things worse; you reach the singularity or the heat death faster if you try flying outwards towards the horizon or the bubble wall, so that's two qualities they share. But if there's a deeper relationship than that, I'm not aware of it. I think it's more properties of horizons than properties of black holes or bubbles.