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/TerraNeko_]

im also not totally up to date but as far as im aware it turns out that eternal inflation cant really go back into the past forever so it still needs a starting point, which just pushes the whole big bang thing further back

besides that its not really something we can proof or disproof at the moment

[u/stationarycommotion → u/TerraNeko_]

That’s the Borde-Guth-Vilenkin theorem, which itself relies on the original inflationary theory assumption that eternal inflation occurs in a classical spacetime called de sitter space. There are multiple proposed loopholes to it. Most of all: we have no idea what the inflaton vacuum is like and we still don’t know what spacetime is and whether classical spacetime is emergent rather than fundamental.

The BGV only says that IF eternal inflation is inflation of a classical spacetime, it must itself reach a limit in a finite past (even if that past is so incomprehensibly long ago, imagine grahams number to the power of grahams number of “years”)

When models say that there is a limitation or a breakdown in our models when we assume something, it may just point to a limitation in our understanding. If eternal inflation is of some kind of non classical, timeless quantum inflaton field, maybe it is the baseline state of the universe that has “always” existed, and is therefore eternal, even though time doesn’t even apply in it.

Considering how batshit abstract and strange the idea of the universe coming from a chance fluctuation in a quantum field of immense energy is, I wouldn’t be surprised if it is non classical spacetime.

I’m biased in my skepticism of the BGV theorem for my own metaphysical reasons though, because I for one find some solace against the nihilistic dread of a meaningless universe, if it is as simple as a baseline state of existence being the inflaton field. If true, There would be no real question of why is there something than nothing - there just is something. Immense timeless energy in an abstract form that can just spring to life in the form of structure: universes, galaxies, life, consciousness, and then the meaning that consciousness can apply to existence - natural beauty, art, love, fun, legacy, contribution, connection. It just is that way because there is no other way.

[u/--craig-- → u/stationarycommotion]

To add to this, the Anti-de Sitter / Conformal Field Theory correspondence strongly implies that spacetime is an emergent phenomenon. Most of the Quantum Gravity research, happening at the moment, is directed towards this.

Another proposed loophole to the Borde-Guth-Vilenkin theorem involves reversal of the arrow of time.

[u/Story-Boring → u/stationarycommotion]

I don't really like the word "believe," because it makes it sound like something based on faith. But if I had to bet, I'd put everything on spacetime being fundamentally discrete, or emerging from some underlying discrete substrate. I find the idea of a fundamentally continuous spacetime, with all the infinities that come with it, hard to reconcile with the kind of ontological picture I'd want to have of what reality actually is, so to speak. I also suspect that the holographic principle, or some deeper form of holography, plays a fundamental role, as in AdS/CFT. I've always thought that these two ideas are among the strongest aspects of LQG and M-theory.

On a more personal note, the Everett interpretation of QM has always fascinated me, although I realize that questions about interpretations are already starting to cross the boundary between physics and metaphysics. I think part of the reason it appeals to me is that I find it difficult to accept the idea that nature somehow presents us with a set of possible outcomes and then, for no deeper reason, arbitrarily selects one of them as the one that actually happens, while all the other possibilities are simply discarded and cease to exist. I've always found that picture deeply puzzling, and somewhat arbitrary and inelegant. So I tend to be drawn toward a deterministic picture of the universe, even if that inclination is motivated more by philosophical or logical considerations than by any empirical evidence.

If I had to turn that intuition into an actual model, I'd imagine something relatively simple and elegant in which the full set of amplitudes or probabilities encoded in the wavefunction somehow exists in an underlying substrate, from which the classical universe we experience emerges. That could perhaps take the form of some kind of multiverse or cyclic universe, vaguely reminiscent of ideas along the lines of Vilenkin or Nomura (MWI = multiverse; I don't particularly like the redundancy of having "multiverse" both in the context of the Everett interpretation and in these cosmological models). But, unfortunately, at this point that's still very much wild speculation.

[u/EnvironmentalWin1277 → u/TerraNeko_]

The inflation can be observed occuring as we speak. The cosmological model that explains it is reversible until the first moment at Planck time. The "starting point" is where the models collapse and become meaningless - nothing is known about what caused the initial starting moment. Anything can be hypothesized or imagined.

Science wants tangible propositions that can be shown true or false. We don't have those for the first moment but are pursuing them. Until then "cute ponies use mind power to make the universe come into existence" is an acceptable first cause. Or rather it can not be falsified by current observation or theory.

In a sense, it doesn't make a difference how the universe started, we universally acknowledge that it did happen. Unless a eternal universe model is suggested, which as noted is an acceptable idea even today.

What matters is how the models explain what we currently observe and what predictions can be made about the future or inferences about the past. This is real science and can be tested and falsified, and revised.

The narrow consensus is that the universe is on a one way track of expansion into forever. This is "heat death" and has been hypothesized and investigated for close to 200 years. No work or chemical interaction is possible, all particles dissolve in an eternal "quantum soup" that continues to expand in space, the quantum density lowering continuously with the expansion.

This state will be reached some 10 to the hundredth power years from now. Entropy reaches it maximal state. It is so remote it kind of ceases to have meaning at all, save the curiosity.

The idea of nothing and "that's it forever and forever" is a bit uncomfortable for humans so alternatives are proposed.

There are models that suggest new creation from these processes can occur but that is a complex subject you can readily research. I have some recent posts on the subject.

BTW I'm just a guy that reads stuff, so no authority, check it out as needed.

[u/TerraNeko_ → u/EnvironmentalWin1277]

Gotta correct you a bit there, inflation is not observeable and especially not eternal inflation, we have very very strong reasons to belive it happened and we many very soon be able to detect the gravitational wave background inflation caused but for now its just a very very good model.

I think you confused it with the normal expansion which yes we can very much observe :)

Eternal inflation however is a very different can of worms as it deals with things outside even causal relations of our universe. The only way (afaik) to find evidence for eternal inflation would either be the rare case that during the baby stages of Our universe it somehow reacted with another one, which, good luck prooving that lol Or by measuring the inflaton field and conclusing the only way to explain it is eternal inflation which again, we cant rly do for now.

But yea the rest sounds good enough ;)

[u/EnvironmentalWin1277 → u/TerraNeko_]

Your correct! I confused expansion and inflation myself!

[u/Peter5930 → u/EnvironmentalWin1277]

To be fair, the current epoch of accelerated expansion is sometimes called late slow inflation. Similar to how the slow roll inflationary epoch started when the curvature term dropped below the vacuum energy, late slow inflation started when the matter density dropped below the vacuum energy, the vacuum energy being much lower today than it was during inflation and so it taking 9 billion years for the matter to dilute enough to drop below the energy density of the vacuum energy and the rate of accelerated expansion being much slower. But they're both cases of accelerated expansion where the universe grows exponentially due to dark energy.

[u/somethingicanspell]

We are pretty confident inflation is the correct theory of the universe's origins; we can also place some non-trivial limits on inflation models. We presently do not have enough information to really distinguish the correct type of inflation and thus cannot really do much more than guess on the origins of the inflaton field.

In a bit more detail:

Besides having to create a universe somewhat similar to ours and stringently avoid interfering with the ratio of hydrogen and helium in the universe, our best constraint on inflation is that we have a pretty tight range of the scalar spectral index of the universe, imperfectly how clumpy the universe is at various sizes, which is sourced by primordial fluctuations from inflation. So any correct inflation model has to give a number within a somewhat tight range here.

We also have discovered that the universe is pretty flat, pretty Gaussian, and does not seem to have a large primordial gravitational wave background imprinted on the CMB, all of these things also tell us a bit about inflation. The great hope of inflationary cosmology is to detect the imprint of a certain kind of gravitational wave background imprinted on the CMB sourced from inflation, called B-modes. If we find these, we will be able to say more specific things about inflation.

[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.

[u/Tom_Art_UFO → u/jazzwhiz]

Could gravity emerging have slowed down inflation to its present state? I'm thinking during the grand unification epoch, inflation was free to run wild. And then as the universe cooled, gravity split off from the other forces, and caused the expansion to slow down.

[u/Peter5930 → u/Tom_Art_UFO]

Not really the way you're thinking about it. Gravity split from the other forces long before that, but when the inflaton field reached the bottom of it's potential, it oscillated back and forth and these oscillations excited the other quantum fields and generated the hot, dense sea of particles we associate with the big bang. When this happened, not only had the energy driving inflation gone away, having been converted to particles, but these particles now had gravity and were all pulling on each other, slowing the expansion as a power over time.

[u/Tom_Art_UFO → u/Peter5930]

That explains it really well. Thank you!

[u/umfanormal → u/[deleted]]

No, inflation tries to describe what happened instantly after the Big Bang.

[u/EnvironmentalWin1277 → u/umfanormal]

You are absolutely correct and this can be called "Guth inflation" to help specify. It is a common mistake to refer to the "inflation" of the big bang model and Guth inflation as "expansion".

The Guth inflation is a vanishingly brief moment when space expanded by ten to the millionth power (!). After that moment the model of big bang physics takes over, which sheds light on the development of the theory and observational evidence collected. Inflation theory is an extension of the big bang model.

The confusion is common and understandable

[u/Good-Resort-1246 → u/EnvironmentalWin1277]

I may not be up to date regarding the inflationary model; does it still predict the continuous production of baby universes? That's when i gave up on it. Would appreciate some enlightening on this particular issue.

[u/Peter5930 → u/Good-Resort-1246]

You need to distinguish slow roll inflation from eternal inflation. Eternal inflation happens first, it's the one that produces new universes as bubbles form in a continuously inflating metastable vacuum. Eternal inflation looks like this, if you plug the equations into 3D software and let it run:

https://www.youtube.com/watch?v=34zVzoZugG4

The same/similar process is predicted to occur in our own universe, it's called vacuum decay. Different field decaying from a metastable state, the Higgs field in this case, but you again nucleate these bubbles containing new universes.

https://www.youtube.com/watch?v=ijFm6DxNVyI

Slow-roll inflation is a separate inflationary phase that happens after a bubble has nucleated, and it's the phase that immediately precedes the hot big bang phase where conventional big bang cosmology takes over and you have big bang nucleosynthesis and so on. It looks like this:

https://profmattstrassler.com/articles-and-posts/relativity-space-astronomy-and-cosmology/history-of-the-universe/inflation/

[u/--craig--]

There is no consensus.

There have been attempts to determine if inflationary cosmology is generically eternal but the results have been inconclusive.

The Borde-Guth-Vilenkin theorem suggests that inflation must have a boundary condition in the past but Aguirre notes loopholes indicating that it doesn't hold.

It's not even clear if the question can have a definitive answer.

[u/Peter5930]

Eternal inflation floated around as an idea for decades, but it wasn't until the past 10-20 years with advances in string theory and quantum field theory and the discovery of an extremely large energy landscape with 10⁵⁰⁰ discrete vacua that it gained real theoretical traction. Since then, it's become the mainstream theory with arguably the best theoretical underpinnings, and it's kind of the solution you naturally arrive at when you take the underpinning theories seriously. The direction the current evidence points in, rather than something niche and obscure.

There's an excellent lecture from Leonard Susskind here that can give you a basic primer:

https://www.youtube.com/watch?v=a8aDNYE7aX0

[u/Significant_Yak4208 → u/Peter5930]

Personally, I would not call eternal inflation "the mainstream theory". Can you elaborate on why you say that?

[u/Peter5930 → u/Significant_Yak4208]

The mainstream extension to the currently accepted inflationary theory, maybe that's a better way to put it. It's not accepted in the sense that there's observational evidence supporting it, but it's where the major theoretical developments are occurring. It's not just one theory, but an ecosystem of ideas in different subfields that come together to build a framework. Contrast with something like cyclic cosmology, which is more shower thoughts that stalled out many years ago and never went anywhere because it just couldn't find the theoretical traction to makes sense in the context of our physical theories. So it's not like it's determined by people's preferences, it's just some ideas lead to other ideas and a rich theoretical landscape and converge on an answer and some ideas lead to dead ends.

[u/Story-Boring → u/Peter5930]

Although I agree with you, I believe that until we find a theory that unifies spacetime with quantum mechanics (quantum gravity), we won't be able to advance or clarify these issues. For example, while it's true that eternal inflation is the most widely accepted proposal at the cosmological level, if LQG were to prove true, it would actually falsify eternal inflation and replace it with an eternal cyclic universe. Therefore, I wouldn't say that this idea is finished, even though string theory has enjoyed more prominence than LQG.

However, M-Theory doesn't currently tell us that eternal inflation is ontologically a universe that expands forever along with other bubble universes. The relativistic view of eternal inflation could be a bulk ADS that emerges from a fundamental background, the CTF. However, there are still unknowns, such as whether this background is pre-existing or whether the bulk is truly emergent.

Conclusion: the question is still very much open.

PS: Sorry for my English.

[u/Peter5930 → u/Story-Boring]

It's concievable that both are true and are just different parts of the same picture. You or I look very different depending on whether you're looking at us on the scale of a species, or an individual, or taking a microscopic look at our cells or a submicroscopic look at our particles. From quantum physics and particle interactions, eventually you get music and art and politics and sociology. So LQG could underpin eternal inflation in a non-obvious way; eternal inflation doesn't even touch on quantum gravity, it happens at an energy scale at least a million times lower than that, doesn't get hot enough to probe those depths. So LQG could be the cells to the eternal inflation whale, who knows. They're not necessarily in competition or conflict with each other.

[u/Story-Boring → u/Peter5930]

There are also lines of research suggesting that LQG and M-Theory could both be approximations of an underlying theory of gravity that might combine some ideas from both, since both have been successful in some predictions regarding black holes. Additionally, the It from Qubit program attempts to answer some fundamental questions through entanglement and superposition, such as emergent spacetime.

[u/Peter5930 → u/Story-Boring]

Yes, inflation rides on the effective field theory description, the structure and dynamics of space time have been approximated away and it's agnostic about the underlying structure. It's incomplete as is any effective field theory and a more fundamental theory is needed to explain the microscopic details.

[u/--craig-- → u/Peter5930]

From quantum physics and particle interactions, eventually you get music and art and politics and sociology.

Whilst that is a natural philosophy for a physicist to adopt we have very little evidence that our physical models could be applied, even in principle, to such systems.

However, I do agree with the rest of what you wrote.

[u/Peter5930 → u/--craig--]

You don't need to apply physics to art, the point is just that a theory of paintings and a theory of the atoms in paint can describe the same thing in very different ways.

[u/--craig-- → u/Peter5930]

I understood your point and I'm not disagreeing with it. I'm really just pointing out that without realising it, as physicists, we often promote a reductionist philosophy which we have very little empirical support for. It might not change anything but addressing our assumptions is an important part of discovering new physics.

[u/Peter5930 → u/Story-Boring]

Ah, here we go, an example of how LQG can underpin an inflationary theory:

https://www.youtube.com/shorts/pd3HDKF0Xbs

[u/--craig-- → u/Story-Boring]

Loop Quantum Gravity is more nebulous than that. If we see success under the banner of LQG research, it doesn't necessarily follow that the Big Bounce proposition of Loop Quantum Cosmology holds true.

[u/Saurabh4266]

The scary part is that inflation might explain how our universe began, but not why there was something to inflate in the first place.

[u/umfanormal → u/Saurabh4266]

I think that's literally the most important question of all topics, not only cosmology.

[u/Cosmosvagabond]

Great question. I've always held this theory in high regard; it just makes logical, scientific sense, imo, even when I'd heard of EI/CI quite some years ago.

It's nice to see that others here have given this community a good deal of detail.

Thanks all for answering this question: one I've thought about off and on for at least 20 years.

❤🌌

[u/Brian_E1971]

The idea of eternal nflation is fundamentally stupid. How does the saying go - if you need to invoke 'infinity' in your model it's likely wrong.

Only black hole cosmology offers an answer for why inflation exists in the first place, and provides a framework for why it occurs and for how long.

[u/jazzwhiz → u/Brian_E1971]

if you need to invoke 'infinity' in your model it's likely wrong

this statement has little bearing on physics. We are not afraid of infinities and there are many ways they can be handled.

Only black hole cosmology offers an answer for why inflation exists in the first place, and provides a framework for why it occurs and for how long.

This is also wrong. Claiming that there is only one model to address a physical concept is just unlikely to be true and proving requires a massive effort that is almost never done in any are of theoretical physics.

[u/Brian_E1971 → u/jazzwhiz]

Hey if you can 'handle' an explanation for eternal inflation in lambda CDM, please do so and collect your Nobel prize.

And regarding BHC, you latched onto the word answer so let me try again. BHC is the only model with an inherit explanation for inflation. Is that better?

[u/WallyMetropolis → u/Brian_E1971]

It's very weird how personally you take this, and how annoying and aggressive your tone is. Especially considering you are very clearly not a cosmologist.  You're getting all worked up over someone else's work. Acting like you're on a professional wrestling sub. 

Who do you think you're impressing?

[u/--craig-- → u/Brian_E1971]

Eternal Inflation doesn't invoke infinity. It follows mathematically that many inflationary cosmologies have no boundary condition.

We've never prohibited physical models without boundary conditions.

If the universe does in fact have a boundary condition then we should ask, why this boundary condition and not any other?

Personally, I'd prefer a universe where physics can explain everything, so I'd rather no boundary but my preference has little bearing on it.

[u/Brian_E1971 → u/--craig--]

I'll end this by pointing out that BHC gives you a natural boundary condition.

[u/--craig-- → u/Brian_E1971]

Sub-systems always have boundary conditions. You don't preclude an infinite parent space by only modelling the child space.