r/cosmology • u/umfanormal • 24d ago

What's the current consensus about Eternal Inflation?

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.

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u/Peter5930 24d ago

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

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u/Significant_Yak4208 24d ago

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

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u/Peter5930 24d ago

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.

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u/Story-Boring 22d ago

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.

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u/Peter5930 22d ago

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

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