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I've been having a hard time understanding bubble universes in eternal inflation. So they are just finite regions of space where inflation has stopped and Hubble expansion has taken over? I just can't understand how a finite universe can work with out current understanding of Cosmology without curvature, which I don't think eternal inflation deals with. Is the multiverse itself flat? Am I missing something?

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  • $\begingroup$ You mean in a continuum formulation? $\endgroup$ – jjack Dec 24 '17 at 21:39
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    $\begingroup$ In classic inflation, the inflaton field moves downhill. In eternal inflation, the idea is that quantum fluctuations can drive the inflaton field "uphill" prolonging inflation in some areas. These would be your "bubble regions". Inflation stops in a region when it thermalizes. So you get domains of inflation continuing and domains where it stops. The problem is that eternal inflation itself is not a well-defined mechanism, and has many issues, see for example: arxiv.org/abs/arXiv:1408.2249 and arxiv.org/abs/1508.02670 (excuse the shameless self-promotion!) $\endgroup$ – Dr. Ikjyot Singh Kohli Dec 24 '17 at 21:51
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The bubbles do have edges, because bubble collisions have been sought after (thus far unsuccessfully, I believe) as evidence of field-based inflation, and the collision of objects without any sort of edge would be indeterminate. The terminology varies with the model: Guth, who's usually credited with originating inflationary cosmology, tends to refer to its spatially or spatial-temporally "local" universes as "pocket universes", whereas Vilenkin often refers to them as "bubble universes". (Guth is very favorable to the "eternal inflation" idea sketched by Dr. Kohli, and may favor the term "pocket universes" for visualization, as it's more suggestive of a universe open to the future, which is usually toward the top of the spacetime diagrams originated by Einstein's teacher and colleague, Minkowski.)

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  • $\begingroup$ I've got to add that inflation definitely assumes that spacetime is curved: It is, so far, entirely a relativistic theory. Inflationary theories take credit for making space as "flat" as it appears to us to be, but you have to understand that time is curved about 300,000 times as much! $\endgroup$ – Edouard Jan 24 '18 at 5:25

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