# Eternal inflation and its implications

Eternal inflation and its implications Alan H. Guth, 2007

A fairly accessible article which opens with a review of evidence (as of 2007) in support of inflation, and then goes on to speculate about its implications.

Since 2007, we only have more evidence in support of inflation. The Planck results fit the model of inflation with $\Lambda$ (figure 1. in the paper) more closely. And very recently, the BICEP2 results are very strong evidence of inflation.

Supporting evidence

• Scale of the universe
• Hubble expansion
• Homogeneity and isotropy
• (approximate) Flatness of the universe
• Absence of magnetic monopoles
• Anisotropy of the cosmic background radiation

## Terms defined in this paper

• Magnetic monopole – A particle predicted by all grand unified theories which is extremely massive and carrying a net magnetic charge.
• Youngness paradox – The rate of new expansion is so high that at any given time almost all the existing universes are very very young. Guth uses this to argue that we’re the only (first) “advanced” species in our universe (but he then states, “I find it more plausible that it is merely a symptom that the synchronous gauge probability distribution is not the right one”).

## Take aways

• Inflation implies eternal inflation
• Eternal inflation implies an infinite multiverse (did I understand this correctly?)
• Eternal inflation (and thus infinite multiverses) is a boon for anthropic reasoning (see above)
• Probability is not well-defined in an infinite and eternally expanding multiverse, as anything and everything is infinitely probable (a.k.a. anything that can happen will happen). This is an open research question. #openquestion
• Inflation (probably) does not extend infinitely into the past, a.k.a. our universe had a beginning. Technical section I did not follow (#unresolved), but the takeaway is that, “new physics (other than inflation) is needed to describe what happens at this boundary”. In other words, don’t extrapolate current physics to think the universe started as a singularity (See Matt Strassler’s post on this issue).