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 (figure 1. in the paper) more closely. And very recently, the BICEP2 results are very strong evidence of inflation.
- 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 used in the paper
- “Standard” FRW cosmology – FRW stands for “Friedman, Robertson, Walker” (and sometimes FLRW with Lemaitre), describes a homogenous and isotropic universe. I think “standard” implies not expanding (contracting) in this case.
- type 1a supernovae – A type of “standard candle” which lets helps to measure distances or occlusions.
- Anisotropy – “non uniform” (opposite of isotropic)
- – scalar field measuring the… rate of expansion? I’m fuzzy on this one, see http://physics.stackexchange.com/questions/105145/what-is-phi-as-refered-to-in-guth-2007
- De Sitter space equation of motion – I don’t understand this. #unresolved
- Anthhropic reasoning – The idea that the parameters of our universe are the way they are because we couldn’t exist anywhere else, so we happen to have evolved (existed) in a universe which matches those parameters.
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”).
- 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).