Inflation

Continuing with chapter #20 of the course “Dark Matter, Dark Energy: The Dark Side of the Universe” by Prof. Sean Carroll, now it is time to talk about “Inflation“. I remember when I was studying General Relativity at University that we didn’t spend much time on this concept. I thought by that time that it was a bit odd and also and old-fashioned idea but it has turned out to be a crucial theory (as it is seems to be quite stablished among many cosmologists) for the understanding of the very first moments of evolution of space-time, of our Universe.

As always, some of the concepts/ideas/people… whatever that Prof. Carroll has mentioned in this chapter are collected here for further reference (at least for myself):

  • Inflation: already mentioned in this previous post. It refers to a extremely short phase of the evolution of the Universe, at the beginning of the Big Bang, in which the Universe could have expanded exponentially fast, rapidly transforming curved space into flat one.
  • Alan Guth (1947 – ): american physicist that proposed the inflationary hypothesis in 1980.
Spectacular realization

Guth’s logbook showing the original idea of Inflation. December 7, 1979.

  • Inflaton: scalar field postulated to be the responsible of the rapid expansion of the Universe, known as inflation.
  • Reheating: this is a poorly understood process by which the temperature of the Universe prior to the inflationary phase gets back to its previous values. It is also known as thermalization. The reheating consists on a decay of the inflaton field into particles and radiation, starting the radiation dominant phase.
  • Multiverse: the multiverse is the hypothetical set of multiple possible universes or bubble universes that are popping into and out of existence and colliding all the time, with the space between them rapidly expanding.
  • Monopole problem:  Grand Unified Theories propose that at high temperatures, such as the ones taking place in the early universe, stable magnetic monopoles would be produced. Nevertheless, this heavy particles, which ought to be present today, haven’t been observed in nature so this is an open question in these theories. Here comes inflation to solve it: if a period of inflation occurred below the temperature where magnetic monopoles could separate from each other as the universe expands, the density of these particles would be highly lowered by many orders of magnitude and this could be the reason why there’s no track of them at the moment.
  • Horizon Problem: this referes to the problem of determining why the Universe appears to be homogeneous and isotropic. In a Big Bang model without inflation we couldn’t explain why two widely separated regions of the observable universe have the same temperature.
  • Flatness Problem: this referes to the problem of determining why the density of matter in the universe is comparable to the critical density necessary for a flat universe (Euclidean),  as recent observations of the cosmic microwave background have demonstrated. Inflationary theory solves this problem as it forces the universe to be very flat (to a very high degree, I mean).
  • Polarization of the CMB (Cosmic Microwave Background): one of the predictions of the inflationary universe is that primordial gravitational waves were created during the inflation era. These waves can be accessed by measuring the CMB polarization. Experiments to detect these perturbations are ongoing.

Although I recommend purchasing the original videos from The Teaching Company, this chapter can be seen on YouTube here (part 1) and here (part 2) and here (part 3).

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