Why is 10^-36 seconds considered the starting point of inflation?

[zeebo17]

I've seen in many places that inflation is believed to begin around 10^{-36} s corresponding to the end of the grand unification epoch. Why do we believe that this is the time that it started? Isn't the only requirement that it start sometime after Planck time 10^{-44} s?

Thanks!

 

[hamster143]

I'm not an expert on inflation, but here's my understanding.

Inflation models assume some kind of GUT (perhaps a variation of SU(5)) which is spontaneously broken around 10^15 GeV, which corresponds to 10^-36 s.

If inflation occurs before GUT symmetry breaking, it does not solve the monopole problem (we should see lots of magnetic monopoles but we don't).

If inflation occurs too late after GUT symmetry breaking, we have a problem of baryogenesis. Baryogenesis requires the availability of baryon number changing interactions, we know that there aren't any in the explored region of energies, but most GUTs allow such process near GUT scale. Problem is, inflation wipes any traces of baryogenesis that occurs before its onset, just as it scatters magnetic monopoles.

Therefore the solution is to have inflation right around the GUT scale, it spreads out magnetic monopoles sufficiently to make them virtually unobservable, and then, when inflation is over, the system experiences reheating and gets close enough to the GUT scale again to generate baryons.

 

[clamtrox]

You can also fix the energy scale on some specific inflationary models. For example, a free inflaton field V \sim \phi^2 produces density perturbations of the order of \delta \rho / \rho \sim 10^{-5} (which is what we observe from the CMB) if and only if the energy density of the field is approximately (10^{15} \mathrm{GeV})^4.

 

[mikeph]

Only a certain period of expansion is required to flatten the universe to the present degree, so my understanding is that 10^-36 is a lower bound on the period