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Inflation can explain why the Cosmic Microwave Background Radiation (CMBR) is so isotropic to a high degree despite that many regions of the universe were never in contact with each other. According to it, regions that are not in causal contact with each other right now were in the past, and could communicate. Then the universe expanded, the physical scales increased in size, and photons from those regions never came into causal contact again.

In slow roll models, inflation is driven by the potential energy of a scalar field, the inflaton, which slowly rolls down the potential. When the slow-rolling stops, inflation comes to an end, and the inflaton field oscillates around the minimum of potential energy. In this period, it produces the elementary particles known to us, in a process called reheating.

Here's what I don't understand - if photons are only produced in this period, after inflation ends, how can they have "talked" before when the universe was expanding, if only the inflaton field was present?

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The photons that you see as the CMB$^\dagger$ were not created during reheating, but rather at recombination, when the temperature had dropped sufficiently.

What inflation did (among other things) was to ensure that everything was in thermodynamical equilibrium. That is, regardless what kind of particle we consider, they shared the same energy distribution. The matter and the radiation kept sharing this (decreasing) energy, until the photons were "released" at decoupling.

Without inflation, one region of the Universe could have had one energy distribution, and another region could have had another, which later would have led to the necessary temperature for recombination (roughly 3000 K) being reached at different epochs. This, in turn, would then lead to the CMB arriving from different parts of the sky having been traveling for different amounts of times through the expanding Universe, and hence different redshifts.

Since the same redshift is observed in all directions (within one part in $\sim10^5$), this cannot be the case.


$^\dagger$Today, "CMB" seems to be the preferred term, while "CMBR" is somewhat outdated. I've seen some Russians still use it, though.

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  • $\begingroup$ What constitutes "creating" a photon is arguably a matter of taste (does scattering produce a new photon or is it the same photon that scatters). I think in the context of the question the OP is perfectly within their rights to say that the photons were created at reheating (and subsequent annihilation processes) in the sense that there were no photons during inflation and many after reheating. $\endgroup$ – Philo Jan 9 '18 at 16:06
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    $\begingroup$ @Philo I'd say that for scattering, where the photon retains some "memory" of its former state, it makes sense to think of it as the same photon before and after the event. But when a photon is absorbed because it ionizes an atom, it is lost and gone forever. At a later point, the ion meets another electron, and a photon it emitted with a frequency and direction which it entirely independent of the first photon. And the originally ejected electron meets with another ion, emitting yet another photon. So while I agree that photons were created at reheating, it is not the same as the CMB photons. $\endgroup$ – pela Jan 10 '18 at 15:28
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It isn't necessary that the particles used for contact and homogenizing before slow-roll started were photons. Any fields present, like the inflaton field, could homogenize to the same potential before inflation. That would mean that slow-roll progresses at much the same rate almost everywhere and, once slow-roll ends and reheating commences, the density of the inflaton field can create a more or less homogeneous density of elementary particles and photons everywhere.

Basically, the homogenization of any primordial fields will lead to a universe after inflation that looks isotropic and homogeneous. It doesn't have to be photons

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