# 'There shall be reheating' and other inflation-related questions [closed]

I've some basic questions about inflation:

1. What is reheating exactly? What triggered the reheating? Is it something that one pulls out of a hat? Through which force did the decay of the inflation occur ?

2. How can I picture the inflaton? It is a scalar field, like the Higgs, could we ever hope to observe the inflaton? Can we assign to the field excitation a mass?

3. On the blog Quantum Frontier I read the following:

"A possible solution the the super-Planckian value of the change of the inflaton field during the inflation is that the field didn't go in a straight line but somehow turned in circles." (Preskill's post on Quantum Frontiers)

What is meant by "going into circles" ? That it oscillated around a minimum of the potential (but isn't that the phenomenon associated to reheating ?) Or is it an identification, e.g. $\phi(x)= \phi (x+2\pi)$? (This seems even stranger :S.)

1. Reheating is the decaying of the inflaton into the particles that we are currently observing. In the context of quantum field theory this happens simply because there is a coupling of the inflaton field to either the Standard Model (and possibly other, yet unobserved, particles) directly, or to a field $\chi$ which then couples to the Standard Model and possible extensions. In the context of quantum theory, the concept of a 'force' is perhaps not always the best way of thinking about particle interactions.
2. The inflaton is usually taken to be a scalar field. Some models take the Higgs field to be the inflaton, although there has been a recent discussion whether the new BICEP2 data rules out such Higgs inflaton scenarios. Alternatively, one can use a large number $N\gg 1$ of scalar fields that together generate inflation. This seems like a very reasonable option if one starts from string theory, which typically gives rise to many scalar fields (if someone knows more about this any input would be much appreciated).
3. Many people are trying to think about a way for the inflaton field to undergo super-Planckian displacement, something which seems favored by the BICEP2 results. The oscillations around a minimum that you mention are something different. In the context of (p)reheating, people often consider a so-called curvaton scenario. The equation of motion for the scalar field(s) (assuming the standard potential $V(\phi)=\frac{1}{2}m^2\phi^2$) is $$\phi''+3H\phi'+m^2\phi=0$$ Therefore, if $H\ll m^2$, the field(s) will oscillate. This happens when the field(s) is/are near the minimum of the potential, because $H$ is related (under slow-roll conditions) to the inflaton potential via the Friedmann equation: $$H^2=\frac{V}{3M^2_\text{Planck}}$$ Around the minimum, therefore, $H$ is small and the solution to the equation of motion is oscillatory. This is the curvaton scenario; it is one of the possible ways to model (p)reheating which is currently under investigation.