'There shall be reheating' and other inflation-related questions I've some basic questions about inflation:


*

*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 ? 

*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? 

*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.)
Thanks in advance.
 A: *

*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.

*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).

*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.
