As I discussed a couple of weeks back, if inflation is the correct description of the very early universe, then reheating is the mechanism by which the universe is populated with matter, and the origin of the “hot” part of the hot big bang. In that post, I described how, in a range of models, depending on the couplings between the inflaton field and other matter fields, reheating can occur in an extremely non-equilibrium fashion, through parametric resonance, known as preheating.
As I said, although preheating is an out of equilibrium phenomenon, eventually almost all the energy produced equilibrates, and produces a plasma at a given equilibrium temperature. One might therefore wonder how there could be any observational consequences of this hypothesized early cosmic phase (and hence whether such considerations are scientific at all). However, as it turns out, some of the energy may never equilibrate, and there are therefore a number of possible fascinating consequences of preheating. Here’s a whistle-stop tour of some of these.
One motivation for inflation is that the quasi-exponential expansion will dilute away any unwanted relics of the early universe. Examples of this are magnetic monopoles and other superheavy weakly-interacting particles that may be produced during phase transitions at early times. Monopoles, to take a specific example, will generically form if the universe undergoes a grand-unified phase transition, expected at temperatures of around 1016 GeV. If inflation is to rid us of them, then it better be that the reheating temperature after inflation is lower than this temperature, otherwise the universe will go through the phase transition after inflation, and form the offending monopoles after all. Thus, without considering preheating, this means that we are safe from GUT relics if the universe reheats to a temperature below the GUT scale. But preheating can create a problem here. Parametric resonance means that, depending on the model, the energy of the inflaton can be preferentially transferred into certain wavelengths (modes) and this can lead to the formation of particles with masses higher than the typical temperature associated with the original energy density of the inflaton. Thus, although the ultimate reheat temperature is less than the GUT scale (for example), there can nevertheless be a background of relics of much higher masses.
Generally speaking, this is a bad thing, since the overproduction of such heavy particles can lead to them dominating the universe, and a cosmic evolution dramatically different from the one we know. Thus, if the correct model of inflation permits preheating into dangerous relics, then one is forced to consider that inflation must have taken place at considerably lower energies than one might naively have thought. In other words, one needs to have sufficiently low scale inflation that even parametric resonance cannot be powerful enough to resurrect monopoles and their kin. Such considerations can provide strong constraints on models, sometimes pushing reheat temperatures down to troublesomely low ranges (more about this soon).
Another interesting outcome of preheating might be in explaining dark matter. A popular candidate for dark matter is a weakly interacting massive particle (WIMP), the ultimate abundance of which is set by a competition between the particle’s tendency to acquire its equilibrium density and its difficulty of self-interacting in an expanding universe. These competing effects identify the weak scale as a natural place to expect a dark matter WIMP. However, these considerations arise from equilibrium considerations and, as we’ve already seen, these may not apply during preheating.
Indeed, it is possible for GUT scale particles, which would not ordinarily naturally establish the abundance necessary to be dark matter, to be produced during preheating at a density such that they are candidates to be superheavy dark matter or WIMPzillas.
When people usually speak of the gravitational wave signature of inflation, they are referring to the gravity part of the quantum fluctuations of the inflaton (the scalar counterparts of which give rise to structure formation). But with preheating, it is also possible to produce gravitational waves from parametric resonance through the coupling to the graviton. The result is, as one might expect, somewhat model-dependent. Nevertheless, a successful observation might give us a direct window into the earliest times.
Above, I mentioned that constraints from not overproducing dangerous relics during preheating can lead to the allowed reheat temperature being rather low. So what? Well, one problem with this is that we need to have a way of generating the baryon asymmetry of the universe, and many of our known options operate at or above the electroweak scale. What if we never get that hot? Preheating got us into this corner and, fortunately, it provides a couple of options to get out of it. One that I (and others) have worked on is related to the idea of electroweak baryogenesis. As I discussed in my post on this, the usual way people think about electroweak baryogenesis is that baryon number violating processes (occurring through nonperturbative field configurations known as sphalerons) take place during the electroweak phase transition. If the reheat temperature after inflation is below the electroweak scale then this phase transition doesn’t occur. However, if preheating takes place, then it is possible (although tricky, and highly model dependent in practice) for sphalerons to be generated through parametric resonance, and for their decays to generate a baryon asymmetry. Not the prettiest way to produce an excess of matter over antimatter, but a fascinating possibility.
This isn’t an exhaustive list, of course (I didn’t even mention magnetic fields), and I deliberately haven’t provided references (since I don’t need the emails that may generate). But hopefully it gives a flavor of how rich inflationary physics may be, how it is constrained, and one way in which it may be both testable and the answer to some of our cosmological conundrums.