Another very informative post Marcus. Can you can also explain what an instanton is? i dont think I understand it.More seriously (perhaps) in some models the bulk of the energy density early on in is something called the "curvaton" field.
If curious about "curvaton" see:
"The curvaton is a hypothetical elementary particle which mediates a scalar field in early universe cosmology. It can generate [geometry] fluctuations during inflation, but does not itself drive inflation, instead it generates curvature perturbations at late times after the inflaton field has decayed and the decay products have redshifted away, when the curvaton is the dominant component of the energy density.
The model was proposed by David Wands and David H. Lyth in 2001."
You asked the kind of question that can only be answered speculatively. People have different ideas about how inflation worked and what fields were present during and after.
Damien Easson has been a co-author of George Smoot who got the Nobel for mapping the CMB. Easson has an idea of what field might have played the CURVATON ROLE.
Higgs Boson in RG running Inflationary Cosmology
Yi-Fu Cai, Damien A. Easson
(Submitted on 6 Feb 2012)
An intriguing hypothesis is that gravity may be non-perturbatively renormalizable via the notion of asymptotic safety. We show that the Higgs sector of the SM minimally coupled to asymptotically safe gravity can generate the observed near scale-invariant spectrum of the Cosmic Microwave Background through the curvaton mechanism. The resulting primordial power spectrum places an upper bound on the Higgs mass, which for canonical values of the curvaton parameters, is compatible with the recently released Large Hadron Collider data.
IOW strange as it might seem, back when energy density was very high a lot of the energy might have been in the form of curvature fluctuations---energy might have been in the microscopic geometry itself, rather than in the form of photons (disturbances in the electromag field.) Maybe at very high energies geometric disturbances can interact with matter, there may be scattering reactions we dont know about.