We study particle production at the preheating era in inflation models with nonminimal coupling ξφ 2 R and quartic potential λφ 4 /4 for several cases: real scalar inflaton, complex scalar inflaton and Abelian Higgs inflaton. We point out that the preheating proceeds much more violently than previously thought. If the inflaton is a complex scalar, the phase degree of freedom is violently produced at the first stage of preheating. If the inflaton is a Higgs field, the longitudinal gauge boson production is similarly violent. This is caused by a spike-like feature in the time dependence of the inflaton field, which may be understood as a consequence of the short time scale during which the effective potential or kinetic term changes suddenly. The produced particles typically have very high momenta k √ λM P . The production might be so strong that almost all the energy of the inflaton is carried away within one oscillation for ξ 2 λ O(100). This may partly change the conventional understandings of the (p)reheating after inflation with the nonminimal coupling to gravity such as Higgs inflation. We also discuss the possibility of unitarity violation at the preheating stage.

We study the dual production of helical Abelian gauge fields and chiral fermions through the Chern-Simons (CS) coupling with a pseudo-scalar inflaton in the presence of a chiral anomaly. Through the CS term, the motion of the inflaton induces a tachyonic instability for one of the two helicities of the gauge field. We show that the resulting helical gauge field necessarily leads to the production of chiral fermions by deforming their Fermi sphere into discrete Landau levels. The population of the lowest Landau level leads to a chiral asymmetry as inferred from the chiral anomaly, while the higher levels are populated symmetrically through pair production. From the backreaction of the fermions on the gauge field production we derive a conservative but stringent upper bound on the magnitude of the gauge fields. Consequently, we find that the scalar perturbations sourced by these helical gauge fields, responsible for enhanced structure formation on small scales, get reduced significantly. We also discuss the fate of the primordial chiral asymmetry and of the helical gauge fields after inflation, and show that the instability in the chiral plasma tends to erase these primordial asymmetries. This result may impact scenarios where the baryon asymmetry of the Universe is connected to primordial magnetic fields.

Primordial black holes (PBHs) are one of the candidates to explain the gravitational wave (GW) signals observed by the LIGO detectors. Among several phenomena in the early Universe, cosmic inflation is a major example to generate PBHs from large primordial density perturbations. In this paper, we discuss the possibility to interpret the observed GW events as mergers of PBHs which are produced by cosmic inflation. The primordial curvature perturbation should be large enough to produce a sizable amount of PBHs and thus we have several other probes to test this scenario. We point out that the current pulsar timing array (PTA) experiments already put severe constraints on GWs generated via the second-order effects, and that the observation of the cosmic microwave background (CMB) puts severe restriction on its µ distortion. In particular, it is found that the scalar power spectrum should have a very sharp peak at k ∼ 10 6 Mpc −1to fulfill the required abundance of PBHs while evading constraints from the PTA experiments together with the µ distortion. We propose a mechanism which can realize such a sharp peak. In the future, simple inflation models that generate PBHs via almost Gaussian fluctuations could be probed/excluded.

Following a new microlensing constraint on primordial black holes (PBHs) with ∼ 10 20 -10 28 g [1], we revisit the idea of PBH as all Dark Matter (DM). We have shown that the updated observational constraints suggest the viable mass function for PBHs as all DM to have a peak at 10 20 g with a small width σ 0.1, by imposing observational constraints on an extended mass function in a proper way. We have also provided an inflation model that successfully generates PBHs as all DM fulfilling this requirement.