Non-Gaussianity from massless preheating

Abstract

Preheating can convert superhorizon fluctuations of light scalar fields present at the end of inflation into observable density perturbations. We show in detail how lattice field theory simulations and the separate universes approximation can be used to calculate these perturbations and make predictions for the nonlinearity parameter f NL . We also present a simple approximation scheme that can reproduce these results analytically. Applying these methods to the massless preheating model, we determine the parameter values that are ruled out by too high levels of non-Gaussianity.PACS numbers: 98.80.Cq, 11.15.Kc NL < 111, −151 < f equil. NL < 253 at 95% confidence limit. However, a recent work [8] using the WMAP three-year data found a significant non-Gaussianity: 26.9 < f local NL < 146.7 at 95% confidence limit. Measurements of large scale structure give

On the anisotropy of the gravitational wave background from massless preheating

“…To answer this, we will study preheating numerically, considering a collection of preheating Hubble volumes (which are in causal contact in our current observable universe, but evolved independently during preheating), with different initial values χ i . This separate universe approximation was employed by [16,20] in the context of massless preheating, to study the effect of χ i on the curvature perturbations. Later, more accurate calculations [23] showed that certain initial values χ i lead to spikes in the curvature perturbation from inflation, which would result in cold spots in the CMB.…”

Quantum field thermalization in expanding backgrounds

“…Possible applications include reheating and preheating with multiple fields (including fermions [5]), departure from and return to equilibrium for systems with heavy particles decaying into light ones. In the context of multi-field preheating this may allow to calculate non-gaussian signatures in the CMB [37,38].…”

Use of δ N formalism—difficulties in generating large local-type non-Gaussianity during inflation

“…Recently, non-Gaussianity of the primordial perturbation also has been studied by many authors [7,8,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48]. The main reason for non-Gaussianity to attract much attention is the expectation to future observations mentioned above.…”

“…In this way, the production of large non-Gaussianity at the end of inflation is rather easily achieved. As a mechanism for getting large non-Gaussianity, one can also consider generation of perturbations during preheating phase [49,50,43,44,45,46,47].In this case, the background evolution is strongly coupled to the evolution of small scale fluctuations because the e-folding number changes depending on how fast the energy of coherent oscillation of fields decays into the small scale perturbations. While in the other models the transmutation of homogeneous isocurvature perturbations on each Hubble patch into adiabatic perturbations is determined just by solving the background evolution of various homogeneous and isotropic universe models.…”