A new mechanism for generating density perturbations from inflation

TL;DR

The paper proposes that density perturbations can originate during reheating if the inflaton decay rate to ordinary matter fluctuates spatially due to light scalar fields controlling the coupling. These fluctuations translate into reheating-temperature variations that induce adiabatic density perturbations, potentially dominating the standard inflationary mechanism. This mechanism allows a lower inflation scale, predicts a suppressed gravitational-wave background, and can yield observable non-Gaussianities depending on the dynamics of the fluctuating field. The authors derive the perturbation equations in conformal gauge, show that the metric perturbation Phi scales as Phi ≈ (1/9) delta Gamma/Gamma, and illustrate the idea with a MSSM-based example, highlighting significant implications for inflation model-building and cosmological observations.

Abstract

We propose a new mechanism to generate density perturbations in inflationary models. Spatial fluctuations in the decay rate of the inflaton field to ordinary matter lead to fluctuations in the reheating temperature. We argue that in most realistic models of inflation the coupling of the inflaton to normal matter is determined by the vacuum expectation values of fields in the theory. If those fields are light during inflation (this is a generic situation in the minimal models of supersymmetric inflation) they will fluctuate leading to density perturbations through the proposed mechanism. We show that these fluctuations could easily dominate over the ones generated through the standard mechanism. The new scenario has several consequences for inflation model building and observations. The proposed mechanism allows to generate the observed level of density perturbations with a much lower scale of inflation and thus generically predicts a smaller level of gravitational waves. The relation between the slope of the spectrum of the produced density perturbations and the potential of the inflaton field is different from the standard relations obtained in the context of slow roll inflation. Because the field responsible for the fluctuations is not the inflaton, it can have significantly larger self couplings and thus density perturbations could be non-Gaussian. The non-Gaussianity can be large enough to be detectable by CMB and Large Scale Structure observations.

Figures (1)

• Figure 1: Evolution of the gravitational potential in units of $\delta \Gamma / \Gamma$ as a function of $N=\ln a$. The curves correspond to $H_*/ \Gamma = 5, 10, 20$ (solid,dotted,dashed).