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Non-Gaussianity from Instant and Tachyonic Preheating

Kari Enqvist, Asko Jokinen, Anupam Mazumdar, Tuomas Multamaki, Antti Vaihkonen

TL;DR

This work addresses how non-Gaussianity in the cosmic microwave background can arise during preheating, focusing on instant and tachyonic mechanisms. It develops a multi-field perturbation framework to compute second-order metric perturbations and the resulting f_NL^φ, linking field dynamics to observable non-Gaussian signals. The key results show that instant preheating produces suppressed non-Gaussianity with $f_{NL}^{\phi} \sim 2 g$ for realistic couplings, while tachyonic preheating can generate large non-Gaussianity, yielding tight bounds on the tachyonic energy scale $V_0^{1/4}/M_P$ (down to ≲ $10^{-4}$ for perturbative λ). These findings constrain both standard and string-inspired inflationary models, highlighting non-Gaussianity as a powerful probe of preheating physics.

Abstract

We study non-Gaussianity in two distinct models of preheating: instant and tachyonic. In instant preheating non-Gaussianity is sourced by the local terms generated through the coupled perturbations of the two scalar fields. We find that the non-Gaussianity parameter is given by $f_{NL}^φ\sim 2g < O(1)$, where $g$ is a coupling constant, so that instant preheating is unlikely to be constrained by WMAP or Planck. In the case of tachyonic preheating non-Gaussianity arises solely from the instability of the tachyon matter and is found to be large. We find that for single field inflation the present WMAP data implies a bound $V_{0}^{1/4}/M_{P}\leq 10^{-4}$ on the scale of tachyonic instability. We argue that the tachyonic preheating limits are useful also for string-motivated inflationary models.

Non-Gaussianity from Instant and Tachyonic Preheating

TL;DR

This work addresses how non-Gaussianity in the cosmic microwave background can arise during preheating, focusing on instant and tachyonic mechanisms. It develops a multi-field perturbation framework to compute second-order metric perturbations and the resulting f_NL^φ, linking field dynamics to observable non-Gaussian signals. The key results show that instant preheating produces suppressed non-Gaussianity with for realistic couplings, while tachyonic preheating can generate large non-Gaussianity, yielding tight bounds on the tachyonic energy scale (down to ≲ for perturbative λ). These findings constrain both standard and string-inspired inflationary models, highlighting non-Gaussianity as a powerful probe of preheating physics.

Abstract

We study non-Gaussianity in two distinct models of preheating: instant and tachyonic. In instant preheating non-Gaussianity is sourced by the local terms generated through the coupled perturbations of the two scalar fields. We find that the non-Gaussianity parameter is given by , where is a coupling constant, so that instant preheating is unlikely to be constrained by WMAP or Planck. In the case of tachyonic preheating non-Gaussianity arises solely from the instability of the tachyon matter and is found to be large. We find that for single field inflation the present WMAP data implies a bound on the scale of tachyonic instability. We argue that the tachyonic preheating limits are useful also for string-motivated inflationary models.

Paper Structure

This paper contains 4 sections, 24 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Basic equations
  3. $f_{NL}^{\phi}$ for Instant Preheating
  4. $f_{NL}^{\phi}$ for Tachyonic Preheating

Figures (1)

  • Figure 1: The parameter space in tachyonic preheating allowed by the WMAP data for single field inflation (below the red curve). Black horizontal line is the usual limit $V_0^{1/4} = 10^{16}$ GeV coming from the COBE normalization.