Gravity waves and non-Gaussian features from particle production in a sector gravitationally coupled to the inflaton

TL;DR

This work investigates whether particle production in a hidden sector, gravitationally coupled to the inflaton, can generate observable primordial gravitational waves on CMB scales without spoiling the standard nearly scale-invariant curvature perturbations. A general formalism is developed to compute scalar and tensor correlators sourced by a vector field, enabling precise power spectra and bispectra predictions. Model I (instantaneous vector production during a mass-crossing) yields localized features in the scalar spectrum and bispectrum but yields an undetectably small tensor signal for a single burst. Model II (continuous gauge-field production via a rolling pseudoscalar) can produce a sizable, parity-violating GW signal with potentially detectable B-modes, while keeping scalar non-Gaussianity within current bounds; this scenario could be testable via TB/EB correlations and may be realizable in string-theory-inspired setups with axion-like fields.

Abstract

We study the possibility that particle production during inflation could source observable gravity waves on scales relevant for Cosmic Microwave Background experiments. A crucial constraint on such scenarios arises because particle production can also source inflaton perturbations, and might ruin the usual predictions for a nearly scale invariant spectrum of nearly Gaussian curvature fluctuations. To minimize this effect, we consider two models of particle production in a sector that is only gravitationally coupled to the inflaton. For a single instantaneous burst of massive particle production, we find that localized features in the scalar spectrum and bispectrum might be observable, but gravitational wave signatures are unlikely to be detectable (due to the suppressed quadrupole moment of non-relativistic quanta) without invoking some additional effects. We also consider a model with a rolling pseudoscalar that leads to a continuous production of relativistic gauge field fluctuations during inflation. Here we find that gravitational waves from particle production can actually exceed the usual inflationary vacuum fluctuations in a regime where non-Gaussianity is consistent with observational limits. In this model observable B-mode polarization can be obtained for any choice of inflaton potential, and the amplitude of the signal is not necessarily correlated with the scale of inflation.

Figures (4)

• Figure 1: Left panel: the total observable power spectrum in the model (\ref{['model1']}) for a representative choice of parameters, illustrating the appearance of a localized bump feature. The solid black curve is the total spectrum while the dashed blue line gives the spectrum of the vacuum fluctuations for comparison. Right panel: a comparison of the shape function $S_b$ (solid black curve) and the fitting function $S_{\mathrm{fit}}$ (dashed red curve).
• Figure 2: Left panel: The tensor-to-scalar ratio as a function of $\xi$, for several illustrative choices of $\epsilon$. The horizontal line corresponds to $r=0.1$, the approximate current observational limit. Notice that an observable tensor-to-scalar ratio can be achieved for any inflationary potential, by suitably tuning the dynamics in the hidden sector. Right panel: The effective nonlinearity parameter as a function of $\xi$, for several illustrative choices of $\epsilon$. The horizontal line corresponds to $f_{NL}=266$, the approximate current observational limit on non-Gaussianity.
• Figure 3: Here we plot contours in the $\xi-\epsilon$ plane leading to $f_{NL}=266$ (the current observational bound on non-Gaussianity), $r=0.1$ (the current observational bound on tensor modes), and $r=0.01$ (which may be detectable in the near future). The region above the solid red line is ruled out by producing too much non-Gaussianity while the region above the dashed green line is ruled out by non-detection of tensor fluctuations. We see that the non-Gaussianity bound is weaker, meaning that the dominant signature of the model comes from gravitational waves.
• Figure 4: Red/solid lines: Predictions for $r$ vs $\Delta \chi$ in the model (\ref{['S2']}); each line is obtained for a fixed value of $\epsilon$, and for varying $\xi$, with greater $\xi$ corresponding to greater particle production, and therefore larger signal. Black/dotted lines: $1 \sigma$ detection lines for the Planck (P), SPIDER (S), CMB-Pol (C), and a cosmic-variance limited (CV) experiment. The signal needs to be above a line to be detectable at $1 \sigma$ by that experiment. These experimental forecasts are an approximate copy of the lines shown in Figure 2 of Gluscevic:2010vv.