An inflationary model with small scalar and large tensor nongaussianities

TL;DR

The paper proposes an inflationary scenario where a rolling pseudoscalar coupled to a gauge field amplifies vectors that source both scalar and tensor metric perturbations. Scalar non-Gaussianities remain negligible due to helicity conservation, while tensor non-Gaussianities are enhanced and exhibit a chiral, equilateral shape. Using a flat-sky framework, the authors compute the temperature bispectrum contributions from scalars and tensors, showing tensor-induced non-Gaussianities vastly exceed the scalar ones and can induce parity-violating signatures in the CMB. Observational constraints from Planck bound the parameter space, but there remains a viable region where tensor chirality could be detected by future experiments (e.g., CMBPol or cosmic-variance-limited surveys), offering a distinctive test of this mechanism. Overall, the work demonstrates a concrete model in which tensor non-Gaussianities dominate and imprint unique, parity-violating patterns in the CMB polarization and temperature maps.

Abstract

We study a model of inflation where the scalar perturbations are almost gaussian while there is sizable (equilateral) nongaussianity in the tensor sector. In this model, a rolling pseudoscalar gravitationally coupled to the inflaton amplifies the vacuum fluctuations of a vector field. The vector sources both scalar and tensor metric perturbations. Both kinds of perturbations are nongaussian, but, due to helicity conservation, the tensors have a larger amplitude, so that nongaussianity in the scalar perturbations is negligible. Moreover, the tensors produced this way are chiral. We study, in the flat sky approximation, how constraints on tensor nongaussianities affect the detectability of parity violation in the Cosmic Microwave Background. We expect the model to feature interesting patterns on nongaussianities in the polarization spectra of the CMB.

Figures (4)

• Figure 1: Plot of the shape of the three-point function (\ref{['3ptgen']}) of the right handed gravitons.
• Figure 2: Maximum allowed value of $\frac{H}{M_P}$ for various values of $\xi$. The blue line uses the limit from $r< 0.11$, and the dotted pink line uses the limit from $f_{NL}^{\rm equil}<150$. The parameter space below both these lines is allowed.
• Figure 3: The figure compares the possibilities of various experiments of detecting tensor pertubations. The dotted, blue line is the maximum allowed parameters for the model using a combination of the current limits on $r$ and $f_{NL}^{\hbox{equil}}$. The solid lines correspond to the projected sensitivities to $r$ of top to bottom, Planck with polarization (green), Spider (pink), and CMBPol (black).
• Figure 4: Comparison of detectability limits of chirality of the primordial tensors for various experiments. The dotted pink line is the maximum allowed $H/M_P$ based on current limits on $r$ and $f_{NL}^{\hbox{equil}}$. The solid lines are for $2\sigma$ detectability for the following experiments listed in order top to bottom: Planck, Spider, CMBPol, and a cosmic variance limited experiment. The experimental lines were derived from Figure 2 of Gluscevic:2010vv.