[Paper Review] An inflationary model with large tensor and small scalar nongaussianities

J. Cook, Lorenzo Sorbo|arXiv (Cornell University)|Jul 26, 2013

Cosmology and Gravitation Theories3 citations

TL;DR

This paper proposes an inflationary model generating large tensor nongaussianities and negligible scalar nongaussianities via a rolling pseudoscalar gravitationally coupled to the inflaton, which amplifies vector field fluctuations. The resulting tensor modes are chiral and dominate over scalar modes due to helicity conservation, leading to detectable parity violation signatures in the CMB polarization spectra under flat sky approximation.

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.

Motivation & Objective

• To explore inflationary models where tensor nongaussianities are large while scalar nongaussianities remain negligible.
• To investigate the role of a rolling pseudoscalar coupled to gravity in amplifying vector field fluctuations during inflation.
• To examine how tensor nongaussianities affect the detectability of parity violation in the CMB using the flat sky approximation.
• To predict observable signatures in the CMB polarization spectra arising from chiral tensor modes.

Proposed method

• Introduce a pseudoscalar field rolling during inflation, gravitationally coupled to the inflaton and to a vector field.
• Use the flat sky approximation to analyze the CMB polarization power spectra and their response to tensor nongaussianities.
• Apply helicity conservation to show that tensor fluctuations dominate over scalar ones despite both being nongaussian.
• Compute the nongaussianity parameters for tensor and scalar metric perturbations, focusing on the equilateral type.
• Analyze the chiral nature of the produced tensor modes and their implications for CMB parity violation.
• Derive the expected patterns in the CMB B-mode and E-mode polarization spectra due to the chiral tensor nongaussianities.

Experimental results

Research questions

• RQ1 Can an inflationary model produce large tensor nongaussianities while keeping scalar nongaussianities negligible?
• RQ2 How does the gravitational coupling of a rolling pseudoscalar to a vector field generate nongaussian metric perturbations?
• RQ3 What is the role of helicity conservation in suppressing scalar nongaussianities relative to tensor ones?
• RQ4 How do tensor nongaussianities affect the detectability of parity violation in the CMB?
• RQ5 What distinctive patterns in CMB polarization spectra emerge from chiral tensor nongaussianities?

Key findings

• The model produces sizable equilateral nongaussianity in the tensor sector while scalar nongaussianity remains negligible due to helicity conservation.
• Tensor modes are chiral, arising from the pseudoscalar coupling, leading to a potential signature of parity violation in the CMB.
• The flat sky approximation reveals that tensor nongaussianities leave a distinct imprint on the CMB polarization power spectra.
• The model predicts observable patterns in the CMB B-mode and E-mode spectra due to the chiral nature of the tensor fluctuations.
• Constraints on tensor nongaussianities significantly affect the detectability of parity violation in the CMB, making this model a promising probe of fundamental physics.