Tensor non-Gaussianity in chiral scalar-tensor theories of gravity. (arXiv:2008.01715v2 [astro-ph.CO] UPDATED)

<a href="http://arxiv.org/find/astro-ph/1/au:+Bartolo_N/0/1/0/all/0/1">Nicola Bartolo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Caloni_L/0/1/0/all/0/1">Luca Caloni</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Orlando_G/0/1/0/all/0/1">Giorgio Orlando</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ricciardone_A/0/1/0/all/0/1">Angelo Ricciardone</a>

Violation of parity symmetry in the gravitational sector, which manifests

into unequal left and right circular polarization states of primordial

gravitational waves, represents a way to test high-energy modifications to

general relativity. In this paper we study inflation within recently proposed

chiral scalar-tensor theories of gravity, that extend Chern-Simons gravity by

including parity-violating operators containing first and second derivatives of

the non-minimally coupled scalar (inflaton) field. Given the degeneracy between

different parity-violating theories at the level of the power spectrum

statistics, we make a detailed analysis of the parity violation on primordial

tensor non-Gaussianity. We show, with an explicit computation, that no new

contributions arise in the graviton bispectra if the couplings in the new

operators are constant in a pure de Sitter phase. On the other hand, if the

coupling functions are time-dependent during inflation, the tensor bispectra

acquire non-vanishing contributions from the parity-breaking operators even in

the exact de Sitter limit, with maximal signal in the squeezed and equilateral

configurations. We also comment on the consistency relation of the three-point

function of tensor modes in this class of models and discuss prospects of

detecting parity-breaking signatures through Cosmic Microwave Background

$B$-mode bispectra.

Violation of parity symmetry in the gravitational sector, which manifests

into unequal left and right circular polarization states of primordial

gravitational waves, represents a way to test high-energy modifications to

general relativity. In this paper we study inflation within recently proposed

chiral scalar-tensor theories of gravity, that extend Chern-Simons gravity by

including parity-violating operators containing first and second derivatives of

the non-minimally coupled scalar (inflaton) field. Given the degeneracy between

different parity-violating theories at the level of the power spectrum

statistics, we make a detailed analysis of the parity violation on primordial

tensor non-Gaussianity. We show, with an explicit computation, that no new

contributions arise in the graviton bispectra if the couplings in the new

operators are constant in a pure de Sitter phase. On the other hand, if the

coupling functions are time-dependent during inflation, the tensor bispectra

acquire non-vanishing contributions from the parity-breaking operators even in

the exact de Sitter limit, with maximal signal in the squeezed and equilateral

configurations. We also comment on the consistency relation of the three-point

function of tensor modes in this class of models and discuss prospects of

detecting parity-breaking signatures through Cosmic Microwave Background

$B$-mode bispectra.

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