Can spacetime torsion source an extremely red-tilted cosmological GW background?

Abstract

In the presence of spacetime torsion, any generic $f(R)$ model of gravity is conformally dual to a scalar-tensor theory augmented with a second rank antisymmetric massless degree of freedom. We investigate the stochastic gravitational wave background (SGWB) that may be sourced directly at the second order by such a torsional field, treated perturbatively during an epoch of canonical, single-field, slow-roll inflation. The resulting second-order induced SGWB, which dominates over the primary inflationary GW background at all scales, peaks only at ultra-low frequencies, and is found to be extremely red-tilted with an effective tensor spectral index $\hat{I}{\pm }_{\mathrm{T}}\sim -6$ on matter-dominated scales. The signal is potentially within the reach of upcoming indirect GW probes on very large scales $k\lesssim 10^{-2}{\text{Mpc}}^{-1}$, i.e., next-generation CMB experiments like the LiteBIRD. In the near future, observation of such a markedly red-tilted SGWB on CMB scales could hence provide a novel and unique clue in favour of torsional gravity during the inflationary era.

4 pages (core) plus 2 pages (appendices), 2 sets of figures, comments are welcome