Can spacetime torsion source an extremely red-tilted cosmological GW background?
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| Format: | Preprint |
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2025
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| _version_ | 1866913793124073472 |
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| author | Bhaumik, Arko Mandal, Bhaswati SenGupta, Soumitra |
| author_facet | Bhaumik, Arko Mandal, Bhaswati SenGupta, Soumitra |
| contents | 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 $α_{\rm T}\sim-6$ on matter-dominated scales. The signal is potentially within the reach of upcoming indirect GW probes on very large scales $k\lesssim10^{-2}\:\textrm{Mpc}^{-1}$, 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2504_10402 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Can spacetime torsion source an extremely red-tilted cosmological GW background? Bhaumik, Arko Mandal, Bhaswati SenGupta, Soumitra Cosmology and Nongalactic Astrophysics General Relativity and Quantum Cosmology High Energy Physics - Theory 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 $α_{\rm T}\sim-6$ on matter-dominated scales. The signal is potentially within the reach of upcoming indirect GW probes on very large scales $k\lesssim10^{-2}\:\textrm{Mpc}^{-1}$, 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. |
| title | Can spacetime torsion source an extremely red-tilted cosmological GW background? |
| topic | Cosmology and Nongalactic Astrophysics General Relativity and Quantum Cosmology High Energy Physics - Theory |
| url | https://arxiv.org/abs/2504.10402 |