We investigate the new observational constraints on $f(T)$ gravity that arise from the effects of primordial gravitational waves (GWs) on the cosmic microwave background (CMB) anisotropies and the BB spectrum. We first show that on the GWs propagation in $f(T)$ gravity we obtain only an amplitude modification and not a phase one, comparing to the case of general relativity in the background of $\mathrm{\Lambda }\mathrm{CDM}$ cosmology. Concerning primordial GWs we find that the more the model departs from general relativity the larger is the GW amplitude decay, and thus a possible future detection would bring the viable $f(T)$ gravity models five orders of magnitude closer to $\mathrm{\Lambda }\mathrm{CDM}$ cosmology comparing to standard cosmological constraints. Additionally, we use the class code and both data from the Planck probe, as well as forecasts from the near-future CORE collaboration, and we show that possible nontrivial constraints on the tensor-to-scalar ratio would offer a clear signature of $f(T)$ gravity. Finally, we discuss on the possibility to use the properties of the GWs that arise from neutron stars mergers in order to extract additional constrains on the theory.

• Received 10 October 2018

DOI:https://doi.org/10.1103/PhysRevD.98.104055