# Primordial-tensor-induced stochastic gravitational waves

## TL;DR

This paper demonstrates that a spectator tensor field during cosmological evolution can produce significant stochastic gravitational waves within the sensitivity of future detectors, with unique features such as resonance effects and no associated primordial black holes.

## Contribution

It reveals a novel mechanism where a spectator tensor field induces detectable gravitational waves without forming primordial black holes, contrasting with scalar-induced scenarios.

## Key findings

- Generated GWs can be within future detector sensitivity.
- Resonance can amplify GW amplitude significantly.
- No primordial black holes are expected from this tensor-induced process.

## Abstract

Cosmological stochastic gravitational waves (GWs) induced by a spectator field are usually expected to have an amplitude very small compared with those generated by the curvature perturbation, or equivalently by a field dominating the universe. On the contrary to this expectation, we show that a spectator field that provides a tensor perturbation, on top of the metric tensor perturbation, can generate a significant amount of GWs. The amplitude and frequency of the generated GWs may lie within the sensitivity range of future GW detectors. In particular, if the sound velocities of the two tensor perturbations coincide, the induced GW amplitude may become very large due to resonance by forced oscillation, even in the limit of small coupling between them. A distinct feature of this scenario is that, since tensor modes can hardly lead to the formation of primordial black holes (PBHs), we expect no presence of PBHs, in contrast to the usual scalar-induced case, in which the detection of strong enough induced GWs suggests the existence of PBHs.