Surfing gravitational waves: can bigravity survive growing tensor modes?

Understand

The theory of bigravity offers one of the simplest possibilities to describe a massive graviton while having self-accelerating cosmological solutions without a cosmological constant.

• However, it has been shown recently that bigravity is affected by early-time fast growing modes on the tensor sector.
• Here we argue that we can only trust the linear analysis up to when perturbations are in the linear regime and use a cut-off to stop the growing of the metric perturbations.
• This analysis, although more consistent, still leads to growing tensor modes that are unacceptably large for the theory to be compatible with measurements of the cosmic microwave background (CMB), both in temperature and polarization spectra.