Tensor instabilities at the end of the LCDM universe

Understand

The unphysical spin-2 massive degrees of freedom in higher derivative gravity may be either massive unphysical ghosts or tachyonic ghosts.

• In the last case there is no Planck-scale threshold protecting vacuum cosmological solutions from instabilities.
• Within the anomaly-induced action formalism the photon-driven IR running of the coefficient of the Weyl-squared term makes the ghost eventually becoming tachyon, that should produce a gravitational explosion of vacuum.
• This effect is stable under higher loop corrections and takes place also in known versions of perturbative quantum gravity.