Quantum Spacetime Instability and Breakdown of Semiclassical Gravity

Understand

The semiclassical gravity describes gravitational back-reactions of the classical spacetime interacting with quantum matter fields but the quantum effects on the background is formally defined as higher derivative curvatures.

• These induce catastrophic instabilities and classic solutions become unstable under small perturbations or their evolutions.
• In this paper we discuss validity of the semiclassical gravity from the perspective of the spacetime instabilities and consider cosmological dynamics of the Universe in this theory.
• We clearly show that the homogenous and isotropic flat Universe is unstable and the solutions either grow exponentially or oscillate even in Planckian time $t_{\rm I}=(\alpha_{ 1 }G_N)^{1/2}\approx \alpha_{ 1 }10^{-43}\ {\rm sec}$.