A weak instability in an expanding universe?

TL;DR

This paper investigates how high-frequency metric oscillations interact with the universe's expansion using higher derivative gravity, revealing effects similar to dark energy for certain conditions.

Contribution

It introduces a high-order derivative gravity framework to analyze nonlinear couplings between oscillations and expansion, uncovering restricted dynamics and dark energy-like effects.

Findings

Oscillations grow logarithmically for w>0

Oscillations influence expansion similarly to dark energy

Dynamics are highly constrained at high derivative orders

Abstract

We use higher derivative classical gravity to study the nonlinear coupling between the cosmological expansion of the universe and metric oscillations of Planck frequency and very small amplitude. We derive field equations at high orders in the derivative expansion and find that the nature of the new dynamics is extremely restricted. For the equation of state parameter $w>0$ the relative importance of the oscillations grows logarithmically. Their effect on the cosmological expansion resembles that of dark energy.