The Letter Cosmological Classicalization: Maintaining Unitarity under Relevant Deformations of the Einstein-Hilbert Action by Felix Berkhahn, Dennis D. Dietrich, and Stefan Hofmann has just been published in the prestigious Physical Review Letters journal of the American Physical Society.
At the core of Einstein’s gravity theory is a democratic principle guaranteeing any source the same coupling to spacetime, independent of its nature. Exploiting this principle has been the successful strategy sui generis to infer the vacuum’s
energy density by observing the Universe’s expansion history.
Ever since Cosmology has advanced to a high precision science, the window of opportunity to probe gravity’s rigidity on cosmological scales is wide open. Deforming Einstein’s theory of gravity introduces additional relevant degrees of freedom, which have a profound impact: they violate the gravitational coupling’s universality. Such proposals might be crucial to cure the spacetime impact of technically unnatural sources, such as the vacuum energy density, which is in staggering conflict with the observed expansion history.
In this Letter the authors consider generic relevant deformations of the Einstein-Hilbert action on arbitrary backgrounds at the linear level. Explicit results are shown for Friedmann-Robertson-Walker (FRW) spacetimes. They analyze the classical and quantum stability of the `free theory,’ which is a necessary prerequisite before completing the corresponding deformations at the nonlinear level.
The authors show that the stabilization dynamics for the additional relevant degrees of freedom on a curved background, even when they are `free,’ is as multi-facetted and rich as self-protection mechanisms in certain non-renormalizable interacting systems on a Minkowsk geometry.
In particular, the authors show that the recent classicalization proposal is at work: the dynamics of the additional degrees of freedom protects them against unitarity violations via the formation of classical objects that eventually become the new background spacetime. The transition to the new geometric ground state is heralded by the massive decay of the original vacuum and evolves through a strong coupling regime.