Unimodular Quadratic Gravity and the Cosmological Constant

TL;DR

This paper explores how unimodular gravity, when applied to a quantum quadratic gravity framework, leads to different inflationary predictions but does not solve the cosmological constant problem, with an anthropic multiverse explanation for dark energy.

Contribution

It introduces a non-perturbative quantum unimodular quadratic gravity model and analyzes its implications for inflationary observables and the cosmological constant.

Findings

Unimodular gravity alters certain inflationary quantum predictions.

It does not resolve the cosmological constant problem.

An anthropic multiverse scenario explains dark energy scale.

Abstract

Unimodular gravity addresses the old cosmological constant (CC) problem, explaining why such constant is not at least as large as the largest particle mass scale, but classically it is indistinguishable from ordinary gravity. Conversely, quantum physics may give us a way to distinguish the two theories. Thus, here the unimodular constraint is imposed on a non-perturbative and background-independent quantum version of quadratic gravity, which was recently formulated. It is shown that unimodularity does lead to different predictions for some inflationary quantum observables. Unimodular gravity per se does not solves the new CC problem (why the CC has the observed value?) even in this realization. To address this issue a multiverse made by different eras in a single big bang is considered and the observed scale of dark energy is explained anthropically.