Gravity as the breakdown of conformal invariance

TL;DR

This paper explores a novel cosmological model where gravity originated from the breaking of conformal invariance, explaining scale-invariant perturbations without inflation and suggesting observable signatures in primordial spectra.

Contribution

It introduces a top-down approach linking gravity's origin to conformal invariance breaking, supported by quantum gravity insights and implications for primordial perturbations.

Findings

Gravity may have been conformally coupled or inactive at the universe's start.

Breaking conformal invariance could explain scale-invariant cosmological perturbations.

Predicted signatures include deviations in primordial power spectrum and gravity wave levels.

Abstract

We propose that at the beginning of the universe gravity existed in a limbo either because it was switched off or because it was only conformally coupled to all particles. This picture can be reverse-engineered from the requirement that the cosmological perturbations be (nearly) scale-invariant without the need for inflation. It also finds support in recent results in quantum gravity suggesting that spacetime becomes two-dimensional at super-Planckian energies. We advocate a novel top-down approach to cosmology based on the idea that gravity and the Big Bang Universe are relics from the mechanism responsible for breaking the fundamental conformal invariance. Such a mechanism should leave clear signatures in departures from scale-invariance in the primordial power spectrum and the level of gravity waves generated.