Cosmic acceleration and a natural solution to the cosmological constant problem

TL;DR

This paper proposes a conformal gravity framework that naturally resolves the cosmological constant problem by linking cosmic acceleration to the sign of vacuum energy, allowing large or negative values without conflicting with observations.

Contribution

It introduces an alternative cosmology based on conformal gravity, showing how it can naturally accommodate large vacuum energies and explain cosmic acceleration without fine-tuning.

Findings

Conformal gravity explains cosmic acceleration with negative vacuum energy sign.

Large vacuum energies do not conflict with current cosmological data in this framework.

Standard gravity cannot reconcile large vacuum energies with observations, unlike conformal gravity.

Abstract

We trace the origin of the cosmological constant problem to the assumption that Newton's constant $G$ sets the scale for cosmology. And then we show that once this assumption is relaxed, the very same cosmic acceleration which has served to make the cosmological constant problem so very severe instead then serves to provide us with its potential resolution. We present an alternate cosmology, one based on conformal gravity, and show that once given only that the sign of the vacuum energy density $\Lambda$ is explicitly the negative one associated with spontaneous breakdown of the scale invariance of the conformal theory (this actually being the choice of sign for $\Lambda$ which precisely leads to cosmic acceleration in conformal gravity), then that alone, no matter how big $\Lambda$ might actually be in magnitude, is sufficient to not only make its measurable contribution to current era cosmology naturally be of order one today, but to even do so in a way which is completely compatible with the recent high $z$ supernovae cosmology data. Cosmology can thus live with either a fundamental $G$ or with the large (and even potentially negative) $\Lambda$ associated with elementary particle physics phase transitions but not with both. Also, we distinguish between the free energy and the internal energy, with it being the former which determines cosmological phase transitions and the latter which is the source of the gravitational field. Then we show that, even if we make the standard ad hoc assumption that $\Lambda$ actually is quenched in standard gravity, vacuum energy is nonetheless still found to dominate standard cosmology at the time of the phase transition which produced it. However, within conformal gravity no such difficulty is encountered.