Brane gravity, massless bulk scalar and self-tuning of the cosmological constant

TL;DR

This paper proposes a self-tuning mechanism in a 5D brane-world model with a massless scalar field, allowing the cosmological constant to remain small despite vacuum energy changes, by dynamically adjusting the background fields.

Contribution

It introduces a novel self-tuning approach in a 5D non-compact space with a scalar field and brane-localized gravity, avoiding fine-tuning and singularities.

Findings

The model maintains a flat 4D space-time despite vacuum energy shifts.

Negative brane cosmological constant avoids bulk singularities.

4D Einstein gravity is recovered at long distances with brane-localized terms.

Abstract

We show that a self-tuning mechanism of the cosmological constant could work in 5D non-compact space-time with a $Z_2$ symmetry in the presence of a massless scalar field. The standard model matter fields live only on the 4D brane. The change of vacuum energy on the brane (brane cosmological constant) by, for instance, electroweak and QCD phase transitions, just gives rise to dynamical shifts of the profiles of the background metric and the scalar field in the extra dimension, keeping 4D space-time flat without any fine-tuning. To avoid naked singularities in the bulk, the brane cosmological constant should be negative. We introduce an additional brane-localized 4D Einstein-Hilbert term so as to provide the observed 4D gravity with the non-compact extra dimension. With a general form of brane-localized gravity term allowed by the symmetries, the low energy Einstein gravity is successfully reproduced on the brane at long distances. We show this phenomenon explicitly for the case of vanishing bulk cosmological constant.