Diluting Cosmological Constant via Large Distance Modification of Gravity

TL;DR

This paper discusses a brane-world model with infinite-volume extra dimensions that modifies gravity at large distances, offering a potential solution to the cosmological constant problem by weakly coupling the zero-mode graviton to matter.

Contribution

It introduces a nonlinear covariant theory with a metastable graviton that alters gravity at large scales, addressing the cosmological constant problem without disrupting early cosmology.

Findings

Vacuum energy curves extra space mostly, resulting in small four-dimensional curvature.

Gravity is mediated by a metastable graviton with a Hubble-scale lifetime.

Standard cosmological properties, including inflation, remain unaffected.

Abstract

We review a solution of the cosmological constant problem in a brane-world model with infinite-volume extra dimensions. The solution is based on a nonlinear generally covariant theory of a metastable graviton that leads to a large-distance modification of gravity. From the extra-dimensional standpoint the problem is solved due to the fact that the four-dimensional vacuum energy curves mostly the extra space. The four-dimensional curvature is small, being inversely proportional to a positive power of the vacuum energy. The effects of infinite-volume extra dimensions are seen by a brane-world observer as nonlocal operators. From the four-dimensional perspective the problem is solved because the zero-mode graviton is extremely weakly coupled to localized four-dimensional sources. The observable gravity is mediated not by zero mode but, instead, by a metastable graviton with a lifetime of the order of the present-day Hubble scale. Therefore, laws of gravity are modified in the infrared above the Hubble scale. Large wave-length sources, such as the vacuum energy, feel only the zero-mode interaction and, as a result, curve space very mildly. Shorter wave-length sources interact predominantly via exchange of the metastable graviton. Because of this, all standard properties of early cosmology, including inflation, are intact.