Gravitational Lorentz Violations and Adjustment of the Cosmological Constant in Asymmetrically Warped Spacetimes

TL;DR

This paper explores asymmetrically warped spacetimes where Lorentz invariance is violated in the bulk, leading to potential explanations for the cosmological constant adjustment and cosmic acceleration through gravitational effects in extra dimensions.

Contribution

It introduces a class of solutions with black holes in asymmetrically warped spacetimes that can relax the cosmological constant and possibly explain cosmic acceleration via gravitational Lorentz violations.

Findings

Gravitational waves may travel faster than light on the brane.

The cosmological constant can relax through black hole parameter adjustments.

Models suggest a link between Lorentz violations and universe acceleration.

Abstract

We investigate spacetimes in which the speed of light along flat 4D sections varies over the extra dimensions due to different warp factors for the space and the time coordinates (``asymmetrically warped'' spacetimes). The main property of such spaces is that while the induced metric is flat, implying Lorentz invariant particle physics on a brane, bulk gravitational effects will cause apparent violations of Lorentz invariance and of causality from the brane observer's point of view. An important experimentally verifiable consequence of this is that gravitational waves may travel with a speed different from the speed of light on the brane, and possibly even faster. We find the most general spacetimes of this sort, which are given by AdS-Schwarzschild or AdS-Reissner-Nordstrom black holes, assuming the simplest possible sources in the bulk. Due to the gravitational Lorentz violations these models do not have an ordinary Lorentz invariant effective description, and thus provide a possible way around Weinberg's no-go theorem for the adjustment of the cosmological constant. Indeed we show that the cosmological constant may relax in such theories by the adjustment of the mass and the charge of the black hole. The black hole singularity in these solutions can be protected by a horizon, but the existence of a horizon requires some exotic energy densities on the brane. We investigate the cosmological expansion of these models and speculate that it may provide an explanation for the accelerating Universe, provided that the timescale for the adjustment is shorter than the Hubble time. In this case the accelerating Universe would be a manifestation of gravitational Lorentz violations in extra dimensions.