Article

# Acceleration of the Universe, String Theory and a Varying Speed of Light

06/2001;
Source: arXiv

ABSTRACT

The existence of future horizons in spacetime geometries poses serious problems for string theory and quantum field theories. The observation that the expansion of the universe is accelerating has recently been shown to lead to a crisis for the mathematical formalism of string and M-theories, since the existence of a future horizon for an eternally accelerating universe does not allow the formulation of physical S-matrix observables. Postulating that the speed of light varies in an expanding universe in the future as well as in the past can eliminate future horizons, allowing for a consistent definition of S-matrix observables.

### Full-text

Available from: John Moffat, Jul 15, 2014
• Source
• "Accurate measurement of the speed of propagation of gravitational waves can constrain extra-dimensional " brane-world " theories in which gravity propagates in the bulk of extra dimensions [8] [9] [10] [11] [12] [13] [14], while the particles of the Standard Model are confined to a 3+1 dimensional subspace known as a brane. It has been argued that in many cases Poincaré invariance should be violated in the bulk [15] [16] [17] [18] [19] [20] [21] giving rise to an anomalous dispersion relation for gravitational waves. 2 No Lorentz violation would show up in the standard model, provided our brane is Poincaré invariant. "
##### Article: Lower Bound on the Propagation Speed of Gravity from Gravitational Cherenkov Radiation
[Hide abstract]
ABSTRACT: Recently, interesting 4-D Lorentz violating models have been proposed, in which all particles have a common maximum velocity $c$, but gravity propagates (in the preferred frame) with a different maximum velocity $c_g \neq c$. We show that the case $c_g < c$ is very tightly constrained by the observation of the highest energy cosmic rays. Assuming a galactic origin for the cosmic rays gives a conservative bound of $c-c_g < 2 \times 10^{-15} c$; if the cosmic rays have an extragalactic origin the bound is orders of magnitude tighter, of order $c-c_g < 2 \times 10^{-19} c$. Comment: 8 pages with 1 figure, JHEP style. References added, slight (superficial) changes
Preview · Article · Jun 2001 · Journal of High Energy Physics
• Source
##### Article: Hybrid Quintessence with an End or Quintessence from Branes and Large Dimensions
[Hide abstract]
ABSTRACT: We describe a model of hybrid quintessence in which in addition to the tracker field there is a trigger field which is responsible for ending quintessence. As a result, hybrid quintessence does not suffer from the problems associated with the eternal acceleration of the universe. We derive the hybrid quintessence potential on branes from the interbrane interactions in string theory and show that it requires TeV scale strings and two millimeter size dimensions. This scenario predicts a dark energy density of O(mm−4) and relates the smallness of this energy to the large size of the extra dimensions.
Preview · Article · May 2001 · Journal of High Energy Physics
• Source
##### Article: Quintessence, Cosmological Horizons, and Self-Tuning
[Hide abstract]
ABSTRACT: We point out that quintessence with an exponential potential V0 exp(−βQ/31/2Mp) can account for the present observed acceleration of the universe, without necessarily leading to eternal acceleration. This occurs for 2.4 < β < 2.8. Thus a cosmological horizon, which is supposed to be problematic within the context of string theory, can be avoided. We argue that this class of models is not particularly fine-tuned. We further examine this question in the context of a modified Friedmann equation, H2∝ρ+p, which is suggested by higher dimensional self-tuning approaches to the cosmological constant problem. It is shown that the self-tuning case can also be consistent with observations, if 1.8 < β < 2.4 (where the potential is now parametrized as V0exp (−βQ/2Mp)). Future observations of high-z supernovae will be able to test whether β lies in the desired range.
Preview · Article · May 2001 · Journal of High Energy Physics