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

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John Moffat, Jul 15, 2014 Available from:- [Show abstract] [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) changesJournal of High Energy Physics 06/2001; 2001(09). DOI:10.1088/1126-6708/2001/09/023 · 6.22 Impact Factor - [Show abstract] [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 interaction 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. Comment: 16 pages in phyzzx.tex, many minor corrections and additions, one reference added, typos correctedJournal of High Energy Physics 05/2001; DOI:10.1088/1126-6708/2001/10/025 · 6.22 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**We point out that quintessence with an exponential potential V_0 exp(- beta phi / 3^{1/2} M_p) can account for the present observed acceleration of the universe, without necessarily leading to eternal acceleration. This occurs for 2.4 < beta < 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, H^2 ~ rho + 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 < beta < 2.4. Future observations of high-z supernovae will be able to test whether beta lies in the desired range. Comment: 13 pp., 5 figures; references addedJournal of High Energy Physics 05/2001; DOI:10.1088/1126-6708/2001/08/035 · 6.22 Impact Factor