39 research outputs found
Spacetime Singularities in String and its Low Dimensional Effective Theory
Spacetime singularities are studied in both the -dimensional string
theory and its -dimensional effective theory, obtained by the Kaluza-Klein
compactification. It is found that spacetime singularities in the low
dimensional effective theory may or may not remain after lifted to the
-dimensional string theory, depending on particular solutions. It is also
found that there exist cases in which spacetime singularities appearing in
high/low dimensional spacetimes do not necessarily happen on the same surfaces.Comment: revtex4, 15 pages, 10 figures. Typos are corrected. Version to appear
in Inter. J. Mod. Phys.
Stability of spin-0 graviton and strong coupling in Horava-Lifshitz theory of gravity
In this paper, we consider two different issues, stability and strong
coupling, raised lately in the newly-proposed Horava-Lifshitz (HL) theory of
quantum gravity with projectability condition. We find that all the scalar
modes are stable in the de Sitter background, due to two different kinds of
effects, one from high-order derivatives of the spacetime curvature, and the
other from the exponential expansion of the de Sitter space. Combining these
effects properly, one can make the instability found in the Minkowski
background never appear even for small-scale modes, provided that the IR limit
is sufficiently closed to the relativistic fixed point. At the fixed point, all
the modes become stabilized. We also show that the instability of Minkowski
spacetime can be cured by introducing mass to the spin-0 graviton. The strong
coupling problem is investigated following the effective field theory approach,
and found that it cannot be cured by the Blas-Pujolas-Sibiryakov mechanism,
initially designed for the case without projectability condition, but might be
circumvented by the Vainshtein mechanism, due to the non-linear effects. In
fact, we construct a class of exact solutions, and show explicitly that it
reduces smoothly to the de Sitter spacetime in the relativistic limit.Comment: Some points regarding to strong coupling are further clarified, and
typos corrected. revtex4, 9 figures. Version to appear in Physical Reviews