3 research outputs found
Flux Compactifications: Stability and Implications for Cosmology
We study the dynamics of the size of an extra-dimensional manifold stabilised
by fluxes. Inspecting the potential for the 4D field associated with this size
(the radion), we obtain the conditions under which it can be stabilised and
show that stable compactifications on hyperbolic manifolds necessarily have a
negative four-dimensional cosmological constant, in contradiction with
experimental observations. Assuming compactification on a positively curved
(spherical) manifold we find that the radion has a mass of the order of the
compactification scale, M_c, and Planck suppressed couplings. We also show that
the model becomes unstable and the extra dimensions decompactify when the
four-dimensional curvature is higher than a maximum value. This in particular
sets an upper bound on the scale of inflation in these models: V_max \sim M_c^2
M_P^2, independently of whether the radion or other field is responsible for
inflation. We comment on other possible contributions to the radion potential
as well as finite temperature effects and their impact on the bounds obtained.Comment: 16 pages, 1 figure, LaTeX; v2: typos fixed and references adde
Membrane Instantons and de Sitter Vacua
We investigate membrane instanton effects in type IIA strings compactified on
rigid Calabi-Yau manifolds. These effects contribute to the low-energy
effective action of the universal hypermultiplet. In the absence of additional
fivebrane instantons, the quaternionic geometry of this hypermultiplet is
determined by solutions of the three-dimensional Toda equation. We construct
solutions describing membrane instantons, and find perfect agreement with the
string theory prediction. In the context of flux compactifications we discuss
how membrane instantons contribute to the scalar potential and the
stabilization of moduli. Finally, we demonstrate the existence of meta-stable
de Sitter vacua.Comment: v3: minor clarifications, JHEP version, 38 page
Cosmology with exponential potentials
We examine in the context of general relativity the dynamics of a spatially
flat Robertson-Walker universe filled with a classical minimally coupled scalar
field \phi of exponential potential ~ e^{-\mu\phi} plus pressureless baryonic
matter. This system is reduced to a first-order ordinary differential equation,
providing direct evidence on the acceleration/deceleration properties of the
system. As a consequence, for positive potentials, passage into acceleration
not at late times is generically a feature of the system, even when the
late-times attractors are decelerating. Furthermore, the structure formation
bound, together with the constraints on the present values of \Omega_{m},
w_{\phi} provide, independently of initial conditions and other parameters,
necessary conditions on \mu. Special solutions are found to possess intervals
of acceleration. For the almost cosmological constant case w_{\phi} ~ -1, as
well as, for the generic late-times evolution, the general relation
\Omega_{\phi}(w_{\phi}) is obtained.Comment: RevTex4, 9 pages, 2 figures, References adde