95 research outputs found
Tachyon Effects on the 2-Dim Black Hole Geometry
We study solutions of the tree level string effective action in the presence
of the tachyon mode.We find that the 2-dim. static black hole is stable against
tachyonic perturbations.For a particular ansatz for the tachyon field we find
an exact solution of the equations of motion which exhibits a naked
singularity.In the case of static fields we find numerically that the full
system has a black hole solution,with the tachyon regular at the horizon.Comment: NTUA 45/94, Pages 11, Latex, 3 Figures on reques
Inflation in supergravity with non-minimal superpotentials
We investigate the cosmological inflation in a class of supergravity models
that are generalizations of non-supersymmetric models. Although such
models have been extensively studied recently, especially after the launch of
the PLANCK and BICEP2 data, the class of models that can be constructed has not
been exhausted. In this note, working in a supergravity model that is a
generalization of Cecotti's model, we show that the appearance of new
superpotential terms, which are quadratic in the superfield that
couples to the Ricci supermultiplet, alters substantially the form of the
scalar potential. The arising potential has the form of the Starobinsky
potential times a factor that is exponential in the inflaton field and
dominates for large inflaton values. We show that the well-known Starobinsky
inflation scenario is maintained only for unnaturally small fine-tuned values
of the coupling describing the superpotential terms. A welcome
feature is the possible increase of the tensor to scalar ratio , within the
limits set by the new Planck and BICEP2 data.Comment: 13 pages, 9 figures, text and references added, version submitted to
Phys. Lett.
Cosmological Evolution in a Type-0 String Theory
We study the cosmological evolution of a type-0 string theory by employing
non-criticality, which may be induced by fluctuations of the D3 brane worlds.
We check the consistency of the approach to O(alpha ') in the corresponding
sigma-model. The ten-dimensional theory is reduced to an effective
four-dimensional model, with only time dependent fields. We show that the
four-dimensional universe has an inflationary phase and graceful exit from it,
while the other extra dimensions are stabilized to a constant value, with the
fifth dimension much larger than the others. We pay particular attention to
demonstrating the role of tachyonic matter in inducing these features. The
Universe asymptotes, for large times, to a non-accelerating linearly-expanding
Universe with a time-dependent dilaton and a relaxing to zero vacuum energy a
la quintessence.Comment: 33 pages LATEX, seven eps figures incorporate
On the radion mediation of the Supersymmetry breaking in N=2, D=5 Supergravity Orbifolds
We discuss the on-shell N=1 Supersymmetric coupling of brane chiral
multiplets in the context of N=2, D=5 Supergravity compactified on
orbifolds. Assuming a constant superpotential on the hidden brane we study the
transmission of the supersymmetry breaking to the visible brane. We find that
to lowest order in the five dimensional Newton's constant and gravitino
mass the spinor field of the radion multiplet is responsible of
inducing positive one-loop squared masses to the scalar fields which are localized on the visible
brane with the length scale of the fifth dimension. Considering a cubic
superpotential on the visible brane we also find that non-vanishing soft
trilinear scalar couplings are induced given by .Comment: 14 pages, 2 figures, version to appear in Physics Letters
Acceleration of the Universe in Type-0 Non-Critical Strings
Presently there is preliminary observational evidence that the cosmological constant might be non zero, and hence that our Universe is eternally accelerating (de Sitter). This poses fundamental problems for string theory, since a scattering matrix is not well defined in such Universes. In a previous paper we have presented a model, based on (non-equilibrium) non-critical strings, which is characterized by eventual ``graceful'' exit from a de Sitter phase. The model is based on a type-0 string theory, involving D3 brane worlds, whose initial quantum fluctuations induce the non criticality. We argue in this article that this model is compatible with the current observations. A crucial r\^ole for the correct ``phenomenology'' of the model is played by the relative magnitude of the flux of the five form of the type 0 string to the size of five of the extra dimensions, transverse to the direction of the flux-field. We do not claim, at this stage at least, that this model is a realistic physical model for the Universe, but we find it interesting that the model cannot be ruled out immediately, at least on phenomenological grounds
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