28 research outputs found
Acceleration from M theory and Fine-tuning
The compactification of M theory with time dependent hyperbolic internal
space gives an effective scalar field with exponential potential which provides
a transient acceleration in Einstein frame in four dimensions. Ordinary matter
and radiation are present in addition to the scalar field coming from
compactification. We find that we have to fine-tune the initial conditions of
the scalar field so that our Universe experiences acceleration now. During the
evolution of our Universe, the volume of the internal space increases about 12
times. The time variation of the internal space results in a large time
variation of the fine structure constant which violates the observational
constraint on the variation of the fine structure constant. The large variation
of the fine structure constant is a generic feature of transient acceleration
models.Comment: 9 pages, 3 figures, use iopart, v2; references updated, accepted for
publication in Class. Quantum Gra
N=2 supergravity in five dimensions revisited
We construct matter-coupled N=2 supergravity in five dimensions, using the
superconformal approach. For the matter sector we take an arbitrary number of
vector-, tensor- and hyper-multiplets. By allowing off-diagonal vector-tensor
couplings we find more general results than currently known in the literature.
Our results provide the appropriate starting point for a systematic search for
BPS solutions, and for applications of M-theory compactifications on Calabi-Yau
manifolds with fluxes.Comment: 35 pages; v.2: A sign changed in a bilinear fermion term in (5.7
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
A deformation of AdS_5 x S^5
We analyse a one parameter family of supersymmetric solutions of type IIB
supergravity that includes AdS_5 x S^5. For small values of the parameter the
solutions are causally well-behaved, but beyond a critical value closed
timelike curves (CTC's) appear. The solutions are holographically dual to N=4
supersymmetric Yang-Mills theory on a non-conformally flat background with
non-vanishing R-currents. We compute the holographic energy-momentum tensor for
the spacetime and show that it remains finite even when the CTC's appear. The
solutions, as well as the uplift of some recently discovered AdS_5 black hole
solutions, are shown to preserve precisely two supersymmetries.Comment: 16 pages, v2: typos corrected and references adde
Answering a Basic Objection to Bang/Crunch Holography
The current cosmic acceleration does not imply that our Universe is basically
de Sitter-like: in the first part of this work we argue that, by introducing
matter into *anti-de Sitter* spacetime in a natural way, one may be able to
account for the acceleration just as well. However, this leads to a Big Crunch,
and the Euclidean versions of Bang/Crunch cosmologies have [apparently]
disconnected conformal boundaries. As Maldacena and Maoz have recently
stressed, this seems to contradict the holographic principle. In the second
part we argue that this "double boundary problem" is a matter not of geometry
but rather of how one chooses a conformal compactification: if one chooses to
compactify in an unorthodox way, then the appearance of disconnectedness can be
regarded as a *coordinate effect*. With the kind of matter we have introduced
here, namely a Euclidean axion, the underlying compact Euclidean manifold has
an unexpectedly non-trivial topology: it is in fact one of the 75 possible
underlying manifolds of flat compact four-dimensional Euclidean spaces.Comment: 29 pages, 3 figures, added references and comparison with "cyclic"
cosmology, JHEP versio
Quintessential Maldacena-Maoz Cosmologies
Maldacena and Maoz have proposed a new approach to holographic cosmology
based on Euclidean manifolds with disconnected boundaries. This approach
appears, however, to be in conflict with the known geometric results [the
Witten-Yau theorem and its extensions] on spaces with boundaries of
non-negative scalar curvature. We show precisely how the Maldacena-Maoz
approach evades these theorems. We also exhibit Maldacena-Maoz cosmologies with
[cosmologically] more natural matter content, namely quintessence instead of
Yang-Mills fields, thereby demonstrating that these cosmologies do not depend
on a special choice of matter to split the Euclidean boundary. We conclude that
if our Universe is fundamentally anti-de Sitter-like [with the current
acceleration being only temporary], then this may force us to confront the
holography of spaces with a connected bulk but a disconnected boundary.Comment: Much improved exposition, exponent in Cai-Galloway theorem fixed,
axionic interpretation of scalar explained, JHEP version. 33 pages, 3 eps
figure
Warped Tachyonic Inflation in Type IIB Flux Compactifications and the Open-String Completeness Conjecture
We consider a cosmological scenario within the KKLT framework for moduli
stabilization in string theory. The universal open string tachyon of decaying
non-BPS D-brane configurations is proposed to drive eternal topological
inflation. Flux-induced `warping' can provide the small slow-roll parameters
needed for successful inflation. Constraints on the parameter space leading to
sufficient number of e-folds, exit from inflation, density perturbations and
stabilization of the Kahler modulus are investigated. The conditions are
difficult to satisfy in Klebanov-Strassler throats but can be satisfied in T^3
fibrations and other generic Calabi-Yau manifolds. This requires large volume
and magnetic fluxes on the D-brane. The end of inflation may or may not lead to
cosmic strings depending on the original non-BPS configuration. A careful
investigation of initial conditions leading to a phenomenologically viable
model for inflation is carried out. The initial conditions are chosen on the
basis of Sen's open string completeness conjecture. We find time symmetrical
bounce solutions without initial singularities for k=1 FRW models which are
correlated with an inflationary period. Singular big-bang/big-crunch solutions
also exist but do not lead to inflation. There is an intriguing correlation
between having an inflationary universe in 4 dimensions and 6 compact
dimensions or a big-crunch singularity and decompactification.Comment: 43 pages, 9 figures. v3: Typos correcte
Brane Inflation, Solitons and Cosmological Solutions: I
In this paper we study various cosmological solutions for a D3/D7 system
directly from M-theory with fluxes and M2-branes. In M-theory, these solutions
exist only if we incorporate higher derivative corrections from the curvatures
as well as G-fluxes. We take these corrections into account and study a number
of toy cosmologies, including one with a novel background for the D3/D7 system
whose supergravity solution can be completely determined. This new background
preserves all the good properties of the original model and opens up avenues to
investigate cosmological effects from wrapped branes and brane-antibrane
annihilation, to name a few. We also discuss in some detail semilocal defects
with higher global symmetries, for example exceptional ones, that could occur
in a slightly different regime of our D3/D7 model. We show that the D3/D7
system does have the required ingredients to realise these configurations as
non-topological solitons of the theory. These constructions also allow us to
give a physical meaning to the existence of certain underlying homogeneous
quaternionic Kahler manifolds.Comment: Harvmac, 115 pages, 9 .eps figures; v2: typos corrected, references
added and the last section expanded; v3: Few minor typos corrected and
references added. Final version to appear in JHE