6 research outputs found
From D3-Branes to Lifshitz Space-Times
We present a simple embedding of a z=2 Lifshitz space-time into type IIB
supergravity. This is obtained by considering a stack of D3-branes in type IIB
supergravity and deforming the world-volume by a plane wave. The plane wave is
sourced by the type IIB axion. The superposition of the plane wave and the
D3-branes is 1/4 BPS. The near horizon geometry of this configuration is a
5-dimensional z=0 Schroedinger space-time times a 5-sphere. This geometry is
also 1/4 BPS. Upon compactification along the direction in which the wave is
traveling the 5-dimensional z=0 Schroedinger space-time reduces to a
4-dimensional z=2 Lifshitz space-time. The compactification is such that the
circle is small for weakly coupled type IIB string theory. This reduction
breaks the supersymmetries. Further, we propose a general method to construct
analytic z=2 Lifshitz black brane solutions. The method is based on deforming
5-dimensional AdS black strings by an axion wave and reducing to 4-dimensions.
We illustrate this method with an example.Comment: version 3: version published in Classical and Quantum Gravit
Observational Bounds on Cosmic Doomsday
Recently it was found, in a broad class of models, that the dark energy
density may change its sign during the evolution of the universe. This may lead
to a global collapse of the universe within the time t_c ~ 10^{10}-10^{11}
years. Our goal is to find what bounds on the future lifetime of the universe
can be placed by the next generation of cosmological observations. As an
example, we investigate the simplest model of dark energy with a linear
potential V(\phi) =V_0(1+\alpha\phi). This model can describe the present stage
of acceleration of the universe if \alpha is small enough. However, eventually
the field \phi rolls down, V(\phi) becomes negative, and the universe
collapses. The existing observational data indicate that the universe described
by this model will collapse not earlier than t_c > 10 billion years from the
present moment. We show that the data from SNAP and Planck satellites may
extend the bound on the "doomsday" time to t_c > 40 billion years at the 95%
confidence level.Comment: 11 pages, 6 figures, revtex
An Inflaton Mass Problem in String Inflation from Threshold Corrections to Volume Stabilization
Inflationary models whose vacuum energy arises from a D-term are believed not
to suffer from the supergravity eta problem of F-term inflation. That is,
D-term models have the desirable property that the inflaton mass can naturally
remain much smaller than the Hubble scale. We observe that this advantage is
lost in models based on string compactifications whose volume is stabilized by
a nonperturbative superpotential: the F-term energy associated with volume
stabilization causes the eta problem to reappear. Moreover, any shift
symmetries introduced to protect the inflaton mass will typically be lifted by
threshold corrections to the volume-stabilizing superpotential. Using threshold
corrections computed by Berg, Haack, and Kors, we illustrate this point in the
example of the D3-D7 inflationary model, and conclude that inflation is
possible, but only for fine-tuned values of the stabilized moduli. More
generally, we conclude that inflationary models in stable string
compactifications, even D-term models with shift symmetries, will require a
certain amount of fine-tuning to avoid this new contribution to the eta
problem.Comment: 25 page
Testing the Cosmological Constant as a Candidate for Dark Energy
It may be difficult to single out the best model of dark energy on the basis
of the existing and planned cosmological observations, because many different
models can lead to similar observational consequences. However, each particular
model can be studied and either found consistent with observations or ruled
out. In this paper, we concentrate on the possibility to test and rule out the
simplest and by far the most popular of the models of dark energy, the theory
described by general relativity with positive vacuum energy (the cosmological
constant). We evaluate the conditions under which this model could be ruled out
by the future observations made by the Supernova/Acceleration Probe SNAP (both
for supernovae and weak lensing) and by the Planck Surveyor cosmic microwave
background satellite.Comment: 6 pages, 2 figures, revtex
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
The Bekenstein Formula and String Theory (N-brane Theory)
A review of recent progress in string theory concerning the Bekenstein
formula for black hole entropy is given. Topics discussed include p-branes,
D-branes and supersymmetry; the correspondence principle; the D- and M-brane
approach to black hole entropy; the D-brane analogue of Hawking radiation, and
information loss; D-branes as probes of black holes; and the Matrix theory
approach to charged and neutral black holes. Some introductory material is
included.Comment: 53 pages, LaTeX. v3: Typos fixed, minor updates, references added,
brief Note Added on AdS/CF