577 research outputs found
A note on accelerating cosmologies from compactifications and S-branes
We give a simple interpretation of the recent solutions for cosmologies with
a transient accelerating phase obtained from compactification in hyperbolic
manifolds, or from S-brane solutions of string/M-theory. In the
four-dimensional picture, these solutions correspond to bouncing the radion
field off its exponential potential. Acceleration occurs at the turning point,
when the radion stops and the potential energy momentarily dominates. The
virtues and limitations of these approaches become quite transparent in this
interpretation.Comment: 9 pages, 1 figure. References adde
Comments on Supergravity Description of S-branes
This is a note on the coupled supergravity-tachyon matter system, which has
been earlier proposed as a candidate for the effective space-time description
of S-branes. In particular, we study an ansatz with the maximal
ISO(p+1)xSO(8-p,1) symmetry, for general brane dimensionality p and homogeneous
brane distribution in transverse space \rho_\perp. A simple application of
singularity theorems shows that (for p \le 7) the most general solution with
these symmetries is always singular. (This invalidates a recent claim in the
literature.) We include a few general comments about the possibility of
describing the decay of unstable D-branes in purely gravitational terms.Comment: 19 pages, refs adde
The evolutionary origin of CIPK16: a gene involved in enhanced salt tolerance
Abstract not availableShanika Amarasinghe, Nathan S. Watson-Haigh, Matthew Gilliham, Stuart Roy, Ute Bauman
Hyperbolic Space Cosmologies
We present a systematic study of accelerating cosmologies obtained from
M/string theory compactifications of hyperbolic spaces with time-varying
volume. A set of vacuum solutions where the internal space is a product of
hyperbolic manifolds is found to give qualitatively the same accelerating
four-dimensional FLRW universe behavior as a single hyperbolic space. We also
examine the possibility that our universe is a hyperbolic space and provide
exact Milne type solutions, as well as intersecting S-brane solutions. When
both the usual 4D spacetime and the m-dimensional internal space are
hyperbolic, we find eternally accelerating cosmologies for , with and
without form field backgrounds. In particular, the effective potential for a
magnetic field background in the large 3 dimensions is positive definite with a
local minimum and thus enhances the eternally accelerating expansion.Comment: 33 pages, 2 figures; v2 refs added; v3 minor change in text, JHEP
versio
Regular S-Brane Backgrounds
We construct time-dependent S-brane solutions to the supergravity field
equations in various dimensions which (unlike most such geometries) do not
contain curvature singularities. The configurations we consider are less
symmetric than are earlier solutions, with our simplest solution being obtained
by a simple analytical continuation of the Kerr geometry. We discuss in detail
the global structure and properties of this background. We then generalize it
to higher dimensions and to include more complicated field configurations -
like non vanishing scalars and antisymmetric tensor gauge potentials - by the
usual artifice of applying duality symmetries.Comment: 22 pages, 3 figures. Typos in eq.(2.6) correcte
Non-Standard Intersections of S-Branes in D=11 Supergravity
We construct new intersecting S-brane solutions in 11-dimensional
supergravity which do not have supersymmetric analogs. They are obtained by
letting brane charges to be proportional to each other. Solutions fall into two
categories with respect to whether there is a non-diagonal term to be cancelled
in the field equations or not. In each case we show that they can be
constructed by using a simple set of rules which is similar to the harmonic
function rule of the usual static p-branes. Furthermore, we study an
intersection where the Chern-Simons term makes a non-zero contribution to the
field equations. We show that this configuration has a singularity like other
S-branes.Comment: 13 pages, 2 figures;v2 Section 2.2 is improved with new examples,
references added;v3 typos correcte
Localized Intersections of Non-Extremal p-branes and S-branes
A class of solutions to Supergravity in 10 or 11 dimensions is presented
which extends the non-standard or semi-local intersections of Dp-branes to the
case of non-extremal p-branes. The type of non-extremal solutions involved in
the intersection is free and we provide two examples involving black-branes
and/or D-\bar{D} systems. After a rotation among the time coordinate and a
relatively transverse radial direction the solutions admit the interpretation
of an intersection among D-branes and S-branes. We speculate on the relevance
of these configurations both to study time dependent phenomena in the AdS/CFT
correspondence as well as to construct cosmological brane-world scenarios
within String Theory admitting accelerating expansion of the Universe.Comment: 31 pages, latex file; v2: typos corrected and references adde
BPS pp-wave brane cosmological solutions in string theory
We construct time dependent BPS pp-wave brane solutions in the context of
M-theory and type II supergravity. It is found that N-brane solutions we
considered satisfy the crossing rule as S-brane solutions but 1/8 supersymmetry
remains. By applying them to the cosmological setting, inflationary solutions
are obtained. During this inflation, the size of the extradimensions becomes
smaller than our four-dimensional spacetime dynamically. We also discuss the
mechanism for terminating this inflation and recovering the hot big-bang
universe.Comment: 10 page
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