1,086 research outputs found
Warped brane-world compactification with Gauss-Bonnet term
In the Randall-Sundrum (RS) brane-world model a singular delta-function
source is matched by the second derivative of the warp factor. So one should
take possible curvature corrections in the effective action of the RS models in
a Gauss-Bonnet (GB) form. We present a linearized treatment of gravity in the
RS brane-world with the Gauss-Bonnet modification to Einstein gravity. We give
explicit expressions for the Neumann propagator in arbitrary D dimensions and
show that a bulk GB term gives, along with a tower of Kaluza-Klein modes in the
bulk, a massless graviton on the brane, as in the standard RS model. Moreover,
a non-trivial GB coupling can allow a new branch of solutions with finite
Planck scale and no naked bulk singularity, which might be useful to avoid some
of the previously known ``no--go theorems'' for RS brane-world
compactifications.Comment: 23 pages, typos in Secs. 5 & 6 corrected, expanded/published version
(IJMPA
Reconstructing a model of quintessential inflation
We present an explicit cosmological model where inflation and dark energy
both could arise from the dynamics of the same scalar field. We present our
discussion in the framework where the inflaton field attains a nearly
constant velocity (where
is the e-folding time) during inflation. We show that the model
with and can easily satisfy inflationary constraints,
including the spectral index of scalar fluctuations (),
tensor-to-scalar ratio () and also the bound imposed on
during the nucleosynthesis epoch (). In our
construction, the scalar field potential always scales proportionally to the
square of the Hubble expansion rate. One may thereby account for the two vastly
different energy scales associated with the Hubble parameters at early and late
epochs. The inflaton energy could also produce an observationally significant
effective dark energy at a late epoch without violating local gravity tests.Comment: 18 pages, 7 figures; added refs, published versio
Accelerating universe from warped extra dimensions
Accelerating universe or the existence of a small and positive cosmological
constant is probably the most pressing obstacle as well as opportunity to
significantly improving the models of four-dimensional cosmology from
fundamental theories of gravity, including string theory. In seeking to resolve
this problem, one naturally wonders if the real world can somehow be
interpreted as an inflating de Sitter brane embedded in a higher-dimensional
spacetime described by warped geometry. In this scenario, the four-dimensional
cosmological constant may be uniquely determined in terms of two length scales:
one is a scale associated with the size of extra dimensions and the other is a
scale associated with the expansion rate of our universe. In some specific
cases, these two scales are complementary to each other. This result is
demonstrated here by presenting some explicit and completely non-singular de
Sitter space dS solutions of vacuum Einstein equations in five and ten
dimensions.Comment: 7 pages; extended (from journal) version, minor typos fixed, refs
adde
Warped compactification on curved manifolds
The characterization of a six- (or seven)-dimensional internal manifold with
metric as having positive, zero or negative curvature is expected to be an
important aspect of warped compactifications in supergravity. In this context,
Douglas and Kallosh recently pointed out that a compact internal space with
negative curvature could help to construct four-dimensional de Sitter solutions
only if the extra dimensions are strongly warped or there are large stringy
corrections. That is, the problem of finding 4-dimensional de Sitter solutions
is well posed, if all extra dimensions are physically compact, which is called
a no-go theorem. Here, we show that the above conclusion does not extend to a
general class of warped compactifications in classical supergravity that allow
a non-compact direction or cosmological solutions for which the internal space
is asymptotic to a cone over a product of compact Einstein spaces or spheres.
For clarity, we present classical solutions that compactify higher-dimensional
spacetime to produce a Robertson--Walker universe with de Sitter-type expansion
plus one extra non-compact direction. Such models are found to admit both an
effective four-dimensional Newton constant that remains finite and a
normalizable zero-mode graviton wavefunction. We also exhibit the possibility
of obtaining 4D de Sitter solutions by including the effect of fluxes (p-form
field strengths).Comment: 24 pages, 1 figure; v5 significant changes in the presentation,
published (journal) versio
Phylogenomics of the pantropical tribe Spermacoceae: resolving taxonomic challenges and phylogenetic relationships using whole plastome data
The pantropical tribe Spermacoceae comprises approximately 1000 species and belongs to the largest subfamily, Rubioideae, within the Rubiaceae family. The genera within the Spermacoceae tribe have a long history of taxonomic confusión and disagreement due to the utilization of inconsistent and overlapping characters in their generic delimitations. Previous molecular phylogenetic studies have made significant contributions to resolving many taxonomic inconsistencies, primarily focusing on taxa from Asia-Pacific and Australia. However, challenges persist in understanding the phylogenetic relationships and taxonomy o f Spermacoceae members from the Americas and Africa. In this study, we employ whole-genome plastid data of Spermacoceae members to address these challenges. Our presentation will primarily focus on two objectives: (1) presenting the first plastome phylogeny of Spermacoceae, primarily representing members from North America, and (2) investigating phylogenetic congruence among genes and phylogenetic signáis within genes using Bayesian and other methods. By shedding light on the phylogenetic relationships and resolving taxonomic inconsistencies, our research contributes to a better understanding of the evolutionary history and taxonomy o f the Spermacoceae tribe. This work is part of an ongoing effort to construct a global genome-wide phylogeny of the Spermacoceae tribe through the utilization of high-throughput DNA data
Late-time Cosmic Dynamics from M-theory
We consider the behaviour of the cosmological acceleration for time-dependent
hyperbolic and flux compactifications of M-theory, with an exponential
potential. For flat and closed cosmologies it is seen that a positive
acceleration is always transient for both compactifications. For open
cosmologies, both compactifications can give at late times periods of positive
acceleration. As a function of proper time this acceleration has a power law
decay and can be either positive, negative or oscillatory.Comment: 10 pages, LaTeX, 2 figure
On compatibility of string effective action with an accelerating universe
In this paper, we fully investigate the cosmological effects of the moduli
dependent one-loop corrections to the gravitational couplings of the string
effective action to explain the cosmic acceleration problem in early (and/or
late) universe. These corrections comprise a Gauss-Bonnet (GB) invariant
multiplied by universal non-trivial functions of the common modulus
and the dilaton . The model exhibits several features of cosmological
interest, including the transition between deceleration and acceleration
phases. By considering some phenomenologically motivated ansatzs for one of the
scalars and/or the scale factor (of the universe), we also construct a number
of interesting inflationary potentials. In all examples under consideration, we
find that the model leads only to a standard inflation () when the
numerical coefficient associated with modulus-GB coupling is positive,
while the model can lead also to a non-standard inflation (), if
is negative. In the absence of (or trivial) coupling between the GB term and
the scalars, there is no crossing between the phases, while
this is possible with non-trivial GB couplings, even for constant dilaton phase
of the standard picture. Within our model, after a sufficient amount of e-folds
of expansion, the rolling of both fields and can be small. In
turn, any possible violation of equivalence principle or deviations from the
standard general relativity may be small enough to easily satisfy all
astrophysical and cosmological constraints.Comment: 30 pages, 8 figures; v2 significant changes in notations, appendix
and refs added; v3 significant revisions, refs added; v4 appendix extended,
new refs, published versio
Completely localized gravity with higher curvature terms
In the intersecting braneworld models, higher curvature corrections to the
Einstein action are necessary to provide a non-trivial geometry (brane tension)
at the brane junctions. By introducing such terms in a Gauss-Bonnet form, we
give an effective description of localized gravity on the singular
delta-function branes. There exists a non-vanishing brane tension at the
four-dimensional brane intersection of two 4-branes. Importantly, we give
explicit expressions of the graviton propagator and show that the
Randall-Sundrum single-brane model with a Gauss-Bonnet term in the bulk
correctly gives a massless graviton on the brane as for the RS model. We
explore some crucial features of completely localized gravity in the solitonic
braneworld solutions obtained with a choice (\xi=1) of solutions. The no-go
theorem known for Einstein's theory may not apply to the \xi=1 solution. As
complementary discussions, we provide an effective description of the power-law
corrections to Newtonian gravity on the branes or at the common intersection
thereof.Comment: 19 pages, LaTeX, Revised/Published Versio
Towards inflation and dark energy cosmologies from modified Gauss-Bonnet theory
We consider a physically viable cosmological model that has a field dependent
Gauss-Bonnet coupling in its effective action, in addition to a standard scalar
field potential. The presence of such terms in the four dimensional effective
action gives rise to several novel effects, such as a four dimensional flat
Friedmann-Robertson-Walker universe undergoing a cosmic inflation at early
epoch, as well as a cosmic acceleration at late times. The model predicts,
during inflation, spectra of both density perturbations and gravitational waves
that may fall well within the experimental bounds. Furthermore, this model
provides a mechanism for reheating of the early universe, which is similar to a
model with some friction terms added to the equation of motion of the scalar
field, which can imitate energy transfer from the scalar field to matterComment: 35 pages, 21 eps figs; section 6 expanded improving explanations,
refs added, final in JCA
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