1,187 research outputs found
Black Holes, Entropy Bound and Causality Violation
The gravity/gauge theory duality has provided us a way of studying QCD at
short distances from straightforward calculations in classical general
relativity. Among numerous results obtained so far, one of the most striking is
the universality of the ratio of the shear viscosity to the entropy density.
For all gauge theories with Einstein gravity dual, this ratio is \eta/s=1/4\pi.
However, in general higher-curvature gravity theories, including two concrete
models under discussion - the Gauss-Bonnet gravity and the (Riemann)^2 gravity
- the ratio \eta/s can be smaller than 1/4\pi (thus violating the conjecture
bound), equal to 1/4\pi or even larger than 1/4\pi. As we probe spacetime at
shorter distances, there arises an internal inconsistency in the theory, such
as a violation of microcausality, which is correlated with a classical limit on
black hole entropy.Comment: 8 pages, no figures; Invited contribution to appear in the
Proceedings of the 75 Years since Solvay, Singapore, Nov 2008, (World
Scientific, Singapore, 2009
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
Cosmic Acceleration from M Theory on Twisted Spaces
In a recent paper [I.P. Neupane and D.L. Wiltshire, Phys. Lett. B 619, 201
(2005).] we have found a new class of accelerating cosmologies arising from a
time--dependent compactification of classical supergravity on product spaces
that include one or more geometric twists along with non-trivial curved
internal spaces. With such effects, a scalar potential can have a local minimum
with positive vacuum energy. The existence of such a minimum generically
predicts a period of accelerated expansion in the four-dimensional
Einstein-conformal frame. Here we extend our knowledge of these cosmological
solutions by presenting new examples and discuss the properties of the
solutions in a more general setting. We also relate the known (asymptotic)
solutions for multi-scalar fields with exponential potentials to the
accelerating solutions arising from simple (or twisted) product spaces for
internal manifolds.Comment: 23 pages, 3 figures; added a summary Table, PRD versio
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
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
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
Coexistence of competing orders with two energy gaps in real and momentum space in high-Tc superconductor Bi2Sr2-xLaxCuO6+delta
The superconducting phase of the high-Tc cuprates has been thought to be
described by a single d-wave pairing order parameter. Recently, there has been
growing evidence suggesting that another form of order, possibly inherited from
the pseudogap phase above Tc, may coexist with superconductivity in the
underdoped regime. Through a combined study of scanning tunneling microscopy
and angle-resolved photoemission spectroscopy, we report the observation of two
distinct gaps (a small-gap and a large-gap) that coexist both in real space and
in the anti-nodal region of momentum space in the superconducting phase of
Bi2Sr2-xLaxCuO6+delta. We show that the small-gap is associated with
superconductivity. The large-gap persists to temperatures above the transition
temperature Tc and is found to be linked to short-range charge ordering.
Remarkably, we find a strong, short-ranged correlation between the local small-
and large- gap magnitudes suggesting that the superconductivity and charge
ordering are affected by similar physical processes.Comment: 19 pages, 4 figure
Unusual photoemission resonances of oxygen-dopant induced states in BiSrCaCuO
We have performed an angular-resolved photoemission study of underdoped,
optimally doped and overdoped BiSrCaCuO samples using a
wide photon energy range (15 - 100 eV). We report a small and broad
non-dispersive A peak in the energy distribution curves whose intensity
scales with doping. We attribute it to a local impurity state similar to the
one observed recently by scanning tunneling spectroscopy and identified as the
oxygen dopants. Detailed analysis of the resonance profile and comparison with
the single-layered BiSrCuO suggest a mixing of this local
state with Cu via the apical oxygens.Comment: 4 pages, 4 figure
Inflation and Quintessence: Theoretical Approach of Cosmological Reconstruction
In the first part of this paper, we outline the construction of an
inflationary cosmology in the framework where inflation is described by a
universally evolving scalar field, with the Lagrangian . By considering a generic situation
that inflaton attains a nearly constant velocity, during inflation, (where is the e-folding time), we find
the conditions that have to satisfied by the (reconstructed) scalar potential
to be consistent with the WMAP inflationary data. In the second part of this
paper, we introduce a novel approach of constructing dark energy within the
context of the standard scalar-tensor gravity. The assumption that a scalar
field might roll with a nearly constant velocity, during inflation, can also be
applied to {\it quintessence} or dark energy models. For the minimally coupled
quintessence, (where is the
standard matter-quintessence coupling), the dark energy equation of state in
the range can be obtained for . For
, the model allows for only modest evolution of dark energy density
with redshift. The effect of the matter-quintessence coupling can be
significant only if , while a small coupling
will have almost no effect on cosmological parameters. The
best fit value of in our model is found to be , but it may contain significant numerical errors, viz , which thereby implies the consistency of our model with general
relativity (for which ) at level.Comment: 33 pages, several figures; significant extension (models confronted
with data
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