92 research outputs found
A Causal Entropy Bound
The identification of a causal-connection scale motivates us to propose a new
covariant bound on entropy within a generic space-like region. This "causal
entropy bound", scaling as the square root of EV, and thus lying around the
geometric mean of Bekenstein's S/ER and holographic S/A bounds, is checked in
various "critical" situations. In the case of limited gravity, Bekenstein's
bound is the strongest while naive holography is the weakest. In the case of
strong gravity, our bound and Bousso's holographic bound are stronger than
Bekenstein's, while naive holography is too tight, and hence typically wrong.Comment: 12 pages, no figures, a reference added and minor typos correcte
Tensor perturbations in high-curvature string backgrounds
We derive a generalized equation for the evolution of tensor perturbations in
a cosmological background, taking into account higher-curvature contributions
and a tree-level coupling to the dilaton in the string frame. The equation is
obtained by perturbing the gravi-dilaton string effective action, expanded up
to first order in . The corrections can modify the
low-energy perturbation spectrum, but the modifications are shown to be small
when the background curvature keeps constant in the string frame.Comment: 9 pages, REVTEX, three figures included using EPSFIG. An updated
collection of papers on the pre-big bang scenario in string cosmology is a
available at http://www.to.infn.it/teorici/gasperin
Perturbative stability of the QCD analysis of DIS data
We perform pQCD analysis of the existing DIS data for charged leptons with
account of corrections up to the NNLO. The parton distributions, value of
strong coupling constant, and high-twist terms are extracted and their
stability with respect to account of the NNLO corrections is analyzed. All the
quantities are generally stable within their experimental errors. Obtained
value of the strong coupling constant is with a guess .Comment: 4 pages, LATEX, 3 figures (EPS). Talk presented at the 37th
Rencontres de Moriond, QCD and Hadronic Interactions, Les Arcs 1800 (France),
March 16-23 200
Higher-Derivative Quantum Cosmology
The quantum cosmology of a higher-derivative derivative gravity theory
arising from the heterotic string effective action is reviewed. A new type of
Wheeler-DeWitt equation is obtained when the dilaton is coupled to the
quadratic curvature terms. Techniques for solving the Wheeler-DeWitt equation
with appropriate boundary conditions shall be described, and implications for
semiclassical theories of inflationary cosmology will be outlined.Comment: 11 pages TeX. A term has been removed from equation (13
AdS/CFT and quantum-corrected brane entropy
It is shown that quantum-induced (inflationary) brane Universe occurs in the
bulk 5d AdS black hole in accordance with AdS/CFT correspondence. Brane stress
tensor is induced by quantum effects of dual CFT and brane crosses the horizon
of AdS black hole. Quantum-corrected Hubble constant, Hawking temperature and
entropy are found on the brane (and at the horizon). The similarity between CFT
entropy at the horizon and FRW equations is extended on the quantum level. This
suggests the way to understand cosmological entropy bounds in quantum gravity.Comment: LaTeX file 17 pages, discussion is extended, version to appear in CQ
Electroweak baryogenesis induced by a scalar field
A cosmological pseudoscalar field coupled to hypercharge topological number
density can exponentially amplify hyperelectric and hypermagnetic fields while
coherently rolling or oscillating, leading to the formation of a time-dependent
condensate of topological number density. The topological condensate can be
converted, under certain conditions, into baryons in sufficient quantity to
explain the observed baryon asymmetry in the universe. The amplified
hypermagnetic field can perhaps sufficiently strengthen the electroweak phase
transition, and by doing so, save any pre-existing baryon number asymmetry from
extinction.Comment: 8 pages, 4 figure
A plausible upper limit on the number of e-foldings
Based solely on the arguments relating Friedmann equation and the Cardy
formula we derive a bound for the number of e-folds during inflation for a
standard Friedmann-Robertson-Walker as well as non-standard four dimensional
cosmology induced by a Randall-Sundrum-type model.Comment: 4 pages. Version appearing in the Physical Review
Low energy effective string cosmology
We give the general analytic solutions derived from the low energy string
effective action for four dimensional Friedmann-Robertson-Walker models with
dilaton and antisymmetric tensor field, considering both long and short
wavelength modes of the -field. The presence of a homogeneous -field
significantly modifies the evolution of the scale factor and dilaton. In
particular it places a lower bound on the allowed value of the dilaton. The
scale factor also has a lower bound but our solutions remain singular as they
all contain regions where the spacetime curvature diverges signalling a
breakdown in the validity of the effective action. We extend our results to the
simplest Bianchi I metric in higher dimensions with only two scale factors. We
again give the general analytic solutions for long and short wavelength modes
for the field restricted to the three dimensional space, which produces an
anisotropic expansion. In the case of field radiation (wavelengths within
the Hubble length) we obtain the usual four dimensional radiation dominated FRW
model as the unique late time attractor.Comment: 22 pages, LaTeX, SUSX-TH-94/37, SUSSEX-AST-94/6-2. (Some terminology
and figure captions corrected, references added.
Holographic principle in the BDL brane cosmology
We study the holographic principle in the brane cosmology. Especially we
describe how to accommodate the 5D anti de Sitter Schwarzschild (AdSS)
black hole in the Binetruy-Deffayet-Langlois (BDL) approach of brane cosmology.
It is easy to make a connection between a mass of the AdSS black hole
and a conformal field theory (CFT)-radiation dominated universe on the brane in
the moving domain wall approach. But this is not established in the BDL
approach. In this case we use two parameters in the Friedmann
equation. These arise from integration and are really related to the choice of
initial bulk matter. If one chooses a bulk energy density to account
for a mass of the AdSS black hole and the static fifth dimension, a
CFT-radiation term with comes out from the bulk
matter without introducing a localized matter distribution on the brane. This
means that the holographic principle can be established in the BDL brane
cosmology.Comment: 9 pages, a version to appear in PR
Numerical Study of Inhomogeneous Pre-Big-Bang Inflationary Cosmology
We study numerically the inhomogeneous pre-big-bang inflation in a
spherically symmetric space-time. We find that a large initial inhomogeneity
suppresses the onset of the pre-big-bang inflation. We also find that even if
the pre-big-bang inflationary stage is realized, the initial inhomogeneities
are not homogenized. Namely, during the pre-big-bang inflation
``hairs''(irregularities) do not fall, in sharp contrast to the usual
(potential energy dominated) inflation where initial inhomogeneity and
anisotropy are damped and thus the resulting universe is less sensitive to
initial conditions.Comment: 12 pages + 14 figures, to be published in Phys.Rev.
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