1,620 research outputs found
Low-energy graceful exit in anisotropic string cosmology backgrounds
We discuss the possibility of a smooth transition from the pre- to the
post-big bang regime, in the context of the lowest-order string effective
action (without higher-derivative corrections), taking into account with a
phenomenological model of source the repulsive gravitational effects due to the
back-reaction of the quantum fluctuations outside the horizon. We determine a
set of necessary conditions for a successful and realistic transition, and we
find that such conditions can be satisfied (by an appropriate model of source),
provided the background is higher-dimensional and anisotropic.Comment: 13 pages, latex, 2 figures included using epsfi
Relic Gravitational Waves from String Cosmology
A large class of string-cosmology backgrounds leads to a spectrum of relic
stochastic gravitational waves, strongly tilted towards high frequencies, and
characterized by two basic parameters of the cosmological model. We estimate
the required sensitivity for detection of the predicted gravitational radiation
and show that a region of our parameter space is within reach for some of the
planned gravitational-wave detectors.Comment: 14 pages, latex, 2 figures included as epsf file
String Theory and Pre-big bang Cosmology
In string theory, the traditional picture of a Universe that emerges from the
inflation of a very small and highly curved space-time patch is a possibility,
not a necessity: quite different initial conditions are possible, and not
necessarily unlikely. In particular, the duality symmetries of string theory
suggest scenarios in which the Universe starts inflating from an initial state
characterized by very small curvature and interactions. Such a state, being
gravitationally unstable, will evolve towards higher curvature and coupling,
until string-size effects and loop corrections make the Universe "bounce" into
a standard, decreasing-curvature regime. In such a context, the hot big bang of
conventional cosmology is replaced by a "hot big bounce" in which the bouncing
and heating mechanisms originate from the quantum production of particles in
the high-curvature, large-coupling pre-bounce phase. Here we briefly summarize
the main features of this inflationary scenario, proposed a quarter century
ago. In its simplest version (where it represents an alternative and not a
complement to standard slow-roll inflation) it can produce a viable spectrum of
density perturbations, together with a tensor component characterized by a
"blue" spectral index with a peak in the GHz frequency range. That means,
phenomenologically, a very small contribution to a primordial B-mode in the CMB
polarization, and the possibility of a large enough stochastic background of
gravitational waves to be measurable by present or future gravitational wave
detectors.Comment: 25 pages, five figures. Contribution to the special issue of IL NUOVO
CIMENTO, published in honor of Gaetano Vilasi on the occasion of his 70-th
birthday (Il Nuovo Cimento C, Italian Physical Society, 2015
Inhomogeneous Pre-Big Bang String Cosmology
An inhomogeneous version of pre--Big Bang cosmology emerges, within string
theory, from quite generic initial conditions, provided they lie deeply inside
the weak-coupling, low-curvature regime. Large-scale homogeneity, flatness, and
isotropy appear naturally as late-time outcomes of such an evolution.Comment: 13 pages, Latex, references adde
Graceful exit in quantum string cosmology
We write an O(d,d)-covariant Wheeler-De Witt equation in the
()-dimensional minisuperspace of low-energy cosmological string
backgrounds. We discuss explicit examples of transitions between two
duality-related cosmological phases, and we find a finite quantum transition
probability even when the two phases are classically separated by a curvature
singularity. This quantum approach is completely free from operator ordering
ambiguities as a consequence of the duality symmetries of the string effective
action.Comment: 16 pages, LATEX. A complete collection of papers and references on
the pre-big-bang scenario in string cosmology is available at
http://www.to.infn.it/teorici/gasperini
A new approach to the propagation of light-like signals in perturbed cosmological backgrounds
We present a new method to compute the deflection of light rays in a
perturbed FLRW geometry. We exploit the properties of the Geodesic Light Cone
(GLC) gauge where null rays propagate at constant angular coordinates
irrespectively of the given (inhomogeneous and/or anisotropic) geometry. The
gravitational deflection of null geodesics can then be obtained, in any other
gauge, simply by expressing the angular coordinates of the given gauge in terms
of the GLC angular coordinates. We apply this method to the standard Poisson
gauge, including scalar perturbations, and give the full result for the
deflection effect in terms of the direction of observation and observed
redshift up to second order, and up to third order for the leading lensing
terms. We also compare our results with those presently available in the
literature and, in particular, we provide a new non trivial check of a previous
result on the luminosity-redshft relation up to second order in cosmological
perturbation theory.Comment: 37 pages, no figures. Typos corrected, comments and references added.
Version accepted for publication in JCA
Relic gravitons on Kasner-like branes
We discuss the cosmological amplification of tensor perturbations in a simple
example of brane-world scenario, in which massless gravitons are localized on a
higher-dimensional Kasner-like brane embedded in a bulk AdS background.
Particular attention is paid to the canonical normalization of the quadratic
action describing the massless and massive vacuum quantum fluctuations, and to
the exact mass-dependence of the amplitude of massive fluctuations on the
brane. The perturbation equations can be separated. In contrast to de Sitter
models of brane inflation, we find no mass gap in the spectrum and no
enhancement for massless modes at high curvature. The massive modes can be
amplified, with mass-dependent amplitudes, even during inflation and in the
absence of any mode-mixing effect.Comment: 11 pages, no figures, to appear in Phys. Lett.
Singularity and exit problems in two-dimensional string cosmology
A broad class of two-dimensional loop-corrected dilaton gravity models
exhibit cosmological solutions that interpolate between the string perturbative
vacuum and a background with asymptotically flat metric and linearly growing
dilaton. The curvature singularities of the corresponding tree-level solutions
are smoothed out, but no branch-change occurs. Thus, even in the presence of a
non-perturbative potential, the system is not attracted by physically
interesting fixed points with constant dilaton, and the exit problem of string
cosmology persists.Comment: 11 pages, LATEX, one figure included using epsf. See also
http://www.to.infn.it/teorici/gasperini
An exact Jacobi map in the geodesic light-cone gauge
The remarkable properties of the recently proposed geodesic light-cone (GLC)
gauge allow to explicitly solve the geodetic-deviation equation, and thus to
derive an exact expression for the Jacobi map J^A_B(s,o) connecting a generic
source s to a geodesic observer o in a generic space time. In this gauge J^A_B
factorizes into the product of a local quantity at s times one at o, implying
similarly factorized expressions for the area and luminosity distance. In any
other coordinate system J^A_B is simply given by expressing the GLC quantities
in terms of the corresponding ones in the new coordinates. This is explicitly
done, at first and second order, respectively, for the synchronous and Poisson
gauge-fixing of a perturbed, spatially-flat cosmological background, and the
consistency of the two outcomes is checked. Our results slightly amend previous
calculations of the luminosity-redshift relation and suggest a possible
non-perturbative way for computing the effects of inhomogeneities on
observations based on light-like signals.Comment: 26 pages, no figures. Inconsequential modification of an equation,
comments and references added. Version accepted for publication in JCA
Macroscopic Quantum Tunneling and the "cosmic" Josephson effect
We discuss the possible influence of a cosmic magnetic field on the
macroscopic quantum tunneling process associated, in a cosmological context, to
the decay of the "false vacuum." We find a close analogy with the effects of an
external magnetic field applied to a Josephson junction in the context of
low-temperature/high-temperature superconducting devices.Comment: 4 pages, 1 pdf figure. Added references and an inset in Fig. 1,
results unchanged. To appear in Phys. Rev. D (Brief Report
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