264 research outputs found
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
Squeezed Thermal Vacuum and the Maximum Scale for Inflation
We consider the stimulated emission of gravitons from an initial state of
thermal equilibrium, under the action of the cosmic gravitational background
field. We find that the low-energy graviton spectrum is enhanced if compared
with spontaneous creation from the vacuum; as a consequence, the scale of
inflation must be lowered, in order not to exceed the observed CMB quadrupole
anisotropy. This effect is particularly important for models based on a
symmetry-breaking transition which require, as initial condition, a state of
thermal equilibrium at temperatures of the order of the inflation scale.Comment: 13 pages, plain tex, three figures available upon request, to appear
in Phys.Rev.D, CERN-TH.6836/9
Primordial Magnetic Fields From String Cosmology
Sufficiently large seeds for generating the observed (inter)galactic magnetic
fields emerge naturally in string cosmology from the amplification of
electromagnetic vacuum fluctuations due to a dynamical dilaton background. The
success of the mechanism depends crucially on two features of the so-called
pre-big-bang scenario, an early epoch of dilaton-driven inflation at very small
coupling, and a sufficiently long intermediate stringy era preceding the
standard radiation-dominated evolution.Comment: 12 pages, latex, two figures available by fax upon reques
Quintessence as a run-away dilaton
We consider a late-time cosmological model based on a recent proposal that
the infinite-bare-coupling limit of superstring/M-theory exists and has good
phenomenological properties, including a vanishing cosmological constant, and a
massless, decoupled dilaton. As it runs away to , the dilaton can
play the role of the quintessence field recently advocated to drive the
late-time accelerated expansion of the Universe. If, as suggested by some
string theory examples, appreciable deviations from General Relativity persist
even today in the dark matter sector, the Universe may smoothly evolve from an
initial "focusing" stage, lasting untill radiation--matter equality, to a
"dragging" regime, which eventually gives rise to an accelerated expansion with
frozen .Comment: 31 pages, latex, 5 figures included using epsfig. New references
added and misprints corrected. To appear in Phys. Rev.
Scalar fluctuations in dilatonic brane-worlds
We derive and solve the full set of scalar perturbation equations for a class
of five-dimensional brane--world solutions, with a dilaton scalar field coupled
to the bulk cosmological constant and to a 3-brane. The spectrum contains one
localized massless scalar mode, to be interpreted as an effective dilaton on
the brane, inducing long--range scalar interactions. Two massive scalar modes
yield corrections to Newton's law at short distances, which persist even in the
limit of vanishing dilaton (namely, in the standard Randall--Sundrum
configuration).Comment: 10 pages. Talk presented by V. Bozza at COSMO-01 conference,
Rovaniemi, 200
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
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
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.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. Thanks to the strong coupling there is also an associate production of higher-dimensional branes, which could prepare (and provide the initial conditions for) a subsequent phase of brane-dominated inflation.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
Assisting pre-big bang phenomenology through short-lived axions
We present the results of a detailed study of how isocurvature axion
fluctuations are converted into adiabatic metric perturbations through axion
decay, and discuss the constraints on the parameters of pre-big bang cosmology
needed for consistency with present CMB-anisotropy data. The large-scale
normalization of temperature fluctuations has a non-trivial dependence both on
the mass and on the initial value of the axion. In the simplest, minimal models
of pre-big bang inflation, consistency with the COBE normalization requires a
slightly tilted (blue) spectrum, while a strictly scale-invariant spectrum
requires mild modifications of the minimal backgrounds at large curvature
and/or string coupling.Comment: 14 pages, latex, 1 figure included using epsfig. A few typos
corrected, two references added, the figure slightly improved. To appear in
Phys. Lett.
Constraints on pre-big bang parameter space from CMBR anisotropies
The so-called curvaton mechanism --a way to convert isocurvature
perturbations into adiabatic ones-- is investigated both analytically and
numerically in a pre-big bang scenario where the role of the curvaton is played
by a sufficiently massive Kalb--Ramond axion of superstring theory. When
combined with observations of CMBR anisotropies at large and moderate angular
scales, the present analysis allows us to constrain quite considerably the
parameter space of the model: in particular, the initial displacement of the
axion from the minimum of its potential and the rate of evolution of the
compactification volume during pre-big bang inflation. The combination of
theoretical and experimental constraints favours a slightly blue spectrum of
scalar perturbations, and/or a value of the string scale in the vicinity of the
SUSY-GUT scale.Comment: 63 pages in Latex style with 14 figures include
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