1,503 research outputs found
Non-local dilaton coupling to dark matter: cosmic acceleration and pressure backreaction
A model of non-local dilaton interactions, motivated by string duality
symmetries, is applied to a scenario of "coupled quintessence" in which the
dilaton dark energy is non-locally coupled to the dark-matter sources. It is
shown that the non-local effects tend to generate a backreaction which -- for
strong enough coupling -- can automatically compensate the acceleration due to
the negative pressure of the dilaton potential, thus asymptotically restoring
the standard (dust-dominated) decelerated regime. This result is illustrated by
analytical computations and numerical examples.Comment: 11 pages, 1 figure ep
Repulsive gravity in the very early Universe
I present two examples in which the curvature singularity of a
radiation-dominated Universe is regularized by (a) the repulsive effects of
spin interactions, and (b) the repulsive effects arising from a breaking of the
local gravitational gauge symmetry. In both cases the collapse of an initial,
asymptotically flat state is stopped, and the Universe bounces towards a state
of decelerated expansion. The emerging picture is typical of the pre-big bang
scenario, with the main difference that the string cosmology dilaton is
replaced by a classical radiation fluid, and the solutions are not
duality-invariant.Comment: 9 pages, LATEX, one figure included using epsf. Awarded the Fourth
Prize in the 1998 Awards for Essays on Gravitation (Gravity Research
Foundation, Wellesley Hills, Ma). To appear in Gen. Rel. Grav. An updated
collection of papers on the pre-big bang scenario is available at
http://www.to.infn.it/~gasperi
Quantum Squeezing and Cosmological Entropy Production
The entropy growth in a cosmological process of pair production is completely
determined by the associated squeezing parameter, and is insensitive to the
number of particles in the initial state. The total produced entropy may
represent a significant fraction of the entropy stored today in the cosmic
black-body radiation, provided pair production originates from a change in the
background metric at a curvature scale of the Planck order.Comment: 7 pages, plain TEX, to appear in Class.Quantum Grav., CERN-TH.6954/9
Boundary Conditions in Quantum String Cosmology
We discuss in detail how to consistently impose boundary conditions in
quantum string cosmology. Since a classical time parameter is absent in quantum
gravity, such conditions must be imposed with respect to intrinsic variables.
Constructing wave packets for minisuperspace models from different tree-level
string effective actions, we explain in particular the meaning of a transition
between ``pre-big-bang" and ``post-big-bang" branches. This leads to a scenario
different from previous considerations.Comment: 16 pages, REVTEX, minor changes, two references adde
String cosmology versus standard and inflationary cosmology
This paper presents a review of the basic, model-independent differences
between the pre-big bang scenario, arising naturally in a string cosmology
context, and the standard inflationary scenario. We use an unconventional
approach in which the introduction of technical details is avoided as much as
possible, trying to focus the reader's attention on the main conceptual aspects
of both scenarios. The aim of the paper is not to conclude in favour either of
one or of the other scenario, but to raise questions that are left to the
reader's meditation. Warnings: the paper does not contain equations, and is not
intended as a complete review of all aspects of string cosmology.Comment: 22 pages, Latex (IOP Style), three figures included using epsfig. To
appear in Class. Quantum Grav. (Topical Review Section). Two misprints
correcte
Fully Anisotropic String Cosmologies, Maxwell Fields and Primordial Shear
We present a class of exact cosmological solutions of the low energy string
effective action in the presence of a homogeneous magnetic fields. We discuss
the physical properties of the obtained (fully anisotropic) cosmologies paying
particular attention to their vacuum limit and to the possible isotropization
mechanisms. We argue that quadratic curvature corrections are able to
isotropize fully anisotropic solutions whose scale factors describe accelerated
expansion. Moreover, the degree of isotropization grows with the duration of
the string phase. We follow the fate of the shear parameter in a decelerated
phase where, dilaton, magnetic fields and radiation fluid are simultaneously
present. In the absence of any magnetic field a long string phase immediately
followed by radiation is able to erase large anisotropies. Conversely, if a
short string phase is followed by a long dilaton dominated phase the
anisotropies can be present, in principle, also at later times. The presence of
magnetic seeds after the end of the string phase can induce further
anisotropies which can be studied within the formalism reported in this paper.Comment: 19 pages in Revtex style, 14 Encapsulated figure
Mixmaster Horava-Witten Cosmology
We discuss various superstring effective actions and, in particular, their
common sector which leads to the so-called pre-big-bang cosmology (cosmology in
a weak coupling limit of heterotic superstring). Then, we review the main ideas
of the Horava-Witten theory which is a strong coupling limit of heterotic
superstring theory. Using the conformal relationship between these two theories
we present Kasner asymptotic solutions of Bianchi type IX geometries within
these theories and make predictions about possible emergence of chaos. Finally,
we present a possible method of generating Horava-Witten cosmological solutions
out of the well-known general relativistic pre-big-bang solutions.Comment: 7 pages, 2 figures, based on the talks given at Marcel Grossmann
Meeting IX, Rome 2000 and at "Supersymmetry and Quantum Field Theory" (D.V.
Volkov Memorial), Kharkov 2000, espcrc2.sty include
Peak and end point of the relic graviton background in string cosmology
Using general arguments we determine the allowed region for the end point
frequency and the peak energy density of the stochastic background of gravity
waves expected in string cosmology. We provide an accurate estimate of the
minimal experimental sensitivity required to detect a signal in the Hz to GHz
range.Comment: 11 pages, LATEX, one figure included using eps. 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
Homogeneous magnetic fields in fully anisotropic string cosmological backgrounds
We present new solutions of the string cosmological effective action in the
presence of a homogeneous Maxwell field with pure magnetic component. Exact
solutions are derived in the case of space-independent dilaton and vanishing
torsion background. In our examples the four dimensional metric is either of
Bianchi-type III and VI or Kantowski-Sachs.Comment: 4 page
Homogeneous Conformal String Backgrounds
We present exact solutions characterised by Bianchi-type I,II,III,V,VI
four-dimensional metric, space-independent dilaton, and vanishing torsion
background, for the low energy string effective action with zero central charge
deficit. We show that, in such a context, curvature singularities cannot be
avoided, except for the trivial case of flat spacetime and constant dilaton. We
also provide a further example of the failure of the standard prescription for
connecting conformal string backgrounds through duality transformations
associated to non-semisimple, non-Abelian isometry group.Comment: 20 pages, latex, no figures, to appear in Class. Q. Gra
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