384 research outputs found
Low-energy quantum string cosmology
We introduce a Wheeler-De Witt approach to quantum cosmology based on the
low-energy string effective action, with an effective dilaton potential
included to account for non-perturbative effects and, possibly, higher-order
corrections. We classify, in particular, four different classes of scattering
processes in minisuperspace, and discuss their relevance for the solution of
the graceful exit problem.Comment: Extended version to appear in the Review section of Int. J. Theor.
Phys. A 13 (1998). The new permanent address is added. An updated collection
of papers on the pre-big bang scenario is available at
http://www.to.infn.it/~gasperi
The Andante Regime of Scalar Field Dynamics
The andante regime of scalar field dynamics in the chaotic inflationary
Universe is defined as the epoch when the field is rolling moderately slowly
down its interaction potential, but at such a rate that first-order corrections
to the slow-roll approximation become important. These conditions should apply
towards the end of inflation as the field approaches the global minimum of the
potential. Solutions to the Einstein-scalar field equations for the class of
power law potentials are found in this regime in
terms of the inverse error function.Comment: 11 pages of plain Latex, FNAL-Pub-94/226-
A Field Range Bound for General Single-Field Inflation
We explore the consequences of a detection of primordial tensor fluctuations
for general single-field models of inflation. Using the effective theory of
inflation, we propose a generalization of the Lyth bound. Our bound applies to
all single-field models with two-derivative kinetic terms for the scalar
fluctuations and is always stronger than the corresponding bound for slow-roll
models. This shows that non-trivial dynamics can't evade the Lyth bound. We
also present a weaker, but completely universal bound that holds whenever the
Null Energy Condition (NEC) is satisfied at horizon crossing.Comment: 16 page
Global fits for the spectral index of the cosmological curvature perturbation
Best-fit values of the spectral index of the curvature perturbation are
presented, assuming the CDM cosmology. Apart from the spectral index,
the parameters are the Hubble parameter, the total matter density and the
baryon density. The data points are intended to represent all measurements
which are likely to significantly affect the result. The cosmic microwave
anisotropy is represented by the COBE normalization, and heights of the first
and second peaks given by the latest Boomerang and Maxima data. The slope of
the galaxy correlation function and the matter density contrast on the
8h^{-1}\Mpc scale are each represented by a data point, as are the expected
values of the Hubble parameter and matter density.
The `low-deuterium' nucleosynthesis value of the baryon density provides a
final data point, the fit giving a value about one standard deviation higher.
The reionization epoch is calculated from the model by assuming that it
corresponds to the collapse of a fraction f\gsim 10^{-4} of matter.
We consider the case of a scale-independent spectral index, and also the
scale-dependent spectral index predicted by running mass models of inflation.
In the former case, the result is compared with the prediction of models of
inflation based on effective field theory, in which the field value is small on
the Planck scale. Detailed comparison is made with other fits, and other
approaches to the comparison with theory.Comment: as published in MNRA
Is brane cosmology predictable?
The creation of the inflationary brane universe in 5d bulk Einstein and
Einstein-Gauss-Bonnet gravity is considered. We demonstrate that emerging
universe is ambigious due to arbitrary function dependence of the junction
conditions (or freedom in the choice of boundary terms). We argue that some
fundamental physical principle (which may be related with AdS/CFT
correspondence) is necessary in order to fix the 4d geometry in unique way.Comment: LaTeX file, 4 page
Supersymmetric Quantization of Anisotropic Scalar-Tensor Cosmologies
In this paper we show that the spatially homogeneous Bianchi type I and
Kantowski-Sachs cosmologies derived from the Brans-Dicke theory of gravity
admit a supersymmetric extension at the quantum level. Global symmetries in the
effective one-dimensional actions characterize both classical and quantum
solutions. A wide family of exact wavefunctions satisfying the supersymmetric
constraints are found. A connection with quantum wormholes is briefly
discussed.Comment: In Press, Class. Quantum Grav. 20 pages, Late
Dynamical study of the hyperextended scalar-tensor theory in the empty Bianchi type I model
The dynamics of the hyperextended scalar-tensor theory in the empty Bianchi
type I model is investigated. We describe a method giving the sign of the first
and second derivatives of the metric functions whatever the coupling function.
Hence, we can predict if a theory gives birth to expanding, contracting,
bouncing or inflationary cosmology. The dynamics of a string inspired theory
without antisymetric field strength is analysed. Some exact solutions are
found.Comment: 18 pages, 3 figure
Observing the Inflaton Potential
We show how observations of the density perturbation (scalar) spectrum and
the gravitational wave (tensor) spectrum allow a reconstruction of the
potential responsible for cosmological inflation. A complete functional
reconstruction or a perturbative approximation about a single scale are
possible; the suitability of each approach depends on the data available.
Consistency equations between the scalar and tensor spectra are derived, which
provide a powerful signal of inflation.Comment: 9 pages, LaTeX, FERMILAB--PUB--93/071--A; SUSSEX-AST 93/4-
Symmetric vacuum scalar--tensor cosmology
The existence of point symmetries in the cosmological field equations of
generalized vacuum scalar--tensor theories is considered within the context of
the spatially homogeneous cosmologies. It is found that such symmetries only
occur in the Brans--Dicke theory when the dilaton field self--interacts.
Moreover, the interaction potential of the dilaton must take the form of a
cosmological constant. For the spatially flat, isotropic model, it is shown how
this point symmetry may be employed to generate a discrete scale factor duality
in the Brans--Dicke action.Comment: 10 pages, latex, To appear in Class. Quantum Gra
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