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 $V(\phi) \propto \phi^{2n}$ 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 $\Lambda$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