Related papers: Birth of the Universe in string cosmology
The decay of the string perturbative vacuum, if triggered by a suitable, duality-breaking dilaton potential, can efficiently proceed via the parametric amplification of the Wheeler-De Witt wave function in superspace, and can appropriately…
In a Wheeler-de Witt approach to quantum string cosmology, the present state of the Universe arises from the scattering and reflection of the wave function representing the initial string vacuum in superspace. This scenario is described and…
A string cosmology scenario ("pre-big-bang") postulates that the evolution of the Universe starts from a state of very small curvature and coupling, undergoes a long phase of dilaton-driven kinetic inflation and at some later time joins…
We discuss the possibility of quantum transitions from the string perturbative vacuum to cosmological configurations characterized by isotropic contraction and decreasing dilaton. When the dilaton potential preserves the sign of the Hubble…
We present a short review of possible applications of the Wheeler-De Witt equation to cosmological models based on the low-energy string effective action, and characterised by an initial regime of asymptotically flat, low energy, weak…
We discuss some aspects of string cosmology with an emphasis on the role played by the dilaton. A cosmological scenario based on the assumption that all spatial dimensions are periodic so that winding modes play an important role is…
In this paper we study the description of the Universe based on the low energy superstring theory modified by the Loop Quantum Gravity effects.This approach was proposed by De Risi et al. in the Phys. Rev. D {\bf 76} (2007) 103531. We show…
Some of the phenomenological implications of string cosmological models are reviewed, with particular attention to the spectra of the tensor, scalar and vector modes of the geometry. A class of self-dual string cosmological models is…
We consider new cosmological solutions with a collapsing, an intermediate and an expanding phase. The boundary between the expanding (collapsing) phase and the intermediate phase is seen by comoving observers as a cosmological past (future)…
We propose a mechanism to have a smooth transition from a pre-Big Bang phase to a standard cosmological phase. Such transition is driven by gravitational production of heavy massive string states that backreact on the geometry to stop the…
The scenario that the Universe contracts towards a big crunch and then undergoes a transition to expanding Universe in envisaged in the quantum string cosmology approach. The Wheeler-De Witt equation is solved exactly for an exponential…
I report recent studies on the evolution of perturbations in the context of the ``pre-big-bang" scenario typical of string cosmology, with emphasis on the formation of a stochastic background of relic photons and gravitons, and its possible…
We consider the coupled evolution of density, (scalar) metric and dilaton perturbations in the transition from a ``stringy" phase of growing curvature and gravitational coupling to the standard radiation-dominated cosmology. We show that…
We propose a modified form of the spontaneous birth of the universe by quantum tunneling. It proceeds through topology change and inflation, to eventually become a universe with closed spatial sections of negative curvature and nontrivial…
The topic of cosmic strings provides a bridge between the physics of the very small and the very large. They are predicted by some unified theories of particle interactions. If they exist, they may help to explain some of the largest-scale…
`Old' String Theory is a theory of one-dimensional extended objects, whose vibrations correspond to excitations of various target-space field modes including gravity. In these lectures I will give an introduction to low-energy Effective…
In dilaton gravity theories, we consider a string-like topological defect formed during U(1) gauge symmetry-breaking phase transition in the early Universe, and far from the cosmic string we have vacuum solutions of the generalized Einstein…
We have investigated the cosmological scenarios with a four dimensional effective action which is connected with multidimensional, supergravity and string theories. The solution for the scale factor is such that initially universe undergoes…
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…
Generic cosmological models in non-critical string theory have a time-dependent dilaton background at a late epoch. The cosmological deceleration parameter Q_0 is given by the square of the string coupling, g_s^2, up to a negative sign.…