String theory and cosmology

1996

After recalling a few basic concepts from cosmology and string theory, I will discuss the main ideas/assumptions underlying string cosmology and show how these lead to a two-parameter family of "minimal" models. I will then explain how to compute, in terms of those parameters, the spectrum of scalar, tensor and electromagnetic perturbations, point at their (T and S-type) duality symmetries, and mention their most relevant physical consequences.

1995

After recalling a few basic concepts from cosmology and string theory, I will outline the main ideas/assumptions underlying (our own group's approach to) string cosmology and show how these lead to the definition of a two-parameter family of ``minimal" models. I will then briefly explain how to compute, in terms of those parameters, the spectrum of scalar, tensor and electromagnetic perturbations, and mention their most relevant physical consequences. More details on the latter part of this talk can be found in Maurizio Gasperini's contribution to these proceedings.

arXiv (Cornell University), 1999

We present a minimal model for the Universe evolution fully extracted from effective String Theory. By linking this model with a minimal but well stablished observational information, we prove that it gives realistic predictions on early and current energy density and its results are compatible with General Relativity. Interestingly enough, the predicted current energy density is found Ω = 1 and in anycase with lower limit Ω ≥ 4 9 . On the other hand, the energy density at the exit of inflationary stage gives also Ω| inf = 1. This result shows an agreement with General Relativity (spatially flat metric gives critical energy density) within an unequivalent Non-Einstenian context (string low energy effective equations). The order of magnitude of the energy density-dilaton coupled term at the beginning of radiation dominated stage agrees with GUT scale. Without solving the known problems about higher order corrections and graceful exit of inflation, we find this model closer to the observational Universe properties than the current available string cosmology scenarii. At a more fundamental level, this model is by its construction close to the standard cosmological evolution, and it is driven selfconsistently by the evolution of the string equation of state itself. The inflationary String Driven stage is able to reach an enough amount of inflation, describing a Big Bang like evolution for the metric.

String Gravity and Physics at the Planck Energy Scale, 1996

After recalling a few basic concepts from cosmology and string theory, I will discuss the main ideas/assumptions underlying string cosmology and show how these lead to a two-parameter family of "minimal" models. I will then explain how to compute, in terms of those parameters, the spectrum of scalar, tensor and electromagnetic perturbations, point at their (T and S-type) duality symmetries, and mention their most relevant physical consequences.

Journal of High Energy Physics, 1999

We consider the theory obtained by adding to the usual string frame dilaton gravity action specially constructed higher derivative terms motivated by the limited curvature construction of [1] and determine the spatially homogeneous and isotropic solutions to the resulting equations of motion. All solutions of the resulting theory of gravity with these symmetries are nonsingular and all curvature invariants are bounded. For initial conditions inspired by the pre-big-bang scenario solutions exist which correspond to a spatially flat Universe starting in a dilaton-dominated superinflationary phase withḢ > 0 and having a smooth transition to an expanding Friedmann Universe withḢ < 0. Hence, the graceful exit problem of pre-big-bang cosmology is solved in a natural way.

Physical Review D, 1994

We give the general analytic solutions derived from the low energy string effective action for four dimensional Friedmann-Robertson-Walker models with dilaton and antisymmetric tensor field, considering both long and short wavelength modes of the H-field. The presence of a homogeneous H-field significantly modifies the evolution of the scale factor and dilaton. In particular it places a lower bound on the allowed value of the dilaton. The scale factor also has a lower bound but our solutions remain singular as they all contain regions where the spacetime curvature diverges signalling a breakdown in the validity of the effective action. We extend our results to the simplest Bianchi I metric in higher dimensions with only two scale factors. We again give the general analytic solutions for long and short wavelength modes for the H field restricted to the three dimensional space, which produces an anisotropic expansion. In the case of H field radiation (wavelengths within the Hubble length) we obtain the usual four dimensional radiation dominated FRW model as the unique late time attractor. a edmundjc@central.susx.ac.uk

Arxiv preprint hep-th/0002094

3.1 Generic asymptotically trivial past . . . . . . . . . . . . . . . . . . 596 3.2 The asymptotic past&#x27;s effective action and different (conformal) frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597 3.3 Classical asymptotic symmetries: The importance of SUSY . . . . 598 3.4 Dilaton-driven inflation as ...

Physical Review D, 1999

A qualitative analysis is presented for spatially flat, isotropic and homogeneous cosmologies derived from the string effective action when the combined effects of a dilaton, modulus, two-form potential and central charge deficit are included. The latter has significant effects on the qualitative dynamics. The analysis is also directly applicable to the anisotropic Bianchi type I cosmology. ͓S0556-2821͑99͒01112-1͔

Physical Review D, 1994

We compute the string energy-momentum tensor and derive the string equation of state from exact string dynamics in cosmological spacetimes. 1+1, 2+1 and D-dimensional universes are treated for any expansion factor R. Strings obey the perfect fluid relation p = (γ -1)ρ with three different behaviours: (i) Unstable for R → ∞ with growing energy density ρ ∼ R 2-D , negative pressure, and γ = (D -2)/(D -1); (ii) Dual for R → 0, with ρ ∼ R -D , positive pressure and γ = D/(D -1) (as radiation); (iii) Stable for R → ∞ with ρ ∼ R 1-D , vanishing pressure and γ = 1 (as cold matter). We find the back reaction effect of these strings on the spacetime and we take into account the quantum string decay through string splitting. This is achieved by considering self-consistently the strings as matter sources for the Einstein equations, as well as for the complete effective string equations. String splitting exponentially suppress the density of unstable strings for large R. The self-consistent solution to the Einstein equations for string dominated universes exhibits the realistic matter dominated behaviour R ∼ (X 0 ) 2/(D-1) for large times and the radiation dominated behaviour R ∼ (X 0 ) 2/D for early times. De Sitter universe does not emerge as solution of the effective string equations. The effective string action (whatever be the dilaton, its potential and the central charge term) is not the appropriate framework in which to address the question of string driven inflation.

2020

In this paper, we analyze the inflationary cosmology using string field theory. This is done by using the zero level contribution from string field theory, which is a non-local tachyonic action. We will use the non-local Friedmann equations for this model based on string field theory, and calculate the slow-roll parameters for this model. We will then explicitly obtain the scalar and tensorial power spectrum, their related indices, and the tensor-to-scalar ratio for this model. Finally, we use cosmological data from Planck 2013 to 2018 to constrain the free parameters in this model, and find that string field theory is compatible with them.