Attractions to radiation-like eras in superstring cosmologies

One-Loop Superstring Cosmology and the Non-Singular Universe

We study the cosmological implications of the one-loop terms in the string expansion. In particular, we find non-singular solutions which interpolate between a contracting universe and an expanding universe, and show that these solutions provide a mechanism for removing the initial conditions problem peculiar to spatially closed FRW cosmologies. In addition, we perform numerical calculations to show that the non-singular cosmologies do not require a careful choice of initial conditions, and estimate the likely magnitude of higher order terms in the string expansion.

Attraction to a radiation-like era in early superstring cosmologies

Starting from an initial classical four dimensional flat background of the heterotic or type II superstrings, we are able to determine at the string one-loop level the quantum corrections to the effective potential due to the spontaneous breaking of supersymmetry by "geometrical fluxes". Furthermore, considering a gas of strings at finite temperature, the full "effective thermal potential" is determined, giving rise to an effective non-trivial pressure. The backreaction of the quantum and thermal corrections to the space-time metric as well as to the moduli fields induces a cosmological evolut…

Closed Inhomogeneous String Cosmologies

We present a general algorithm which permits to construct solutions in string cosmology for heterotic and type-IIB superstrings in four dimensions. Using a chain of transformations applied in sequence: conformal, T-duality and SL(2,R) rotations, along with the usual generating techniques associated to Geroch transformations in Einstein frame, we obtain solutions with all relevant low-energy remnants of the string theory. To exemplify our algorithm we present an inhomogeneous string cosmology with S^3 topology of spatial sections, discuss some properties of the solution and point out some subtl…

Slow-roll parameters in braneworld cosmologies

We present a general slow-roll formalism within braneworld-motivated cosmologies with non-standard effective Friedmann equations. Full towers of parameters involving either the inflaton potential or the Hubble parameter are constructed and the dynamics of non-tachyonic and tachyonic fields are considered in detail; exact cosmological solutions and the inflationary attractor condition are provided. We compare scalar-driven and tachyon-driven accelerating eras through slow-roll correspondence and the observational imprint on early-universe structures.

Thermal and quantum induced early superstring cosmology

In this work, we review the results of Refs [1]-[5] dedicated to the description of the early Universe cosmology induced by quantum and thermal effects in superstring theories. The present evolution of the Universe is described very accurately by the standard Lambda-CDM scenario, while very little is known about the early cosmological eras. String theory provides a consistent microscopic theory to account for such missing epochs. In our framework, the Universe is a torus filled with a gas of superstrings. We first show how to describe the thermodynamical properties of this system, namely energ…

Thermal duality and non-singular cosmology in d-dimensional superstrings

We are presenting the basic ingredients of a stringy mechanism able to resolve both the Hagedorn instabilities of finite temperature superstrings as well as the initial singularity of the induced cosmology in arbitrary dimensions. These are shown to be generic in a large class of (4,0) type II superstring vacua, where non-trivial "gravito-magnetic" fluxes lift the Hagedorn instabilities of the thermal ensemble and the temperature duality symmetry is restored. This symmetry implies a universal maximal critical temperature. In all such models there are three characteristic regimes, each with a d…

S-brane to thermal non-singular string cosmology

We present a new class of non-singular string cosmologies in d space-time dimensions. At very early times, τ<< τ_c, the Universe is described by a flat σ-model metric, a constant maximal temperature T_c and super-weak string interactions, g_str << 1. During the evolution, the metric remains flat up to τ_c, while the string coupling grows and reaches a critical value g*_str at τ_c. This phase is characterized by a uniform temporal distribution of spacelike branes. At later times, τ> τ_c, the Universe enters in a new phase of expansion, with radiation. The string coupling decreases due to the di…

Gravitational Anomalies in string-inspired Cosmologies: from Inflation to Axion Dark Matter?

In this talk, I review briefly a scenario for the evolution of a string-inspired cosmological model, in which condensates of primordial gravitational waves (GW), formed at the very early eras after the Big Bang, are considered responsible for inducing inflation and then a smooth exit to a radiation dominated epoch. Primordial axion fields, that exist in the fundamental massless gravitational (bosonic) string multiplet, couple to the non-trivial GW-induced anomalies. As a result of this coupling, there exist axion background configurations which violate (spontaneously) Lorentz symmetry, and rem…

Non-singular string cosmology in a 2d Hybrid model

The existence of non-singular string cosmologies is established in a class of two-dimensional supersymmetric Hybrid models at finite temperature. The left-moving sector of the Hybrid models gives rise to 16 real (N_4=4) spacetime supercharges as in the usual superstring models. The right-moving sector is non-supersymmetric at the massless level, but is characterized by MSDS symmetry, which ensures boson/fermion degeneracy of the right-moving massive levels. Finite temperature configurations, which are free of Hagedorn instabilities, are constructed in the presence of non-trivial "gravito-magne…

新的修改宇宙学从新的广义熵

New modified cosmology from a new generalized entropy

本文提出基于广义熵的新宇宙学模型，通过引入广义熵函数和热力学框架，推导出暗能量密度和方程状态参数，揭示暗能量在不同熵指数下的演化行为。