Dark energy generated from a (super-) string effective action with higher-order curvature corrections and a dynamical dilaton (original) (raw)
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We study the cosmological evolution based upon a DDD-dimensional action in low-energy effective string theory in the presence of second-order curvature corrections and a modulus scalar field (dilaton or compactification modulus). A barotropic perfect fluid coupled to the scalar field is also allowed. Phase space analysis and the stability of asymptotic solutions are performed for a number of models which include ($i$) fixed scalar field, ($ii$) linear dilaton in string frame, and ($iii$) logarithmic modulus in Einstein frame. We confront analytical solutions with observational constraints for deceleration parameter and show that Gauss-Bonnet gravity (with no matter fields) may not explain the current acceleration of the universe. We also study the future evolution of the universe using the GB parametrization and find that big rip singularities can be avoided even in the presence of a phantom fluid because of the balance between the fluid and curvature corrections. A non-minimal coupling between the fluid and the modulus field also opens up the interesting possibility to avoid big rip regardless of the details of the fluid equation of state.
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In this paper we investigate the cosmological effects of modified gravity with string curvature corrections added to Einstein-Hilbert action in the presence of a dynamically evolving scalar field coupled to Riemann invariants. The scenario exhibits several features of cosmological interest for late universe. It is shown that higher order stringy corrections can lead to a class of dark energy models consistent with recent observations. The model can give rise to quintessence without recourse to scalar field potential. The detailed treatment of reconstruction program for general scalar-Gauss-Bonnet gravity is presented for any given cosmology. The explicit examples of reconstructed scalar potentials are given for accelerated (quintessence, cosmological constant or phantom) universe. Finally, the relation with modified F (G) gravity is established on classical level and is extended to include third order terms on curvature. PACS numbers: 11.25.-w, 95.36.+x, 98.80.-k
Cosmological solutions of higher-curvature string effective theories with dilatons
1995
We study the effect of higher-curvature terms in the string low-energy effective actions on the cosmological solutions of the theory, up to corrections quartic in the curvatures, for the bosonic and heterotic strings as well as the type II superstring. We find that cosmological solutions exist for all string types but they always disappear when the dilaton field is included,
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We discuss the main cosmological implications of considering string-loop effects and a potential for the dilaton in the lowest order string effective action. Our framework is based on the effective model arising from regarding homogeneous and isotropic dilaton, metric and Yang-Mills field configurations. The issues of inflation, entropy crisis and the Polonyi problem as well as the problem of the cosmological constant are discussed.
Physical Review D, 2007
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Dark energy cosmological models with cosmic string
Astrophysics and Space Science, 2018
In this paper we have studied the anisotropic Kantowski-Sachs, locally rotationally symmetric (LRS) Bianchi type-I and LRS Bianchi type-III geometries filled with dark energy and one dimensional cosmic string in the Saez-Ballester theory of gravitation. To get physically valid solution we take hybrid expansion law of the average scale factor which is a product of power and exponential type of functions that results in time dependent deceleration parameter (q). The equation of state parameter of dark energy (ω de) has been discussed and we have observed that for the three models it crosses the phantom divide line (ω de = −1) and shows quintom like behavior. The density of dark energy (ρ de) is an increasing function of redshift and remains positive throughout the evolution of the universe for the three models. Moreover in Kantowski-Sachs and LRS Bianchi type-I geometries the dark energy density dominates the string tension density (λ) and proper density (ρ) throughout the evolution of the universe. The physical and geometrical aspects of the statefinder parameters (r, s), squared speed of sound (v 2 s) and ω de-ω de plane are also discussed. Keywords Kantowski-Sachs space-time • LRS Bianchi type-I space-time • LRS Bianchi type-III space time • Dark energy • String tension density • Scalar tensor theory of gravitation Recently, there has been a considerable interest in cosmological models with dark energy due to the accelerated ex-B T. Vinutha
Low energy effective string cosmology
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