Graceful exit in quantum string cosmology

Gasperini, M.; Maharana, Jnanadeva; Veneziano, G.

doi:10.1016/0550-3213(96)00201-5

Cited by 130 publications

(242 citation statements)

References 26 publications

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“…Perhaps the most fundamental is the mechanism by which the singularity present in the high curvature regime can be avoided, bringing the superluminal expansion to an end (the Graceful Exit Problem) [6]. For recent work on possible resolutions see [7,8]. Another important issue is whether density perturbations can be generated that are capable of seeding the observed large-scale structure in the universe.…”

Section: Introductionmentioning

confidence: 99%

S-duality-invariant perturbations in string cosmology

1997

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We investigate the generation of curvature and isocurvature (dilaton, moduli and axion) perturbations in a general class of axion-dilaton-moduli models, including the pre-big bang scenario. Allowing for an arbitrary coupling constant λ between the dilaton field and the axion field, we exploit the SL(2,R) symmetry of the theory to obtain the spectral indices of the field perturbations in a pre-big bang type scenario. Axion field fluctuations about a homogeneous background field can yield a scale-invariant (Harrison-Zel'dovich) spectrum. As an example we present a string-motivated case with SL(2,R)×SL(2,R) symmetry, where a second axion field arises from the compactification of the ten-dimensional theory to four dimensions.

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Section: Introductionmentioning

confidence: 99%

S-duality-invariant perturbations in string cosmology

1997

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“…Brief discussions on the violation/ conservation of the Energy Conditions and their role in avoiding the singularity theorems can be also found in [17], [18].…”

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confidence: 99%

Scale factor duality and the energy condition inequalities

Kar

1997

Physics Letters B

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We demonstrate, by a simple analysis, that cosmological line elements related by scale factor duality also exhibit a duality with respect to the conserva- Stringy cosmologies (for reviews, see [8], [9] ) have certain characteristic features apart from the symmetry of scale factor duality. Firstly, one does not need to bring in an ad-hoc scalar field to get the necessary inflationary phase. The dilaton field which arises as one of the massless excitations of the string world sheet serves the purpose. Moreover, there exists the notion of a phase termed as pre-big-bang (t < 0) during which we have a rapidly inflating universe with a scale factor generically obeying a pole law ( a(t) = (−t) β ; (β < 0 ). This new phase is expected to end at t = 0 where a FRW evolution (a(t) = t β ; 0 < β < 1) takes over and we finally end up with our universe today. Unfortunately, the transition from the pre-big-bang epoch to the FRW phase is plagued by the presence of a singularity, which, if absent would have solved the problem of singularities in GR, at least, in a cosmological setting. The generic presence of a singularity in the transition epoch which spoils a smooth crossover has been termed as the graceful exit problem in string cosmology [10]. Recently, Rey [11] has claimed that by introducing quantum back reaction it is possible to avoid the graceful exit problem at least within the limits of a two dimensional model like that of CGHS [12]. Whether an extension of this to four dimensions is possible or not is still an open question. On the other hand, a quantum cosmology approach to graceful exit has been advocated in [13]. The low energy effective theory that emerges out of string theory is much like Einstein gravity. The equations are derivable from an action which resembles that of a Brans-Dicke 2 theory with the ω parameter set to −1. Specifically, one has :This is the bosonic sector of the genus-zero, low energy action for closed superstrings in the limit when inverse string tension (or α ′ ) goes to zero. Here, H µνλ is the third-rank antisymmetric tensor field, φ the dilaton field and V contains contributions from the dilaton potential and the cosmological constant. The β function equations which are obtained by imposing quantum conformal invariance in the worldsheet sigma model can be derived from this action by performing appropriate variations.Obviously, there are two frames in which the metrics look very different-the string (Brans-Dicke) and the Einstein frame. These frames are related to each other by a conformal transformation. We shall exclusively work in the string frame.The Einstein-like equations can be written in the form G µν = e 2φ T µν where T µν = T φ µν + T M µν (M denotes matter fields other than the dilaton). These equations are exactly those for Brans-Dicke theory with the parameter ω = −1. One might argue that it is not proper to rewrite the β function equations in an explicit Einstein form because at the level of the worldsheet sigma model g µν , φ or B µν -all have the same status-they ...

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“…In Refs. [68] It is amusing that, from such a point of view, the initial state of the Universe is described by a right-moving plane wave, which later encounters a potential, giving rise to both a transmitted and a reflected (i.e. left-moving) wave.…”

Section: Exit Via Quantum Tunnellingmentioning

confidence: 99%

“…Several groups [68] have attempted to describe the transition from the pre-to the post-big bang without modifying the low-energy tree-level effective action, by exploiting the quantum cosmology approach based on the Wheeler-De Witt (WDW) equation. In Refs.…”

Section: Exit Via Quantum Tunnellingmentioning

confidence: 99%