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28
Coupling of rolling tachyon to closed strings
 Nucl. Phys. B
, 2002
"... We study the late time behavior of the boundary state representing the rolling tachyon constructed by Sen. It is found that the coupling of the rolling tachyon to massive modes of the closed string grows exponentially as the system evolves. We argue that the description of rolling tachyon by a bound ..."
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Cited by 54 (1 self)
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We study the late time behavior of the boundary state representing the rolling tachyon constructed by Sen. It is found that the coupling of the rolling tachyon to massive modes of the closed string grows exponentially as the system evolves. We argue that the description of rolling tachyon by a boundary state is valid during the finite time determined by string coupling, and that energy could be dissipated to the bulk beyond this time. We also comment on the relation between the rolling tachyon boundary state and the spacelike Dbrane boundary state. 1
Caustics in tachyon matter and other BornInfeld scalars
 JHEP 0209
"... Abstract: We consider scalar BornInfeld type theories with arbitrary potentials V (T) of a scalar field T. We find that for models with runaway potentials V (T) the generic inhomogeneous solutions after a short transient stage can be very well approximated by the solutions of a HamiltonJacobi equa ..."
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Cited by 41 (3 self)
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Abstract: We consider scalar BornInfeld type theories with arbitrary potentials V (T) of a scalar field T. We find that for models with runaway potentials V (T) the generic inhomogeneous solutions after a short transient stage can be very well approximated by the solutions of a HamiltonJacobi equation that describes free streaming wave front propagation. The analytic solution for this wave propagation shows the formation of caustics with multivalued regions beyond them. We verified that these caustics appear in numerical solutions of the original scalar BI nonlinear equations. Our results include the scalar BI model with an exponential potential, which was recently proposed as an effective action for the string theory tachyon in the approximation where highorder spacetime derivatives of T are truncated. Since the actual string tachyon dynamics contain derivatives of all orders, the tachyon BI model with an exponential potential becomes inadequate when the caustics develop because high order spatial derivatives of T become divergent. BI type tachyon theory with a potential decreasing at large T could have interesting cosmological applications because the tachyon field rolling towards its ground state at infinity acts as pressureless dark matter. We find that inhomogeneous cosmological tachyon fluctuations
Constraining dark energy anisotropic stress. arXiv:0708.0830 [astroph
, 2007
"... Abstract. We investigate the possibility of using cosmological observations to probe and constrain an imperfect dark energy fluid. We consider a general parameterization of the dark energy component accounting for an equation of state, speed of sound and viscosity. We use present and future data fro ..."
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Cited by 13 (7 self)
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Abstract. We investigate the possibility of using cosmological observations to probe and constrain an imperfect dark energy fluid. We consider a general parameterization of the dark energy component accounting for an equation of state, speed of sound and viscosity. We use present and future data from the cosmic microwave background radiation (CMB), large scale structures and supernovae type Ia. We find that both the speed of sound and viscosity parameters are difficult to nail down with the present cosmological data. Also, we argue that it will be hard to improve the constraints significantly with future CMB data sets. The implication is that a perfect fluid description might ultimately turn out to be a phenomenologically sufficient description of all the observational consequences of dark energy. The fundamental lesson is however that even then one cannot exclude, by appealing to observational evidence alone, the possibility of imperfectness in dark energy. PACS numbers: 98.80.k,98.80.JkConstraining Dark Energy Anisotropic Stress 2 1.
PowerLaw Inflation from the Rolling Tachyon
, 2002
"... Modeling the potential by an inverse square law in terms of the tachyon field (V (T) = βT −2) we find exact solution for spatially flat isotropic universe. We show that for β> 2 √ 3/3 the model undergoes powerlaw inflation. A way to construct other exact solutions is specified and exemplified. ..."
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Cited by 9 (0 self)
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Modeling the potential by an inverse square law in terms of the tachyon field (V (T) = βT −2) we find exact solution for spatially flat isotropic universe. We show that for β> 2 √ 3/3 the model undergoes powerlaw inflation. A way to construct other exact solutions is specified and exemplified. Several interesting attempts to reconcile the inflationary paradigm with string theories have resulted recently in much work ( see for example [1]). These efforts, however, did not produce a completely successful union between the string theory and cosmology. It is fair to say, at this stage, that string inspired cosmologies are not a match to standard inflation. Yet, since many believe that superstrings or their generalisations provide an adequate description of fundamental interactions including gravity, it is worthwhile to pursue exploring the connections between the string theories and cosmology. Much has been written and emphasised about the role of the fundamental dilaton field in the context of string cosmology. Less is known about tachyon component of the string theory. Recently, several works have appeared studying such a field in the cosmological context [2], [3]. The main purpose of this note is to show that under the assumption that the tachyon potential behaves as an inverse square in terms of the field, the Einstein equations lead to simple exact analytic solutions in the case of spatially flat isotropic universe. These solutions undergo the socalled powerlaw inflationary expansion if the “slope ” of the potential β> 2 √ 3/3, and decelerate otherwise. We also indicate and exemplify as to how more general solutions can be constructed. To this end we consider spatially flat FRW line element given by: ds 2 = dt 2 − a 2 (t)(dx 2 + dy 2 + dz 2) (1) As shown by Sen [4], [5] a rolling tachyon condensate may be described by an effective fluid with energy density and pressure given by V (T) ρ = √
Aspects of stability and phenomenology in type IIA orientifolds with intersecting D6branes,” Fortsch. Phys
"... Intersecting branes have been the subject of an elaborate string model building for several years. After a general introduction into string theory, this work introduces in detail the toroidal and ZNorientifolds. The picture involving D9branes with Bfluxes is shortly reviewed, but the main discuss ..."
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Cited by 9 (2 self)
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Intersecting branes have been the subject of an elaborate string model building for several years. After a general introduction into string theory, this work introduces in detail the toroidal and ZNorientifolds. The picture involving D9branes with Bfluxes is shortly reviewed, but the main discussion employs the Tdual picture of intersecting D6branes. The derivation of the RR and NSNS tadpole cancellation conditions in the conformal field theory is shown in great detail. Various aspects of the open and closed chiral and nonchiral massless spectrum are discussed, involving spacetime anomalies and the generalized GreenSchwarz mechanism. An introduction into possible gauge breaking mechanisms is given, too. Afterwards, both N=1 supersymmetric and nonsupersymmetric approaches to low energy model building are treated. Firstly, the problem of complex structure instabilities in toroidal ΩRorientifolds is approached by a Z3orbifolded model. In particular, a stable nonsupersymmetric standardlike model with three fermion generations is discussed. This model features the standard model gauge groups at the same time as having a massless hypercharge, but possessing an additional global BL
A)symmetric tachyon rolling in de Sitter spacetime: A universe devoid of Planck density
 Nucl. Phys. B
, 2006
"... In a recent paper we studied rolling tachyon flat FRW cosmologies, but those admitting only timereversal asymmetric boundary conditions. The timereversal symmetric cosmologies have been studied by Sen previously. We show explicitly here that through appropriate choice of initial conditions, the ti ..."
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Cited by 4 (1 self)
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In a recent paper we studied rolling tachyon flat FRW cosmologies, but those admitting only timereversal asymmetric boundary conditions. The timereversal symmetric cosmologies have been studied by Sen previously. We show explicitly here that through appropriate choice of initial conditions, the time evolution of the Hubble parameter in these two types of solutions can be made completely identical for t> 0, except near t = 0. The rolling tachyon solution also gives rise to necessary inflation. We find that universe does start as a string size object (with string scale 10 15 GeV) with a string mass density ρs ≃ 10 78 gm.cm −3 and not with Planck density. 1 1
More on tachyon cosmology in de Sitter gravity
 JHEP
, 2006
"... We aim to study rolling tachyon cosmological solutions in de Sitter gravity. The solutions are taken to be flat FRW type and these are not timereversal symmetric. We find that cosmological constant of our universe has to be finetuned at the level of the action itself, as in KKLT string compactific ..."
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Cited by 3 (2 self)
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We aim to study rolling tachyon cosmological solutions in de Sitter gravity. The solutions are taken to be flat FRW type and these are not timereversal symmetric. We find that cosmological constant of our universe has to be finetuned at the level of the action itself, as in KKLT string compactification. The rolling tachyon can give rise to required inflation with suitable choice of the initial conditions which include nonvanishing Hubble constant. We also determine an upper bound on the volume of the compactification manifold. 1 1
Spacelike and Time Dependent Branes from DBI
 J. High Energy Phys
"... Spacelike branes are new timedependent systems to explore and it has been observed that related supergravity solutions can be obtained by analytically continuing known Dbrane solutions. Here we show that analytic continuation of known solutions of the DiracBornInfeld equations also lead to inter ..."
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Cited by 3 (0 self)
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Spacelike branes are new timedependent systems to explore and it has been observed that related supergravity solutions can be obtained by analytically continuing known Dbrane solutions. Here we show that analytic continuation of known solutions of the DiracBornInfeld equations also lead to interesting analogs of time dependent gravity solutions. Properties of these new solutions, which are similar to the Witten bubble of nothing and Sbranes, are discussed. We comment on how these new bubble solutions seem relevant to the tachyon condensation process of nonBPS branes, and remark on their application to cosmological scenarios. Unstable brane configurations which resemble Sbrane type solutions are also discussed.
Causality and the Skyrme model
 Physics Letters B
"... It is shown that despite the possibility of a breakdown of hyperbolicity, the SU(2) Skyrme model can never exhibit bulk violations of Einstein causality because its energy momentum tensor satisfies the dominant energy condition. It also satisfies the strong energy condition. The BornInfeldSkyrme m ..."
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It is shown that despite the possibility of a breakdown of hyperbolicity, the SU(2) Skyrme model can never exhibit bulk violations of Einstein causality because its energy momentum tensor satisfies the dominant energy condition. It also satisfies the strong energy condition. The BornInfeldSkyrme model also satisfies both the dominant and strong energy conditions. 1 1
Parameterizing the Effect of Dark Energy Perturbations on the Growth of Structures,” Phys
 Lett
, 2008
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