arXiv subjects

S. Kalyana Rama

Publications and source records attributed to S. Kalyana Rama.

At least 55 records · Page 3Linked to original sources

arXiv2002

On the Emergence of the Microcanonical Description from a Pure State

We study, in general terms, the process by which a pure state can ``self-thermalize'' and {\em appear} to be described by a microcanonical density matrix. This requires a quantum mechanical version of the Gibbsian coarse graining that conceptually underlies classical statistical mechanics. We introduce some extra degrees of freedom that are necessary for this. Interaction between these degrees and the system can be understood as a process of resonant absorption and emission of ``soft quanta''. This intuitive picture allows one to state a criterion for when self thermalization occurs. This paradigm also provides a method for calculating the thermalization rate using the usual formalism of atomic physics for calculating decay rates. We contrast our prescription for coarse graining, which is somewhat dynamical, with the earlier approaches that are intrinsically kinematical. An important motivation for this study is the black hole information paradox.

cond-mat.stat-mech↗

arXiv2001

Some Consequences of the Generalised Uncertainty Principle: Statistical Mechanical, Cosmological, and Varying Speed of Light

We study the dynamical consequences of Maggiore's unique generalised uncertainty principle (GUP). We find that it leads naturally, and generically, to novel consequences. In the high temperature limit, there is a drastic reduction in the degrees of freedom, of the type found, for example, in strings far above the Hagedorn temperature. In view of this, the present GUP may perhaps be taken as the new version of the Heisenberg uncertainty principle, conjectured by Atick and Witten to be responsible for such reduction. Also, the present GUP leads naturally to varying speed of light and modified dispersion relations. They are likely to have novel implications for cosmology and black hole physics, a few of which we discuss qualitatively.

hep-th↗

arXiv2001

Some Phenomenological Aspects of the (n + m + 1) dimensional Brane World Scenario with an m-form Field

In the D = (n + m + 1) dimensional brane world scenario with m compact dimensions, the radion modulus can be stabilised by a massive bulk m-form antisymmetric field. We analyse some of the phenomenological aspects of this scenario. We find that the radion mass is smaller than the TeV scale, but larger than that in the case where the radion modulus is stabilised by a bulk scalar field. From the macroscopic n dimensional spacetime point of view, the m-form field mimics a set of p-form fields. We analyse the mass spectrum of these fields. The lowest mass is \stackrel{>}{_\sim} TeV whereas, for any bulk or brane field, the excitations in the compact space have Planckian mass and are likely to reintroduce the hierarchy problem. Also, we analyse the couplings of the m-form field to the matter fields living on a brane. The present results are applicable to more general cases also.

hep-ph↗

arXiv2000

Brane World Scenario with m-form field: Stabilisation of Radion Modulus and Self Tuning Solutions

We consider (n + m + 1) dimensional bulk spacetime, containing flat (n + m - 1) dimensional parallel branes, with topology R^{n - 1} \times T^m. We assume that the graviton and an m-form field are the only bulk fields and that the m-form field has non vanishing components along the T^m directions only. We then find that the m-form field, with suitable bulk and brane potentials, can stabilise the radion modulus at the required value with no fine tuning. We find self tuning solutions also.

hep-th↗

arXiv2000

Tsallis Statistics: Averages and a Physical Interpretation of the Lagrange Multiplier $β$

Tsallis has proposed a generalisation of the standard entropy, which has since been applied to a variety of physical systems. In the canonical ensemble approach that is mostly used, average energy is given by an unnromalised, or normalised, $q$-expectation value. A Lagrange multiplier $β$ enforces the energy constraint whose physical interpretation, however, is lacking. Here, we use a microcanonical ensemble approach and find that consistency requires that only normalised $q$-expectation values are to be used. We then present a physical interpretation of $β$, relating it to a physical temperature. We derive this interpretation by a different method also.

cond-mat.stat-mech↗

arXiv1999

Supergravity with Self-dual B fields and Instantons in Noncommutative Gauge Theory

We study Type IIB supergravity in the presence of (euclidean) D3 branes and nonzero self-dual B-fields. We point out that the Einstein frame metric is identical to the full geometry for D3 branes without B fields turned on. Furthermore, in a decoupling limit in which the theory is conjectured to be dual to noncommutative Yang-Mills theory, the entire Einstein metric remains intact, and in particular, is asymptotically flat. We construct D-instanton solutions in this geometry. We show that in the decoupling limit the D-instanton action agrees with the action of the corresponding instanton in the noncommutative Yang-Mills theory and is expressed in terms of the open string coupling. Some other aspects of this correspondence, which have unusual features because the underlying metric is asymptotically flat, are explored.

hep-th↗

arXiv1999

On the Role of Chaos in the AdS/CFT Connection

The question of how infalling matter in a pure state forms a Schwarzschild black hole that appears to be at non-zero temperature is discussed in the context of the AdS/CFT connection. It is argued that the phenomenon of self-thermalization in non-linear (chaotic) systems can be invoked to explain how the boundary theory, initially at zero temperature self thermalizes and acquires a finite temperature. Yang-Mills theory is known to be chaotic (classically) and the imaginary part of the gluon self-energy (damping rate of the gluon plasma) is expected to give the Lyapunov exponent. We explain how the imaginary part would arise in the corresponding supergravity calculation due to absorption at the horizon of the black hole.

hep-th↗

arXiv1999

Holographic Principle in the Closed Universe : a Resolution with Negative Pressure Matter

A closed universe containing pressureless dust, more generally perfect fluid matter with pressure-to-density ratio w in the range (1/3, - 1/3), violates holographic principle applied according to the Fischler-Susskind proposal. We show, first for a class of two-fluid solutions and then for the general multifluid case, that the closed universe will obey the holographic principle if it also contains matter with w < - 1/3, and if the present value of its total density is sufficiently close to the critical density. It is possible that such matter can be realised by some form of `quintessence', much studied recently.

hep-th↗

arXiv1998

Holographic Principle during Inflation and a Lower Bound on Density Fluctuations

We apply the holographic principle during the inflationary stage of our universe. Where necessary, we illustrate the analysis in the case of new and extended inflation which, together, typify generic models of inflation. We find that in the models of extended inflation type, and perhaps of new inflation type also, the holographic principle leads to a lower bound on the density fluctuations.

hep-th↗

arXiv1998

The Hagedorn Transition, Deconfinement and the AdS/CFT Correspondence

A connection between the Hagedorn transition in string theory and the deconfinement transition in (non-supersymmetric) Yang-Mills theory is made using the AdS/CFT correspondence. We modify the model of zero temperature QCD proposed by Witten by compactifying an additional space-time coordinate with supersymmetry breaking boundary conditions thus introducing a finite temperature in the boundary theory. There is a Hagedorn-like transition associated with winding modes around this coordinate, which signals a topology changing phase transition to a new AdS/Schwarzschild blackhole where this coordinate is the time coordinate. In the boundary gauge theory time like Wilson loops acquire an expectation value above this temperature.

hep-th↗

arXiv1998

Matrix Theory Description of Schwarzschild Black Holes in the Regime N >> S

We study the description of Schwarzschild black holes, of entropy S, within matrix theory in the regime $N \ge S \gg 1$. We obtain the most general matrix theory equation of state by requiring that black holes admit a description within this theory. It has a recognisable form in various cases. In some cases a D dimensional black hole can plausibly be thought of as a $\tilde{D} = D + 1$ dimensional black hole, described by another auxiliary matrix theory, but in its $\tilde{N} \sim S$ regime. We find what appears to be a matrix theory generalisation to higher dynamical branes of the normalisation of dynamical string tension, seen in other contexts. We discuss a further possible generalisation of the matrix theory equation of state. In a special case, it is governed by $N^3$ dynamical degrees of freedom.

hep-th↗

arXiv1997

Boosts, Schwarzschild Black Holes and Absorption cross-sections in M theory

$D$ dimensional neutral black strings wrapped on a circle are related to $(D-1)$ dimensional charged black holes by boosts. We show that the boost has to be performed in the covering space and the boosted coordinate has to be compactified on a circle with a Lorentz contracted radius. Using this fact we show that the transition between Schwarzschild black holes to black p-branes observed recently in M theory is the well-known black hole- black string transition viewed in a boosted frame. In a similar way the correspondence point where an excited string state goes over to a neutral black hole is mapped exactly to the correspondence point for black p-branes. In terms of the $p$ brane quantities the equation of state for an excited string state becomes identical to that of a 3+1 dimensional massless gas for all $p$. Finally, we show how boosts can be used to relate Hawking radiation rates. Using the known microscopic derivation of absorption by extremal 3-branes and near-extremal 5D holes with three large charges we provide a microscopic derivation of absorption of 0-branes by seven and five dimensional Schwarzschild black holes in a certain regime.

hep-th↗

arXiv1997

Size of Black Holes through Polymer Scaling

We imagine the strings on the stretched horizon of any $d$ space-time dimensional black hole to be bits of polymer. Then, proposing an interaction between these bits we obtain the size of the configuration, and thus of the black hole, using the scaling laws. The transition from a typical black hole state to a typical string state has a simple explanation, which also holds for the extremal black holes.

hep-th↗

arXiv1997

Graviton-Dilaton Cosmology

We show that the evolution of the universe is singularity free in a class of graviton-dilaton models.

hep-th↗

arXiv1997

Can String Theory Avoid Cosmological Singularities?

We consider the effective action for strings and describe in detail the evolution of a four dimensional homogeneous isotropic universe with matter included. We find that the evolution, which is singular in general, becomes singularity free if during certain Phase of the evolution, when the scale factor increases and the effective string coupling becomes strong, the universe is dominated by solitonic p-branes, p = 0 and/or - 1, or by `matter' for which $(pressure) \le - \frac{1}{\sqrt{3}} (density)$. The mechanism in the case of branes is reminiscent of the recently discovered field theory mechanism where heavy states become light and resolve the moduli space singularities.

hep-th↗

arXiv1997

Singularity Free (Homogeneous Isotropic) Universe in Graviton-Dilaton Models

We present a class of graviton-dilaton models in which a homogeneous isotropic universe, such as our observed one, evolves with no singularity at any time. Such models may stand on their own as interesting models for singularity free cosmology, and may be studied further accordingly. They may also arise from string theory. We discuss critically a few such possibilities.

hep-th↗

arXiv1996

Early Universe Evolution in Graviton-Dilaton Models

We present a class of graviton-dilaton models which leads to a singularity free evolution of the universe. We study the evolution of a homogeneous isotropic universe. We follow an approach which enables us to analyse the evolution and obtain its generic features even in the absence of explicit solutions, which are not possible in general. We describe the generic evolution of the universe and show, in particular, that it is singularity free in the present class of models. Such models may stand on their own as interesting models for singularity free cosmology, and may be studied accordingly. They may also arise from string theory. We discuss critically a few such possibilities.

hep-th↗

arXiv1996

Some Cosmological Consequences of Non Trivial PPN Parameters $β$ and $γ$

We study homogeneous isotropic universe in a graviton-dilaton theory obtained, in a previous paper, by a simple requirement that the theory be able to predict non trivial values for $β$ and/or $γ$ for a charge neutral point star, without any naked singularities. We find that in this universe the physical time can be continued indefinitely into the past or future, and that all the physical curvature invariants are always finite, showing the absence of big bang singularity. Adding a dilaton potential, we find again the same features. As a surprising bonus, there emerges naturally a Brans-Dicke function, which has precisely the kind of behaviour needed to make $ω_{bd} ({\rm today}) > 500$ in hyperextended inflation.

hep-th↗