19 research outputs found
Stochastic thermodynamics of active Brownian particles
Examples of self propulsion in strongly fluctuating environment is abound in
nature, e.g., molecular motors and pumps operating in living cells. Starting
from Langevin equation of motion, we develop a fluctuating thermodynamic
description of self propelled particles using simple models of velocity
dependent forces. We derive fluctuation theorems for entropy production and a
modified fluctuation dissipation relation, characterizing the linear response
at non-equilibrium steady states. We study these notions in a simple model of
molecular motors, and in the Rayleigh-Helmholtz and energy-depot model of self
propelled particles.Comment: 8 pages, version accepted in Phys. Rev.
Quasi-isotropic cycles and non-singular bounces in a Mixmaster cosmology
A Bianchi IX Mixmaster spacetime is the most general spatially homogeneous
solution of Einstein's equations and it can represent the space-averaged
Universe. We introduce two novel mechanisms resulting in a Mixmaster Universe
with non-singular bounces which are quasi-isotropic. A fluid with a non-linear
equation of state allows non-singular bounces. Using negative anisotropic
stresses successfully isotropises this Universe and mitigates the well known
Mixmaster chaotic behaviour. Thus the Universe can be an eternal Mixmaster,
going through an infinite series of different cycles separated by bounces, with
a sizable fraction of cycles isotropic enough to be well approximated by a
standard Friedmann-Lema\^itre-Robertson-Walker model from the radiation era
onward.Comment: 5 pages, 4 figure
Quantum field theory in de Sitter and quasi-de Sitter spacetimes: Revisited
It is possible to associate temperatures with the non-extremal horizons of a
large class of spherically symmetric spacetimes using periodicity in the
Euclidean sector and this procedure works for the de Sitter spacetime as well.
But, unlike e.g., the black hole spacetimes, the de Sitter spacetime also
allows a description in Friedmann coordinates. This raises the question of
whether the thermality of the de Sitter horizon can be obtained, working
entirely in the Friedmann coordinates, without reference to the static
coordinates or using the symmetries of de Sitter spacetime. We discuss several
aspects of this issue for de Sitter and approximately de Sitter spacetimes, in
the Friedmann coordinates (with a time-dependent background and the associated
ambiguities in defining the vacuum states). The different choices for the
vacuum states, behaviour of the mode functions and the detector response are
studied in both (1+1) and (1+3) dimensions. We compare and contrast the
differences brought about by the different choices. In the last part of the
paper, we also describe a general procedure for studying quantum field theory
in spacetimes which are approximately de Sitter and, as an example, derive the
corrections to thermal spectrum due to the presence of pressure-free matter.Comment: 26 page
Cosmic no-hair theorems for viscous contracting Universes
Abstract: A cosmic no-hair theorem for all initially contracting, spatially homogeneous, orthogonal Bianchi Cosmologies is derived - which shows that all such Universes asymptote to a spatially flat, isotropic Universe with the inclusion of a shear viscous stress. This establishes a new mechanism of isotropisation in a contracting Universe, which does not take recourse to an ekpyrosis-like mechanism using an effective ultra-stiff equation of state fluid, that is, one in which the pressure is much greater than the energy density
Evolution of initially contracting Bianchi Class A models in the presence of an ultra-stiff anisotropic pressure fluid
We study the behaviour of Bianchi class A universes containing an ultra-stiff isotropic ghost field and a fluid with anisotropic pressures which is also ultra-stiff on the average. This allows us to investigate whether cyclic universe scenarios, like the ekpyrotic model, do indeed lead to isotropisation on approach to a singularity (or bounce) in the presence of dominant ultra-stiff pressure anisotropies. We specialise to consider the closed Bianchi type IX universe and show that when the anisotropic pressures are stiffer on average than any isotropic ultra-stiff fluid then, if they dominate on approach to the singularity, it will be anisotropic. We include an isotropic ultra-stiff ghost fluid with negative energy density in order to create a cosmological bounce at finite volume in the absence of the anisotropic fluid. When the dominant anisotropic fluid is present it leads to an anisotropic cosmological singularity rather than an isotropic bounce. The inclusion of anisotropic stresses generated by collisionless particles in an anisotropically expanding universe is therefore essential for a full analysis of the consequences of a cosmological bounce or singularity in cyclic universes.JDB is supported by the STFC. CG is supported by the Jawaharlal Nehru Memorial Trust Cambridge International Scholarship.TThis is the final version of the article. It first appeared from the Institute of Physics via https://doi.org/10.1088/0264-9381/33/12/12500
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Anisotropic Cyclic Cosmologies
Standard models of cosmology use inflation as a mechanism to resolve the isotropy and
homogeneity problem of the universe as well as the flatness problem. However, due to various
well known problems with the inflationary paradigm, there has been an ongoing search for
alternatives. Perhaps the most famous among these are the cyclic universe scenarios which
incorporate bounces. As these scenarios have a contracting phase in the evolution of the
universe, anisotropies and inhomogeneities would be expected to blow up on approach to the
bounce. Thus, it is reasonable to ask whether the problems of homogeneity and isotropy can
still be resolved in these scenarios. In this thesis, I will focus on the problem of the resolution
of the isotropy problem.
I begin with a brief review of anisotropic, spatially homogeneous geometries of cosmological
interest. Next, I review the existing literature on bouncing cosmologies, and discuss
the mechanism of bounce studied in previously proposed models, as well as their theoretical
and observational advantages and disadvantages.
I then discuss the process of isotropisation in the contracting phase of each bounce. In this
phase of the evolution, the mechanism of ekpyrosis is used in most cosmological scenarios
which incorporate a contracting phase to mitigate the problem of anisotropies blowing up
on approaching the bounce. I start by studying anisotropic universes and I then examine the
effect of the addition of ultra-stiff anisotropic pressures on the ekpyrotic phase.
I then consider evolving such anisotropic universes through several cycles with increasing
expansion maxima at each successive bounce. This eventually leads to flatness in the isotropic
case. My aim is to see if the resolution of the flatness problem also leads to a simultaneous
resolution of the isotropy problem.
In the next chapter, I consider the effect of non comoving velocities on the shape of this
anisotropic bouncing universe.
In the final section of my thesis, I consider anisotropic cosmological models within
the context of canonical quantum cosmology and investigate the quantum behaviour of
anisotropies.Cambridge Trust, Jawaharlal Nehru Memorial Trus
Penrose process in a charged axion–dilaton coupled black hole
Using the Newman–Janis method to construct the axion–dilaton coupled charged rotating black holes, we show that the energy extraction from such black holes via the Penrose process takes place from the axion/Kalb–Ramond field energy responsible for rendering the angular momentum to the black hole. Determining the explicit form for the Kalb–Ramond field strength, which is argued to be equivalent to spacetime torsion, we demonstrate that at the end of the energy extraction process, the spacetime becomes torsion free with a spherically symmetric non-rotating black hole remnant. In this context, applications to physical phenomena, such as the emission of neutral particles in astrophysical jets, are also discussed. It is seen that the infalling matter gains energy from the rotation of the black hole, or equivalently from the axion field, and that it is ejected as a highly collimated astrophysical jet