395 research outputs found
Friction forces on phase transition fronts
In cosmological first-order phase transitions, the microscopic interaction of
the phase transition fronts with non-equilibrium plasma particles manifests
itself macroscopically as friction forces. In general, it is a nontrivial
problem to compute these forces, and only two limits have been studied, namely,
that of very slow walls and, more recently, ultra-relativistic walls which run
away. In this paper we consider ultra-relativistic velocities and show that
stationary solutions still exist when the parameters allow the existence of
runaway walls. Hence, we discuss the necessary and sufficient conditions for
the fronts to actually run away. We also propose a phenomenological model for
the friction, which interpolates between the non-relativistic and
ultra-relativistic values. Thus, the friction depends on two friction
coefficients which can be calculated for specific models. We then study the
velocity of phase transition fronts as a function of the friction parameters,
the thermodynamic parameters, and the amount of supercooling.Comment: 34 pages, 7 figures. v2: minor correction
Stability of cosmological deflagration fronts
In a cosmological first-order phase transition, bubbles of the stable phase
nucleate and expand in the supercooled metastable phase. In many cases, the
growth of bubbles reaches a stationary state, with bubble walls propagating as
detonations or deflagrations. However, these hydrodynamical solutions may be
unstable under corrugation of the interface. Such instability may drastically
alter some of the cosmological consequences of the phase transition. Here, we
study the hydrodynamical stability of deflagration fronts. We improve upon
previous studies by making a more careful and detailed analysis. In particular,
we take into account the fact that the equation of motion for the phase
interface depends separately on the temperature and fluid velocity on each side
of the wall. Fluid variables on each side of the wall are similar for weakly
first-order phase transitions, but differ significantly for stronger phase
transitions. As a consequence, we find that, for large enough supercooling, any
subsonic wall velocity becomes unstable. Moreover, as the velocity approaches
the speed of sound, perturbations become unstable on all wavelengths. For
smaller supercooling and small wall velocities, our results agree with those of
previous works. Essentially, perturbations on large wavelengths are unstable,
unless the wall velocity is higher than a critical value. We also find a
previously unobserved range of marginally unstable wavelengths. We analyze the
dynamical relevance of the instabilities, and we estimate the characteristic
time and length scales associated to their growth. We discuss the implications
for the electroweak phase transition and its cosmological consequences.Comment: 45 pages, 13 figures. v2: Minor corrections, references added. v3:
Typos corrected, minor modifications and references added (version accepted
in PRD
Hydrodynamics of phase transition fronts and the speed of sound in the plasma
The growth of bubbles in cosmological first-order phase transitions involves
nontrivial hydrodynamics. For that reason, the study of the propagation of
phase transition fronts often requires several approximations. A frequently
used approximation consists in describing the two phases as being composed only
of radiation and vacuum energy (the so-called bag equation of state). We show
that, in realistic models, the speed of sound in the low-temperature phase is
generally smaller than that of radiation, and we study the hydrodynamics in
such a situation. We find in particular that a new kind of hydrodynamical
solution may be possible, which does not arise in the bag model. We obtain
analytic results for the efficiency of the transfer of latent heat to bulk
motions of the plasma, as a function of the speed of sound in each phase.Comment: 44 pages, 19 figures. v2: some comments and references added. v3:
Some discussions and a figure added. Results unchanged. Matches published
versio
Stability of cosmological detonation fronts
The steady state propagation of a phase transition front is classified,
according to hydrodynamics, as a deflagration or a detonation, depending on its
velocity with respect to the fluid. These propagation modes are further divided
into three types, namely, weak, Jouguet, and strong solutions, according to
their disturbance of the fluid. However, some of these hydrodynamic modes will
not be realized in a phase transition. One particular cause is the presence of
instabilities. In this work we study the linear stability of weak detonations,
which are generally believed to be stable. After discussing in detail the weak
detonation solution, we consider small perturbations of the interface and the
fluid configuration. When the balance between the driving and friction forces
is taken into account, it turns out that there are actually two different kinds
of weak detonations, which behave very differently as functions of the
parameters. We show that the branch of stronger weak detonations are unstable,
except very close to the Jouguet point, where our approach breaks down.Comment: 34 pages, 11 figures. v2: typos corrected and minor change
Analytic approach to the motion of cosmological phase transition fronts
We consider the motion of planar phase-transition fronts in first-order phase
transitions of the Universe. We find the steady state wall velocity as a
function of a friction coefficient and thermodynamical parameters, taking into
account the different hydrodynamic modes of propagation. We obtain analytical
approximations for the velocity by using the thin wall approximation and the
bag equation of state. We compare our results to those of numerical
calculations and discuss the range of validity of the approximations. We
analyze the structure of the stationary solutions. Multiple solutions may exist
for a given set of parameters, even after discarding non-physical ones. We
discuss which of these will be realized in the phase transition as the
stationary wall velocity. Finally, we discuss on the saturation of the friction
at ultra-relativistic velocities and the existence of runaway solutions.Comment: 25 pages, 9 figures. The title has changed. A discussion on the
saturation of the friction and the possibility of runaway walls has been
adde
Strings With Axionic Content and Baryogenesis
We describe different electroweak strings with axionic content, including
non-topological configurations calculated numerically, and show their possible
influence on baryogenesis indicating that they may constitute a mechanism
competitive to that of bubble nucleation with two Higgs-doublets.Comment: 12 page
Scalar field confinement as a model for accreting systems
We investigate the possibility to localize scalar field configurations as a
model for black hole accretion. We analyze and resolve difficulties encountered
when localizing scalar fields in General Relativity. We illustrate this ability
with a simple spherically symmetric model which can be used to study features
of accreting shells around a black hole. This is accomplished by prescribing a
scalar field with a coordinate dependent potential. Numerical solutions to the
Einstein-Klein-Gordon equations are shown, where a scalar filed is indeed
confined within a region surrounding a black hole. The resulting spacetime can
be described in terms of simple harmonic time dependence.Comment: 18 pages; accepted for publication in Classical and Quantum Gravit
Numerical evolution of squeezed and non-Gaussian states in loop quantum cosmology
In recent years, numerical simulations with Gaussian initial states have
demonstrated the existence of a quantum bounce in loop quantum cosmology in
various models. A key issue pertaining to the robustness of the bounce and the
associated physics is to understand the quantum evolution for more general
initial states which may depart significantly from Gaussianity and may have no
well defined peakedness properties. The analysis of such states, including
squeezed and highly non-Gaussian states, has been computationally challenging
until now. In this manuscript, we overcome these challenges by using the
Chimera scheme for the spatially flat, homogeneous and isotropic model sourced
with a massless scalar field. We demonstrate that the quantum bounce in this
model occurs even for states which are highly squeezed or are non-Gaussian with
multiple peaks and with little resemblance to semi-classical states. The
existence of the bounce is found to be robust, being independent of the
properties of the states. The evolution of squeezed and non-Gaussian states
turns out to be qualitatively similar to that of Gaussian states, and satisfies
strong constraints on the growth of the relative fluctuations across the
bounce. We also compare the results from the effective dynamics and find that,
although it captures the qualitative aspects of the evolution for squeezed and
highly non-Gaussian states, it always underestimates the bounce volume. We show
that various properties of the evolution, such as the energy density at the
bounce, are in excellent agreement with the predictions from an exactly
solvable loop quantum cosmological model for arbitrary states.Comment: 26 pages, 16 figures. v2: Discussion of the main results expande
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