711 research outputs found
Is scalar-tensor gravity consistent with polytropic stellar models?
We study the scalar field potential in the scalar-tensor gravity
with self-consistent polytropic stellar configurations. Without choosing a
particular potential, we numerically derive the potential inside various
stellar objects. We restrict the potential to conform to general relativity or
to gravity inside and require the solution to arrive at SdS vacuum at
the surface. The studied objects are required to obtain observationally valid
masses and radii corresponding to solar type stars, white dwarfs and neutron
stars. We find that the resulting scalar-tensor potential for the
numerically derived polytrope that conforms to general relativity, in each
object class, is highly dependent on the matter configuration as well as on the
vacuum requirement at the boundary. As a result, every stellar configuration
arrives at a potential that is not consistent with the other stellar
class potentials. Therefore, a general potential that conforms to all these
polytropic stellar classes could not be found.Comment: 6 pages, 2 figures, text revised and some concepts clarified, results
unchange
Baryon asymmetry and dark matter from soft leptogenesis
The framework for soft leptogenesis minimally extended with a DM sector is
studied. A heavy singlet neutrino superfield acts as the source for (s)lepton
asymmetry and by coupling to the singlet DM superfield it produces a DM
particle density through decays. The nature of DM generated is twofold
depending on whether the Yukawa and DM couplings are either small or large.
With sufficiently small Yukawa and DM couplings DM annihilations into MSSM
particles are slow and as a consequence all DM particles form the DM component.
The solutions to Boltzmann equations are given and the dependence between the
DM masses and coupling are presented in this weak coupling regime. Also, the
behavior of the efficiency of producing asymmetric DM is determined with weak
couplings. We note that a different outcome arises if the couplings are larger
because then the ADM component is dominant due to the effectiveness of DM
decays into the MSSM sector.Comment: 22 pages, 8 figure
Short-range correlations in binary alloys: Spin model approach to AgPd
Short-range correlations in Ag-Pd alloys are investigated by analyzing the
{\em ab initio} total energy of fcc based random AgPd. Since the
information on the atomic interactions is incorporated in the energetics of
alloys it is possible with a suitable model, Bethe-Peierls-Weiss model is used
in the present work, to invert the problem, i.e.\ to obtain information on the
short-range correlation from the total energy of a random system. As an example
we demonstrate how site correlations can be extracted from random alloy data.
Bethe-Peierls-Weiss model predicts positive first neighbor correlator and
mixing energy for substitutional face centered cubic (fcc) Ag-Pd alloys at low
temperature which can be related to the optimal structures of
AgPd.Comment: 6 page
Induced Nucleation in Weak First Order Phase Transition
We study induced nucleation by considering the accumulation rate of shrinking
subcritical bubbles. We derive the probability for a collection of subcritical
bubbles to form a critical bubble, and argue that this mechanism could well
play a role in electroweak phase transitions if the Higgs is heavy.Comment: 11 pages in latex, HU-TFT-94-41 and TURKU-FL-P1
Particle decay in expanding Friedmann-Robertson-Walker universes
The lack of energy conservation introduces new particle processes in curved
spacetime that are forbidden in flat space. Therefore one has to be very
cautious about using the results calculated in Minkowskian space in early
universe applications. This is true for particle decay rates in particular,
which need to be calculated using quantum field theory in curved spacetime.
Previous studies are usually restricted to using minimal or conformal coupling
for the decaying particle, while using a more general coupling would give
deeper insight into particle decay. This paper presents the results we obtained
for a massive particle decaying in a general power-law universe with arbitrary
coupling to gravity. We find that depending on the value of the gravitational
coupling, the effect of gravitation may either strengthen or weaken the decay.
The analysis further reveals that, apart from radiation dominated universe,
there are values of the coupling constant for which the decay rate is exactly
Minkowskian for all universe types. Because the decay rate may be considerably
modified in curved space, these issues need to be considered when doing precise
cosmological calculations.Comment: 9 pages, 5 figure
Decay of a Massive Particle in a Stiff Matter Dominated Universe
In the presence of a gravitational field decay rates may significantly differ
from flat space equivalent. By studying mutually interacting quantum fields the
decay rates can be calculated on a given spacetime. This paper presents the
calculation of the transition probability for the decay of a massive scalar
particle in a stiff matter dominated universe. We find that due to the precence
of a gravitational field a finite correction to the transition probability is
added which depends inversely on the mass. Moreover the decay rate is smaller
and lifetime of the particles is longer compared to flat space. The mass
dependence is such that the lifetime of lighter particles is prolonged more
compared to heavier particles. This result may be of significance when studying
cosmological situations involving stiff matter.Comment: 6 pages, 1 figur
f(R) gravity constraints from gravitational waves
The recent LIGO observation sparked interest in the field of gravity wave
signals. Besides the gravity wave observation the LIGO collaboration used the
inspiraling black hole pair to constrain the graviton mass. Unlike general
relativity, theories have a characteristic non-zero mass graviton. We
apply the constraint on the graviton mass to viable models to find the
effects on model parameters. We find it possible to constrain the parameter
space with the gravity wave based observations. We make a case study for the
popular Hu-Sawicki model and find a parameter bracket. The result generalizes
to other theories and can be used to contain the parameter space.Comment: 10 pages, in the revised version added discussion on different modes
to section 2, some typos and terminology correcte
Inhomogeneity of the LTB models
The Lema\'itre-Toman-Bondi (LTB) models have reported to suffer from
incompatibility with cosmological observations and fine-tuning of the
observer's location. Further analysis of these issues indicates that they could
be resolved by models that are compatible with the supernova Ia data, but less
inhomogeneous than those that have been presented in the literature so far. We
study if such models exist by employing the degrees of freedom of the LTB
models in a novel manner. We discovered two scenarios which may meet the
expectations, but extensive numerical and analytical investigation showed them
inviable. We extended our studies to the LTB models, which generalizes
the LTB models by including a non-zero cosmological constant in
Einsteins equations. This adds an additional degree of freedom for the earlier
scenarios and introduces a new scenario capable of meeting the expectations.
However, extensive numerical and analytical investigation reveals that
inclusion of does not enhance the viability of the models. We
identify the lack of degrees of freedom to be the reason for the unviability.
However, the method presented here can be generalized to models including more
degrees of freedom, like the Szekeres models, which have more promise to
overcome the issues in the LTB models.Comment: 15 pages including 1 appendix and 4 figure
Jeans Analysis of Bok globules in gravity
We examine the effects of gravity on Jeans analysis of collapsing dust
clouds. We provide a method for testing modified gravity models by their
effects on star formation as the presence of gravity is found to modify
the limit for collapse. In this analysis we add perturbations to a de Sitter
background. Depending on the characteristics of a chosen model, the
appearance of new limits is possible. The physicality of these limits is
further examined. We find the asymptotic Jeans masses for theories
compared to standard Jeans mass. Through this ratio, the effects of the
modified Jeans mass for viable theories are examined in molecular clouds. Bok
globules have a mass range comparable to Jeans masses in question and are
therefore used for comparing different models. Viable theories are found
to assist in star formation.Comment: 22 pages, 1 figure, new version 23.8.2016: added discussion and
corrected typo
Decaying Massive Particle in Matter and Radiation Dominated Eras
According to the standard model of cosmology, the early universe has been
dominated by radiation or non-relativistic matter in several eras of its
history. However, many cosmological calculations involving particle processes
are commonly done using Minkowskian results for them, although, for more
precise treatment, quantum field theory in curved spacetime is needed. This
paper aims to fill this gap by presenting decay rates for matter and radiation
dominated universes in this more precise treatment. We provide a study of the
average decay rates for a process where a conformally coupled massive scalar
field decays into massless scalar particles. It is found that the presence of a
curved spacetime modifies the Minkowskian result considerably for early times
but asymptotically only by an additive term proportional to the inverse of mass
and interaction time. Thus, the correction is small for large time scales, but
on the time scales of the order of , the relative correction term may
be of importance.Comment: 6 pages, 2 figure
- …