125 research outputs found
Nonperturbative models of quark stars in gravity
Quark star models with realistic equation of state in nonperturbative
gravity are considered. The mass-radius relation for model
is obtained. Considering scalar curvature as an independent function, one
can find out, for each value of central density, the unique value of central
curvature for which one has solutions with the required asymptotic
for . In another words, one needs a
fine-tuning for to achieve quark stars in gravity. We consider also
the analogue description in corresponding scalar-tensor gravity. The
fine-tuning on is equivalent to the fine-tuning on the scalar field
in this description. For distant observers, the gravitational mass of the star
increases with increasing () but the interpretation of this
fact depends on frame where we work. Considering directly gravity, one
can say that increasing of mass occurs by the "gravitational sphere" outside
the star with some "effective mass". On the other hand, in scalar-tensor
theory, we also have a dilaton sphere (or "disphere") outside the star but its
contribution to gravitational mass for distant observer is negligible. We show
that it is possible to discriminate modified theories of gravity from General
Relativity due to the gravitational redshift of the thermal spectrum emerging
from the surface of the star.Comment: 9 pages, 5 figures, accepted for publication in Phys. Lett.
The Cosmological Models with Jump Discontinuities
The article is dedicated to one of the most undeservedly overlooked
properties of the cosmological models: the behaviour at, near and due to a jump
discontinuity. It is most interesting that while the usual considerations of
the cosmological dynamics deals heavily in the singularities produced by the
discontinuities of the second kind (a.k.a. the essential discontinuities) of
one (or more) of the physical parameters, almost no research exists to date
that would turn to their natural extension/counterpart: the singularities
induced by the discontinuities of the first kind (a.k.a. the jump
discontinuities). It is this oversight that this article aims to amend. In
fact, it demonstrates that the inclusion of such singularities allows one to
produce a number of very interesting scenarios of cosmological evolution. For
example, it produces the cosmological models with a finite value of the
equation of state parameter even when both the energy density and
the pressure diverge, while at the same time keeping the scale factor finite.
Such a dynamics is shown to be possible only when the scale factor experiences
a finite jump at some moment of time. Furthermore, if it is the first
derivative of the scale factor that experiences a jump, then a whole new and
different type of a sudden future singularity appears. Finally, jump
discontinuities suffered by either a second or third derivatives of a scale
factor lead to cosmological models experiencing a sudden dephantomization -- or
avoiding the phantomization altogether. This implies that theoretically there
should not be any obstacles for extending the cosmological evolution beyond the
corresponding singularities; therefore, such singularities can be considered a
sort of a cosmological phase transition.Comment: 27 pages, 5 figures. Inserted additional references; provided in
Introduction a specific example of a well-known physical field leading to a
cosmological jump discontinuity; seriously expanded the discussion of
possible physical reasons leading to the jump discontinuities in view of
recent theoretical and experimental discoverie
Dynamical analysis of the Tsallis holographic dark energy models with event horizon as cut-off and interaction with matter
The model of generalized Tsallis holographic dark energy (which is known to
be particular representative of Nojiri-Odintsov HDE) with event horizon as
cut-off is investigated using methods of dynamical analysis. We take into
consideration possible interaction with dark energy and matter in various
forms. Critical points are determined. Cosmological evolution of the Universe
depends from interaction parameters. If we use event horizon scale as cutoff
quasi-de Sitter expansion is possible only for interaction of type (where is the Hubble parameter). For
interactions and Universe eventually stops () or ends its existence in final singularity (). In first
case fraction of dark energy tends to or constant value lesser than 1
because dynamical equilibrium between matter and dark energy is established on
late times.Comment: 23 pp., 9 figs., to appear in Int. J. Mod. Phys.
Brane cosmology from observational surveys and its comparison with standard FRW cosmology
Several dark energy models on the brane are investigated. They are compared
with corresponding theories in the frame of 4d Friedmann-Robertson-Walker
cosmology. To constrain the parameters of the models considered, recent
observational data, including SNIa apparent magnitude measurements, baryon
acoustic oscillation results, Hubble parameter evolution data and matter
density perturbations are used. Explicit formulas of the so-called {\it
state-finder} parameters in teleparallel theories are obtained that could be
useful to test these models and to establish a link between Loop Quantum
Cosmology and Brane Cosmology. It is concluded that a joint analysis as the one
developed here allows to estimate, in a very convenient way, possible deviation
of the real universe cosmology from the standard Friedmann-Robertson-Walker
one.Comment: 19 pages, 6 figures. arXiv admin note: text overlap with
arXiv:1206.219
Astronomical bounds on future big freeze singularity
Recently it was found that dark energy in the form of phantom generalized
Chaplygin gas may lead to a new form of the cosmic doomsday, the big freeze
singularity. Like the big rip singularity, the big freeze singularity would
also take place at a finite future cosmic time, but unlike the big rip
singularity it happens for a finite scale factor.Our goal is to test if a
universe filled with phantom generalized Chaplygin gas can conform to the data
of astronomical observations. We shall see that if the universe is only filled
with generalized phantom Chaplygin gas with equation of state
with , then such a model cannot be matched
to the data of astronomical observations. To construct matched models one
actually need to add dark matter. This procedure results in cosmological
scenarios which do not contradict the data of astronomical observations and
allows one to estimate how long we are now from the future big freeze doomsday.Comment: 8 page
Causal Limit of Neutron Star Maximum Mass in Gravity in View of GW190814
We investigate the causal limit of maximum mass for stars in the framework
of gravity. We choose a causal equation of state, with variable speed of
sound, and with the transition density and pressure corresponding to the SLy
equation of state. The transition density is chosen to be equal to twice the
saturation density , and also the analysis is performed for
the transition density, chosen to be equal to the saturation density
. We examine numerically the combined effect of the stiff
causal equation of state and of the sound speed on the maximum mass of static
neutron stars, in the context of Jordan frame of gravity. This yields
the most extreme upper bound for neutron star masses in the context of extended
gravity. As we will evince for the case of model, the upper causal mass
limit lies within, but not deeply in, the mass-gap region, and is marginally
the same with the general relativistic causal maximum mass, indicating that the
general relativistic limit is respected. In view of the
modified gravity perspective for the secondary component of the GW190814 event,
we also discuss the strange star possibility. Using several well established
facts for neutron star physics and the Occam's razor approach, although the
strange star is exciting, for the moment, it remains a possibility for
describing the secondary component of GW190814. We underpin the fact that the
secondary component of the compact binary GW190814 is probably a neutron star,
a black hole or even a rapidly rotating neutron star, but not a strange star.
We also discuss, in general, the potential role of the extended gravity
description for the binary merging.Comment: PLB Accepte
Effect of Absorbing Layer Thickness on Efficiency Solar Cells Based on Cu(In,Ga)(S,Se)2
On the basis of one-dimensional model of the solar cell (SC) the influence of photoactive layer thickness on the photocurrent (a key
parameter for the collection efficiency of photogenerated charge carriers) of a has been studied. It is shown that for a typical Cu(In,Ga)(S,Se)2 - based SC the optimal value of the photoactive layer thickness is about 1.5 mkm. Reducing the thickness of the photoactive layer leads to a sharp decrease in the photocurrent and increase the series resistance
The apparent decay of pulsar magnetic fields
© Published under licence by IOP Publishing Ltd. Neutron stars are extremely strong cosmic magnets which fields are expected to decay with time. Here we report on the simple test of this process. Adopting a novel approach, we have estimated surface magnetic fields B for 76 radiopulsars (the most numerous subclass of the known isolated neutron stars) which ages t are known independently. Focusing on the accurate evaluation of the precision of both quantities, we determined a significant power-law trend B(t) ∝ t -β with index β = 0.19 +0.05 -0.06 at 95% C.L. The effects of the observational selection turn this value into the upper limit for the intrinsic field decay rate. If so, then neutron star crusts are close to the "impurity-free crystals", which results in a relatively slow magnetic fields decay
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