39 research outputs found
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
Phantom Cosmology without Big Rip Singularity
We construct phantom energy models with the equation-of-state parameter
such that , but finite-time future singularity does not occur. Such
models can be divided into two classes: (i) energy density increases with time
("phantom energy" without "Big Rip" singularity) and (ii) energy density tends
to constant value with time ("cosmological constant" with asymptotically de
Sitter evolution). The disintegration of bound structure is confirmed in Little
Rip cosmology. Surprisingly, we find that such disintegration (on example of
Sun-Earth system) may occur even in asymptotically de Sitter phantom universe
consistent with observational data. We also demonstrate that non-singular
phantom models admit wormhole solutions as well as possibility of big trip via
wormholes.Comment: LaTeX 13 pages, to appear in PL
Further stable neutron star models from f(R) gravity
Neutron star models in perturbative gravity are considered with
realistic equations of state. In particular, we consider the FPS, SLy and other
equations of state and a case of piecewise equation of state for stars with
quark cores. The mass-radius relations for model and
for models with logarithmic and cubic corrections are obtained. In the
case of gravity with cubic corrections, we obtain that at high central
densities (, where g/cm
is the nuclear saturation density), stable star configurations exist. The
minimal radius of such stars is close to km with maximal mass (SLy equation). A similar situation takes place for AP4 and BSK20
EoS. Such an effect can give rise to more compact stars than in General
Relativity. If observationally identified, such objects could constitute a
formidable signature for modified gravity at astrophysical level. Another
interesting result can be achieved in modified gravity with only a cubic
correction. For some EoS, the upper limit of neutron star mass increases and
therefore these EoS can describe realistic star configurations (although, in
General Relativity, these EoS are excluded by observational constraints).Comment: 18 pages, 17 figures, revised version significally expanded, to
appear in JCA
Maximal neutron star mass and the resolution of hyperon puzzle in modified gravity
The so-called hyperon puzzle in the theory of neutron stars is considered in
the framework of modified gravity. We show that for simple hyperon
equations of state, it is possible to obtain the maximal neutron star mass
which satisfies the recent observational data for PSR J1614-2230, in
higher-derivative models with power-law terms as . The soft hyperon equation of state under consideration is usually treated
as non-realistic in the standard General Relativity. The numerical analysis of
Mass-Radius relation for massive neutron stars with hyperon equation of state
in modified gravity turns out to be consistent with observations. Thus, we show
that the same modified gravity can solve at once three problems: consistent
description of the maximal mass of neutron star, realistic Mass-Radius relation
and account for hyperons in equation of state.Comment: 10 pages, 6 figures, some misprints are fixe
Supermassive Neutron Stars in Axion Gravity
We investigated realistic neutron stars in axion gravity. The
coupling between curvature and axion field is assumed in the simple form
. For the axion mass in the range
eV the solitonic core within neutron star and corresponding halo with size
km can exist. Therefore the effective contribution of term
grows inside the star and it leads to change of star parameters (namely, mass
and radius). We obtained the increase of star mass independent from central
density for wide range of masses. Therefore, maximal possible mass for given
equation of state grows. At the same time, the star radius increases not so
considerably in comparison with GR. {Hence, our model may predict possible
existence of supermassive compact stars with masses and
radii km for realistic equation of state (we considered APR
equation of state). In General Relativity one can obtain neutron stars with
such characteristics only for unrealistic, extremely stiff equations of state.}
Note that this increase of mass occurs due to change of solution for scalar
curvature outside the star. In GR curvature drops to zero on star surface where
. In the model under consideration the scalar curvature dumps more
slowly in comparison with vacuum gravity due to axion "galo" around the
star.Comment: to appear in MNRAS, 9pp., 6 figure