Nowadays the major problem in cosmology is in the choice of the valid gravity theory for interpretation of the observational data. Usually in cosmology it is used the Einstein general theory of relativity and the corresponding Friedman-Robertson-Walker equations in the strong field limit (when gravitational potential is of the order of square of the light velocity). Meanwhile, the general...
The founders of quantum geometrodynamics assumed that spacetime may have nontrivial topology: John Wheeler suggested the idea of spacetime foam; Stephen Hawking wrote that one would expect that quantum gravity would allow all possible topologies of spacetime. Later, A. D. Sakharov put forward yet more exotic hypothesis that metric signature may change. Then the question arises, do we really...
We study the stability of classical solutions in Horndeski theory, which is the most general scalar-tensor theory of gravity with an additional scalar field and second-order motion equations, which in turn provides absence of Ostrogradski ghosts. In this work, we address a general dynamical spherically symmetric background. We derive the set of stability conditions in the cubic subclass of...
The $4d$ gravitational model with real scalar field $\varphi$, Einstein and Gauss-Bonnet terms
is considered. The action contains potential term $U(\varphi)$ and Gauss-Bonnet coupling function $f(\varphi)$.
For a special (static) spherically symmetric metric
$ds^2 = \left(A(u)\right)^{-1}du^2 - A(u)dt^2 + u^2 d\Omega^2$
with $A(u) > 0$ ($u > 0$ is a radial coordinate)
we verify and correct ...
Cosmological models based on scalar-torsion gravity with non-minimal coupling between the scalar field and torsion are considered. These models were obtained as a consequence of the generalized exact solutions of cosmological dynamic equations for an arbitrary Hubble parameter and an arbitrary scalar field evolution.
An inflation models classification according to the expansion order of the ...
This work considers two classes of cosmological models based on scalar-tensor theories of gravity. The models are obtained using certain functional relationships between the dynamics of the expansion of the Universe and the type of scalar-tensor theories of gravity. The consistency of the models with observational constraints according to Planck-2018 data is considered.
The emergence of relativistic quantum gravitational effects is well anticipated when the Planck density ($m_p/\ell_p^3$, where $m_p$ is the Planck mass and $\ell_p$ is the Planck length), regardless of whether the prediction of an infinite-density singularity is viewed as a flaw in general relativity. Over the past century, the orthogonality between quantum mechanics and general relativity has...
Massive primordial black holes may have formed in the early universe, accounting for a small fraction of dark matter. Most of dark matter, however, may be composed of elementary particles or black holes with smaller masses. These objects could form dense spikes around the large black holes during the radiation-dominated phase of the universe's evolution. Dark matter particles can annihilate in...
This work is devoted to the analysis of the influence of tidal effects on the shape of the gravitational signal from the fusion of neutron stars. The main part of the work is divided into theoretical information, analytical and numerical models, and practical implementation.
The theoretical section includes a description of the basic model for black holes and neutron stars, as well as the...
The influence of non-minimal coupling between the scalar field and the Gauss-Bonnet term on the background parameters of cosmological models and the parameters of cosmological perturbations is considered by comparison with the case of the Einstein gravity. The possibility of parametrization of this influence is shown.
A procedure for reconstructing solutions of the cosmological dynamic...
We discuss a class of solutions of multidimensional gravity which are formally related to black-hole solutions but can observationally look like stars whose surface reflects back particles or signals getting there. Some particular examples of such solutions are presented and studied.
A hypothetical particle known as the axion holds the potential to resolve both the cosmic dark matter riddle and particle physics' long-standing, strong CP dilemma. Unusual strong 21 cm absorption feature associated with the initial star formation era, maybe due to ultralight axion dark matter ($\sim 10^{-22}$ eV) at this time. The radio wave observation's 21 cm absorption signal can be...
Astrophysics traditionally deals with phenomena and objects in which quantum effects are not expected to be important or even significant – too large, too massive, too hot. Therefore, does it make sense to speak about “quantum astrophysics” as a separate discipline, and what would its key features and ideology be?
We consider a general relativistic model of a self-interacting complex scalar...
This project investigates the dissipative properties of an accelerated relativistic medium and their connection to the Unruh effect and effective black hole radiation. The thermodynamic properties in spaces with a horizon is one of the most discussed in modern fundamental physics. A notable 2005 string theory limit sets a minimum shear viscosity. We calculated viscosity in an accelerated frame...
In the framework of the quartet-metric gravity [1] the so called ``dark holes'' combining a central black hole and a peripheral scalar graviton dark halo are considered. It is shown that the dark holes can quite naturally explain asymptotically flat rotation curves, otherwise explained by presence of dark matter in galaxies. Possibilities of further modification of basic dark hole solutions...
In this presentation, I will delve into the behavior and impact of tidal forces within the realm of Newtonian gravity. I will provide concrete examples that illustrate solutions and outcomes by analyzing the Newtonian deviation equation. Furthermore, I will extensively explore the conditions under which tidal forces can exhibit compressive or disruptive effects by leveraging different model...
Dwarf spheroidal (dSph) satellites of the Milky Way (MW) are the
mostly dark matter (DM) dominated systems. Their dynamical parameters
such as line-of-sight (LOS) velocities are measured very accurately, so they
are very good probes of DM properties. We have constructed nearly
self-consistent stars-halo model of the dSph Fornax galaxy using
Schwarzschild orbit-superposition code of the AGAMA...
