arXiv subjects

S. Myrzakul

Publications and source records attributed to S. Myrzakul.

6 recordsLinked to original sources

arXiv2024

Metric-affine Myrzakulov gravity theories with Gauss-Bonnet and boundary term scalars

In this paper, we consider some metric-affine Myrzakulov gravity (MG) theories with Gauss-Bonnet scalars. Also we consider the MG theories with the boundary term scalars. Note that these MG theories with the Gauss-Bonnet and boundary term scalars were proposed in [arXiv:1205.5266]. Some examples of Metric-Affine Gravity (MAG) theories are reviewed in the context of the $F(R,T,Q,{\cal T}, {\cal D})$ type models. Then the generalized MAG theory with the curvature, torsion and nonmetricity (the so-called MG-VIII) was studied. For the FRW spacetime case, in particular, the Lagrangian, Hamilatonian and gravitational equations are obtained. The particular case $F(R,T)=αR+βT+μQ+ν{\cal T}$ is investigated in detail. In quantum case, the corresponding Wheeler-DeWitt equation is obtained. Finally, some gravity theories with the curvature, torsion and nonmetricity are presented.

gr-qc↗

arXiv2024

On the dilation current in metric-affine gravity

We review $F(R,\mathcal{D})$ gravity in the metric-affine framework, where $\mathcal{D}$ is the divergence of the dilation current appearing in the hypermomentum tensor. We assume only linear couplings between the general affine connection and the matter fields (minimal coupling) and break projective invariance to preserve a nonvanishing dilation current. For $F(R,\mathcal{D})$ linear in $\mathcal{D}$ the dilation current dependence in the function $F(R,\mathcal{D})$ does not contribute to the field equations of the theory. We show that, on the other hand, in more complicated cases (e.g., considering the function $F(R,\mathcal{D})=R+α\mathcal{D}^2$), the $\mathcal{D}$ contribution to the metric field equations is nontrivial and can affect the cosmology of the theory.

hep-th↗

arXiv2021

The bound mass of Dehnen models with centrally peaked star formation efficiency

Understanding the formation of star clusters with low star-formation efficiency (SFE) is very important to know about the star-formation history. In N-body models of star cluster evolution after gas expulsion, the Plummer model with outer power law density profile has been used massively. We study the impact of the density profile slopes on the survivability of the low-SFE star clusters after instantaneous gas expulsion. We compare cases when stellar cluster has Plummer profile and Dehnen profiles with cusp of different slopes at the time of formation. We determine the corresponding density profile of the residual gas for a given global SFE, assuming that our model clusters formed with a constant efficiency per free-fall time and hence have shallower density profile of gas than that of stars. We perform direct $N$-body simulations of evolution of clusters initially in virial equilibrium within gas potential after gas removal. We find that the violent relaxation lasts no longer than 20~Myr independently of the density profile power law slopes. Dehnen model clusters survive after violent relaxation with significantly lower SFEs when the global SFE measured within the Jacobi radius or within a half-mass radius. Dehnen $γ=0$ model clusters show similar final bound fraction with the Plummer model clusters if global SFE is measured within 10 scale radii. The final bound fraction increases with $γ$ values for a given global SFE. We conclude that Dehnen clusters better resist the consequences of the violent relaxation followed the instantaneous gas expulsion than the Plummer clusters. Thus the shallower the outer density slope of the low-SFE clusters, the better for their survivability after gas expulsion. Among Dehnen clusters we find that the steeper the inner slope (cusp) the higher the bound mass fraction is retained after violent relaxation for a given global SFE.

astro-ph.GA↗

arXiv2017

Warm inflation in Horndeski gravity

In this paper, we investigate a class of Horndeski scalar-tensor theory of gravity for warm inflation. We present some models where the early-time acceleration is realized in the weak and in the strong dissipation regime. Cosmological perturbations are analyzed.

gr-qc↗

arXiv2016

Reconstruction of cosmic history from a simple parametrization of H

In this paper, we propose a simple parametrization of the Hubble parameter H in order to explain the late time cosmic acceleration. We show that our proposal covers many models obtained in different schemes of parametrization under one umbrella. We demonstrate that a simple modification in the functional form of Hubble parameter can give rise to interesting cosmological phenomena such as big rip singularity, bounce and others. We have also constrained the model parameters using the latest 28 points of H(z) data for three cases which admit transition from deceleration to acceleration.

gr-qc↗

arXiv2008

Usual and phantom scalar fields in five dimensions: compactification and flat thick brane solutions

In the model of a gravitating system with two scalar fields (one of which is phantom), two new types of regular solutions are found: mechanism for compactification of an extra dimension and a flat thick brane solution. It is shown that the first model has solutions oscillating over the extra coordinate and giving a finite radius of compactification of the fifth dimension and the second model is a flat thick brane embedded in the 5D Minkowski spacetime. Geometry of both models corresponds to a five-dimensional Minkowski space-time. Consideration of linear perturbations shows stability of the obtained solutions.

gr-qc↗