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Maxim Khlopov

Publications and source records attributed to Maxim Khlopov.

At least 37 records · Page 2Linked to original sources

Stochastic gravitational wave background due to core collapse resulting in neutron stars

The stochastic background of gravitational wave signals arising from the core-collapse supernovae is produced through various complex mechanisms that need detailed and careful investigation. We proposed a simplified multi-peak waveform of the amplitude spectrum. The corresponding energy spectra of our model fit the energy spectra obtained from different numerical simulations of various types of core-collapse events such as non-rotating, slow and fast rotating massive progenitors resulting in neutron stars. The maximum dimensionless energy density $Ω_{gw}$ corresponding to our model is of $\mathcal{O}(10 {^{-12})} $ around 650 Hz. Assuming some degree of uncertainty, we estimated the parameters for the core collapse waveform using BILBY. We studied the detectability of the signal of our model against gravitational-wave detectors like the Einstein Telescope, advanced LIGO and Virgo. Our study indicates that these detectors have to gain more sensitivity to pick up the gravitational wave signals of stochastic background.

gr-qc↗

Collapsing scenarios of K-essence generalized Vaidya spacetime under $f(\bar{R},\bar{T})$ gravity

The paper investigates the collapse of the generalized emergent Vaidya spacetime in the setting of $f(\bar{R}, \bar{T})$ gravity, specifically in K-essence theory. In this study, the Dirac-Born-Infeld type non-standard Lagrangian is used to calculate the emergent metric $\bar{G}_{μν}$, which is not conformally equivalent to the conventional gravitational metric. We use the function $f(\bar{R}, \bar{T})$ to reflect the additive nature of the emergent Ricci scalar ($\bar{R}$) and the trace of the emergent energy-momentum tensor ($\bar{T}$). Our study demonstrates that certain choices of $f(\bar{R}, \bar{T})$ may result in the existence of a naked singularity caused by gravitational collapse. The alternative $f(\bar{R}, \bar{T})$ values resulted in an accelerating universe dominated by dark energy. Moreover, the investigation showed the presence of both positive and negative masses, which might suggest the coexistence of dark matter and dark energy. Furthermore, for a given quantity of kinetic part of the K-essence scalar field, mass is completely changed into energy, meaning that spacetime is Minkowskian. The K-essence theory may also be employed as a dark energy framework and a basic gravitational theory, making it possible for researchers to investigate a wide ranges of cosmic phenomena.

gr-qc↗

Inflection Point of Minimally Coupled Tachyonic Scalar Field

In this paper, we investigate the behaviour of a minimally coupled tachyonic scalar field at inflection points in an accelerating universe. We consider the different expansion factors and obtain potentials of tachyonic scalar field. Inflection points of homogeneous tachyonic scalar field is calculated for these potentials. We employ the tools of dynamical system analysis for the considered potentials and obtain the stable points.

gr-qc↗

Cosmological singularities in $f(T,ϕ)$ gravity

The pursuit of understanding the mysteries surrounding dark energy has sparked significant interest within the field of cosmology. While conventional approaches, such as the cosmological constant, have been extensively explored, alternative theories incorporating scalar field-based models and modified gravity have emerged as intriguing avenues. Among these, teleparallel theories of gravity, specifically the $f(T,ϕ)$ formulation, have gained prominence as a means to comprehend dark energy within the framework of teleparallelism. In this study, we investigate two well-studied models of teleparallel dark energy and examine the presence of cosmological singularities within these scenarios. Using the Goriely-Hyde procedure, we examine the dynamical systems governing the cosmological equations of these models. Our analysis reveals that both models exhibit Type IV singularities, but only for a limited range of initial conditions. These results could indicate a potential edge for teleparallel cosmological models over their other modified gravity counterparts, as the models we examine seem to be only allowing for weak singularities that too under non general conditions.

gr-qc↗

Holographic dark energy with Granda-Oliveros cutoff and ansatz based approach: An extended look

Holographic dark energy models have proven to be a very interesting way to study various aspects of late-time acceleration of the universe. In this work we extensively study HDE models with the Granda-Oliveros cutoff with an ansatz based approach. We consider the Tsallis, Barrow and PLEC HDE models in this regard and consdier simple power law, emergent universe, intermediate and logamediate forms of for the universe. Studying various cosmologically interesting parameters alongside the thermodynamical aspects in these models, we show that the Logamediate models are the best fit out of the other possibilites, followed by the emergent universe model, intermediate model and the simple power law models at the very last in terms of feasibility.

gr-qc↗

Tsallis holographic dark energy with power law ansatz approach

Holographic principles have proven to be a very interesting approach towards dealing with the issues of the late-time acceleration of the universe, which has resulted in a great amount of work on holographic dark energy models. We consider one such very interesting holographic scenario, namely the Tsallis Holographic dark energy model and consider an ansatz based approach to such models. We consider three cosmological scenarios in such models, namely those with viscous , non-viscous and Chaplygin gas scenarios, discussing various crucial aspects related to these models. We discuss various crucial properties of the Tsallis model in such scenarios and see how the phantom divide is crossed in each case, but it's only the Chaplygin gas models which give a better view on stability issues.

gr-qc↗

Recent advances of Beyond the Standard model cosmology

BSM physics, underlying the now standard inflationary cosmology with baryosynthesis and dark matter/energy, inevitably leads to features, going beyond this standard cosmological paradigm. The signatures of these features can be considered in the hints for the existence of antihelium component of cosmic rays and massive Primordial Black Holes (PBH), in the discovery of stochastic gravitational wave background (SGWB) and evidence for early galaxy formation as well as in the dark atom solution for the puzzles of direct dark mater searches. We discuss open problems of physics of dark atoms, which can explain positive result of DAMA/NaI and DAMA/LIBRA direct searches for dark matter, of the origin and evolution of antimatter domains in baryon asymmetrical Universe, of the Axion Like Particle (ALP) physics, which may explain the origin of SGWB, discovered by Pulsar Timing Array (PTA) facilities and early galaxy formation, favored by James Webb Space Telescope.

physics.gen-ph↗

Geodesic structure of generalized Vaidya spacetime through the K-essence

This article investigates on the radial and non-radial geodesic structures of the generalized K-essence Vaidya spacetime. Within the framework of K-essence geometry, it is important to note that the metric does not possess conformal equivalence to the conventional gravitational metric. This study employs a non-canonical action of the Dirac-Born-Infeld kind. In this work, we categorize the generalized K-essence Vaidya mass function into two distinct forms. Both the forms of the mass functions have been extensively utilized to analyze the radial and non-radial time-like or null geodesics in great details inside the comoving plane. Indications of the existence of wormhole can be noted during the extreme phases of spacetime, particularly in relation to black holes and white holes, which resemble the Einstein-Rosen bridge. In addition, we have also detected the distinctive indication of the quantum tunneling phenomenon around the central singularity.

gr-qc↗

Can Sterile Neutrino Explain Very High Energy Photons from GRB221009A?

The LHAASO collaboration has reported their observation of very high energy photons ($E^{max}_γ\simeq 18$ TeV) from the gamma-ray burst GRB221009A. The sterile neutrino that involves both mixing and transition magnetic moment may be a viable explanation for these high energy photon events. However, we demonstrate that such a solution is strongly disfavored by the cosmic microwave background (CMB) and Big Bang nucleosynthesis (BBN) in the standard cosmology.

hep-ph↗

Singularity formation in asymptotically safe cosmology with inhomogeneous equation of state

Interest in cosmological singularities has remarkably grown in recent times, particularly on future singularities with the discovery of late-time acceleration of the universe and dark energy. While such explorations have previously been done in various modified gravity and quantum gravitationally inspired cosmologies (besides standard general relativistic cosmology), no such an endeavour has been taken up till now in the realms of renormalization group approaches to cosmology and we have hence took up on this journey. In this work, we consider the formation of cosmological singularities in an asymptotically safe cosmology where the cut off scale is proportional to the Hubble parameter. We consider a well motivated inhomogeneous form of the equation of state(EOS) as well. We firstly delve into some basics of this cosmology and show that such a scenario permits a transition between phantom and quintessence forms of universal evolution. We then show that one can have Type I - Type IV singularities in such a cosmology for various version of the EOS and that the singularities can occur both in finite and infinite time. The conditions in which these singularities occur is significantly different than how they occur in the standard cosmology, with the formulations being even more involved. Interestingly this difference comes about without the need of any free parameters in the cosmological models, which is usually what one observes when one goes for the same pursuit via various modified gravity/ QG inspired cosmological approaches. Furthermore, we showed that usual singularity removal methods like conformal anomaly effects and f(R) gravity effects do not offer much hope for singularity removal in this cosmology.

gr-qc↗

On finite time singularities in scalar field dark energy models based in the RS-II Braneworld

The quest of deciphering the true nature of dark energy has proven to be one of the most exciting in recent times in cosmology. Various ideas have been put forward in this regard besides the usual cosmological constant approach, ranging from scalar field based models like Quintessence and Phantom dark energy to various modified gravity approaches as well. A very interesting idea then is to consider scalar field dark energy models in quantum gravitationally corrected cosmologies with the RS-II Braneworld being one of the most well known in this regard. So in this work, we consider RS-II Braneworld based scalar field dark energy models and try to look out for the existence of finite time singularities in these regimes both through a dynamical system perspective, for which we employ the Goriely-Hyde singularity analysis method, and a physical perspective. Our approach is general in the sense that it is not limited to any particular class of potentials or for any constrained parameter region for the brane tension and is valid for both Quintessence and phantom dark energy regimes. We firstly show through Goriely-Hyde procedure that finite time singularities can exist in these models for a limited set of initial conditions and that this result would hold irrespective of any consideration given to the swampland dS conjecture. We then discuss the physical nature of the singularities that can occur in this regime, where we use a well motivated ansatz for the Hubble parameter and show that these models of dark energy can allow for weak singularities like those of Type III and Type IV and can also allow for strong singularities like the Big Rip (Type I).

gr-qc↗

Type V singularities in non-standard cosmological backgrounds

Interest in cosmological singularities has remarkably grown in recent times, particularly on future singularities with the discovery of late-time acceleration of the universe and dark energy. Recent work has seen a proper classification of such singularities into strong and weak based on their strength, with weak singularities being the likes of sudden, w and big freeze singularities and strong singularities like the big rip. While there has been an expansive literature which has discussed the occurrence of Type I-IV in many non-standard cosmologies, w-singularities have not yet been explored in exotic cosmological settings. So in this work we pursue the same and discuss the status quo of w-singularities in a variety of non-standard cosmologies. We consider the RS-II Braneworld cosmology, an F(R) gravity cosmology which gives viable late time acceleration. We also consider cosmologies due to modified area-entropy relations, generalized uncertainty principles, holographic renormalization and Chern-Simons gravity( all of which can be coincidentally described by the same form of the modified Friedmann equation). We show that w-singularities will occur in exactly the same conditions in all these vividly different cosmological settings as they do in the usual general relativistic cosmology if one considers a power series expansion ansatz for the scale factor. We also show that if one considers an exponential form of the scale factor then while Type V singularities in the RS-II Braneworld and Chern-Simons cosmologies occur in the same conditions as in the standard cosmology case, there is a significant difference in the conditions when one considers the f(R) gravity case. These results are surprising overall, as one would usually not expect cosmological singularities to occur in almost the same conditions in non-standard cosmologies as they do in the usual standard cosmology.

gr-qc↗

An eccentric binary blackhole in post-Newtonian theory

Gravitational waves radiated during binary black hole coalescence is a perfect probe for studying the characteristics of strong gravity. Advanced techniques for creating numerical relativity substitute models for eccentric binary black hole systems are presumed to become more crucial in existing and anticipated gravitational wave detectors. The imprint on the observation data of the gravitational wave emitted by the binary coalescence enhances the two body system studies. The aim of this study is to present an overview of the change in characteristics behaviours of hierarchical massive astrophysical objects merger, which are the data bank of the early universe. We present results from numerical relativity simulations of equal-mass and unequal mass nonspinning inspiral binary-black-hole system in the Post Newtonian framework. We also consider the time evolution of eccentricity for the initial eccentric system. The eccentric Post Newtonian equations are expanded in the form of frequency related variable $x = (M ω)^{2/3}$. The model is restricted to the (2, 2) spin-weighted spherical harmonic modes. We conclude that for higher eccentricity as well as mass ratio there exist higher oscillation in orbital radius and in eccentricity.

gr-qc↗

The Large High Altitude Air Shower Observatory (LHAASO) Science Book (2021 Edition)

Since the science white paper of the Large High Altitude Air Shower Observatory (LHAASO) published on arXiv in 2019 [e-Print: 1905.02773 (astro-ph.HE)], LHAASO has completed the transition from a project to an operational gamma-ray astronomical observatory LHAASO is a new generation multi-component facility located in Daocheng, Sichuan province of China, at an altitude of 4410 meters. It aims at measuring with unprecedented sensitivity the spectrum, composition, and anisotropy of cosmic rays in the energy range between 10$^{12}$ and 10$^{18}$~eV, and acting simultaneously as a wide aperture (one stereoradiant) continuously operating gamma-ray telescope in the energy range between 10$^{11}$ and $10^{15}$~eV with the designed sensitivity of 1.3\% of the Crab Unit (CU) above 100 TeV. LHAASO's capability of measuring simultaneously different shower components (electrons, muons, and Cherenkov/fluorescence light), will allow it to investigate the origin, acceleration, and propagation of CR through measurement of the energy spectrum, elemental composition, and anisotropy with unprecedented resolution. The remarkable sensitivity of LHAASO will play a key role in CR physics and gamma-ray astronomy for a general and comprehensive exploration of the high energy universe and will allow important studies of fundamental physics (such as indirect dark matter search, Lorentz invariance violation, quantum gravity) and solar and heliospheric physics. The LHAASO Collaboration organized an editorial working group and finished all editorial work of this science book, to summarize the instrumental features and outline the prospects of scientific researches with the LHAASO experiment.

astro-ph.HE↗

Gravitational waves in the modified gravity

We have taken a modified version of the Einstein Hilbert action, $ f(R, T^ϕ) $ gravity under consideration, where $T^ϕ$ is the energy-momentum tensor trace for the scalar field under consideration. The structural behaviour of the scalar field considered varies with the form of the potential. The number of polarization modes of gravitational waves in modified theories has been studied extensively for the corresponding fields. There are two additional scalar modes, in addition to the usual two transverse-traceless tensor modes found in general relativity: a massive longitudinal mode and a massless transverse mode (the breathing mode).

gr-qc↗

Gravitational waves in the extended theory of gravity

Extended Theories of Gravity are considered as a new approach for solving the infrared and ultraviolet scale problems of the Standard Theory of Gravity (General Relativity). Observational evidence of gravitational waves and subsequent identification of the number of existing polarizations are an effective tool for testing general relativity and extended theories of gravity. The Newman-Penrose method is used to characterize the polarization modes for specific forms of $ f(R) $ in the present study. Both the forms of the $ f(R) $ theory belong to a far more general variational class of gravitational waves, capable of presenting up to six separate polarizations states. We have introduced new $ f(R) $ gravity models as an attempt to have a theory with more parametric regulations so that the model can be used to describe existing issues and discover different directions in gravity physics. The primary goal of this research is to look into the properties of gravitational waves with new cases. The model shows the existence of scalar degrees of freedom in $ f(R) $ gravity metric notation.

gr-qc↗

Anisotropic compact stars: Constraining model parameters to account for physical features of tidal Love numbers

In this paper, we develop a new class of models for a compact star with anisotropic stresses inside the matter distribution. By assuming a linear equation of state for the anisotropic matter composition of the star we solve the Einstein field equations. In our approach, for the interior solutions we use a particular form of the ansatz for the metric function $g_{rr}$. The exterior solution is assumed as Schwarzschild metric and is joined with the interior metric obtained across the boundary of the star. These matching of the metrices along with the condition of the vanishing radial pressure at the boundary lead us to determine the model parameters. The physical acceptability of the solutions has verified by making use of the current estimated data available from the pulsar 4U1608-52. Thereafter, assuming anisotropy due to tidal effects we calculate the Love numbers from our model and compare the results with the observed compact stars, viz. KS 1731- 260,4U 1608- 52,4U 1724- 207,4U 1820- 30,SAX J1748.9-2021 and EXO 1745-268. The overall situation confirms physical viability of the proposed approach,which can shed new light on the interior of the compact relativistic objects.

gr-qc↗

The stochastic gravitational wave background from magnetars

Magnetars have already been a potential candidate as gravitational wave sources that could be detected by current and future terrestrial as well as ground based gravitational wave detectors. In this article, we focus on the gravitational wave emission from the distorted rotating neutron stars. The deformation is assumed to be symmetric around an axis that is perpendicular to the rotation axis. The form is applied in the context of a neutron star whose magnetic field has been deformed on its own. By introducing the effects from all magnetars in the Universe, based on various proposed magnetic field configurations, such as poloidal, toroidal; stochastic gravitational wave background (SGWB) can be generated. We choose to figure out exactly how the observations of the stochastic gravitational-wave background should be used to understand much more about physics correlated with the magnetar behaviour, based on the restriction on the eccentricity of the magnetar.

gr-qc↗