International Journal of Modern Physics D

Over the years, de Sitter spacetime has been a central focus in studies involving quantum fields, for its importance in the early and late expansion stages of the universe. While de Sitter spacetime closely mimics characteristics of the inflationary and dark energy dominated universe, it does not help to understand the radiation and matter dominated expansions. In this paper, we revisit some recent works involving the author which study gravitational particle creation beyond the de Sitter stage. Specifically, we present novel aspects of particle creation in the radiation dominated universe and then provide an analysis of time evolution of the primordial (Bunch–Davies) vacuum state, its particle excitation and quantum versus classical behavior of field modes in a multi-stage universe, comprising of (i) the inflationary de Sitter (ii) the radiation dominated and (iii) the late dark energy dominated stages.

We consider a unified barotropic dark fluid model with dissipation. Our fluid asymptotes between two power laws and so can interpolate between the dust and dark energy (DE) equations-of-state at early and late times. The dissipative part is a bulk viscous part with constant viscosity coefficient. The model is analyzed using the phase-space methodology which helps to understand the dynamical behavior of the model in a robust manner without reference to the system solution. The parameters of the model are constrained through many observational constraints. The model is tested through many physical and observational tests. We first considered the model independent $Om(z)$ test. Results for $Om(z)$ are plotted against the BAO data for this quantity from different authors, which shows that the model is consistent with the data points for the full redshift range. The ${\chi }^2$ statistics results in the value of ${{\chi }^2}_{\mathrm{\text{red}}}=0.30$ with a $p$-value of $0.95$. The Hubble parameter equation is solved numerically and results are plotted against the recent set of Hubble data. The ${\chi }^2$ test with the Hubble data resulted in the ${{\chi }^2}_{\mathrm{\text{red}}}$ value of $0.672$ with a $p$-value of $0.94$. The distance modulus at different values of redshift is calculated numerically and results are compared to the newest set of SNe Ia data, the Pantheon Sample. We obtained a ${{\chi }^2}_{\mathrm{\text{red}}}$ value of $1.04$ with a $p$-value of $0.17$. These results show that our model is efficiently consistent with observations. The model expectations for the evolution of the universe are also studied by testing the evolution of the deceleration parameter, the density of the universe, and the effective equation-of-state parameter of the model and of its underlying dark energy candidate. The value of the present day viscosity coefficient of the cosmic fluid, ${\zeta }_0$, is estimated. It is found to be $8\times 10^6$Pas. We argue that this model is able to explain the behavior of the universe evolution.

We compute the lowest frequency nonradial oscillation modes of dilute axion stars. The effective potential that enters into the Schrödinger-like equation, several associated eigenfunctions and the large as well as the small frequency separations are shown as well.

We have investigated dynamical evolution of electromagnetic perturbation in a scalar hairy black hole spacetime, which belongs to solutions in Horndeski theory with a logarithmic cubic term. Our results show that the parameter $\alpha$ affects the existence of event horizon and modifies the asymptotical structure of spacetime at spatial infinity, which imprints on the quasinormal frequency of electromagnetic perturbation. Moreover, we find that the late-time tail of electromagnetic perturbation in this background depends also on the parameter $\alpha$ due to the existence of solid angle deficit. These imply that the spacetime properties arising from the logarithmic cubic term in the action play important roles in the dynamical evolutions of the electromagnetic perturbation in the background of a scalar hairy black hole.

Using the quasinormal modes of a massless scalar perturbation, we investigate the small/large black hole phase transition in the Lorentz symmetry breaking massive gravity. We mainly focus on two issues: (i) the sign change of slope of the quasinormal mode frequencies in the complex-$\omega$ diagram; (ii) the behaviors of the imaginary part of the quasinormal mode frequencies along the isobaric or isothermal processes. For the first issue, our result shows that, at low fixed temperature or pressure, the phase transition can be probed by the sign change of slope. While increasing the temperature or pressure to certain values near the critical point, there will appear the deflection point, which indicates that such method may not be appropriate to test the phase transition. In particular, the behavior of the quasinormal mode frequencies for the small and large black holes tend to be the same at the critical point. For the second issue, it is shown that the nonmonotonic behavior is observed only when the small/large black hole phase transition occurs. Therefore, this property can provide us with an additional method to probe the phase transition through the quasinormal modes.

We carry out the stability analysis of the Schwarzschild black hole in Gauss–Bonnet and Chern–Simons-scalar theory. Here, we introduce two quadratic scalar couplings (${\varphi }_1^2,{\varphi }_2^2$) to Gauss–Bonnet and Chern–Simons terms, where the former term is parity-even, while the latter one is parity-odd. The perturbation equation for the scalar ${\varphi }_1$ is the Klein–Gordon equation with an effective mass, while the perturbation equation for ${\varphi }_2$ is coupled to the parity-odd metric perturbation, providing a system of two coupled equations. It turns out that the Schwarzschild black hole is unstable against ${\varphi }_1$ perturbation, leading to scalarized black holes, while the black hole is stable against ${\varphi }_2$ and metric perturbations, implying no scalarized black holes.

We describe a new actuation technique for gravity experiments based on a liquid field mass. The characterizing idea is to modulate the gravity force acting on a test mass by controlling the level of a liquid in a suitable container. This allows to obtain a periodical gravity force without moving parts (except the liquid level) close to the TM. We describe in detail the most relevant aspects of the liquid actuator and discuss how it can be used in gravity experiments. In particular, we analyze an application to test the inverse square law in the mm to cm distance region.

This paper discusses a generalized model for compact stars, assumed to be anisotropic in nature due to the presence of highly dense and ultra-relativistic matter distribution. After embedding the 4D Riemannian space locally and isometrically into a 5D pseudo-Euclidean space, we solve the Einstein equations by employing a class of physically acceptable metric functions. The physical properties determined include the anisotropic factor showing that the anisotropy is zero at the center and maximum at the surface. Other boundary conditions yield the values of various parameters needed for rendering the numerous plots and also led to the EOS parameters. It is further determined that the usual energy conditions are satisfied and that the compact structures are stable, based on several criteria, viz., the equilibrium of forces, Herrera cracking concept, adiabatic index, etc. We note that the proposed stellar model satisfies the Buchdahl condition. Finally, the values of the numerous constants and physical parameters are determined, specifically for the compact stellar object $\mathrm{\text{LMC}}X-4$, which we choose as a representative of the compact stars to present the analysis of the obtained results. Finally, we show that the present generalized model can justify most of the compact stars including white dwarfs and ultra-dense compact stars for a suitable tuning of the parametric values of $n$.

We investigate higher-dimensional spherically symmetric anisotropic collapsing solutions of the field equations. Our aim is to check the effects of higher dimensions on the density and pressures profile of the collapsing fluid. It has been observed that the energy density, radial and tangential pressures of the collapsing system are strongly affected by higher dimensions. It also comes out that the anisotropy of the collapsing system becomes constant in higher dimensions.

In order to explain the current acceleration of the universe, the fine-tuning problem of the cosmological constant $\mathrm{\Lambda }$ and the cosmic coincidence problem, different alternative models have been proposed in the literature. We use the most recent observational data from CMB (Planck 2018 final data release) and LSS (SDSS, WiggleZ, VIPERS) to constrain dynamical dark energy (DE) models. The CMB shift parameter, which traditionally has been used to determine the main cosmological parameters of the standard model $\mathrm{\Lambda }\mathrm{\text{CDM}}$, is employed in addition to data from redshift-space distortions through the growth parameter $A(z)=f(z){\sigma }_8(z)$ to constrain the mass variance ${\sigma }_8$. BAO data are also used to study the history of the cosmological expansion and the main properties of DE. From the evolution of $q(z)$, we found a slowdown of acceleration behavior at low redshifts, and by using the Akaike and Bayesian Information Criteria (AIC, BIC), we discriminate different models to find those that are better suited to the observational data, finding that the interacting dark energy (IDE) model is the most favored by observational data, including information from SNIa and Hz. The analysis shows that the IDE model is followed closely by EDE and $\mathrm{\Lambda }\mathrm{\text{CDM}}$ models, which in some cases fit better the observational data with individual probes.

We develop an action principle to construct the dynamics that gives rise to a minimal generalization of Einstein’s equations, where the speed of light ($c$), the gravitational constant ($G$) and the cosmological constant ($\mathrm{\Lambda }$) are allowed to vary. Our construction preserves general covariance of the theory, which yields a general dynamical constraint on $c$, $G$ and $\mathrm{\Lambda }$. This action is general and can be applied to describe different cosmological solutions. We apply this formulation to the initial condition puzzles of the early universe and show that it generates a dynamical mechanism to obtain the homogeneous and flat universe we observe today. We rewrite the conditions necessary to solve the horizon and flatness problems in this framework, which does not necessarily lead to an accelerated expansion as in inflation. Then, we show how the dynamics of the scalar field that represents $c$ or $G$ (and $\mathrm{\Lambda }$) can be used to solve the problems of the early universe cosmology by means of different ways to $c$-inflate the horizon in the early universe. By taking $\mathrm{\Lambda }=0$, we show that the dynamics of the scalar field representing $c$ can be described once a potential is given.

Within the context of nonlinear electromagnetism, we consider the Yukawa extension of a Reissner–Nordström black hole. Exact solution is given which modifies certain characteristics of the latter. Some thermodynamical aspects are given for comparison. The model which is based on a nonpolynomial Lagrangian implicitly may be considered as a useful agent to describe a short-ranged, charged and massive interaction.

TianQin is a geocentric space-based gravitational-wave observatory mission consisting of three drag-free controlled satellites in an equilateral triangle with an orbital radius of $10^5$km. The constellation faces the white-dwarf binary RX J0806.3$+$1527 located slightly below the ecliptic plane, and is subject to gravitational perturbations that can distort the formation. In this study, we present combined methods to optimize the TianQin orbits so that a set of 5-year stability requirements can be met. Moreover, we discuss slow long-term drift of the detector pointing due to orbital precession, and put forward stable orbits with six other pointings along the lunar orbital plane. Some implications of the findings are pointed out.

The possible existence of $\mathrm{\Delta }$ resonances is inspected in the cold dense matter of neutron star (NS) core in the presence of hyperons. The diverse effects of variation in $\mathrm{\Delta }$ mass on their formation and the equation of state (EoS) are studied in this work with an effective chiral model and the resultant NS properties are calculated with the help of parameterized Tolman–Oppenheimer–Volkoff (PTOV) equations to bring out the two important features of pressure in the context of massive NSs. The $\mathrm{\Delta }$ puzzle is re-explored and resolved taking into account the concept of modified/parametrized inertial pressure and self-gravity in case of massive pulsars like PSR J1614−2230 and PSR J0348-0432. It is seen that although the presence of exotic matter like the hyperons and $\mathrm{\Delta }s$ softens the EoS considerably, their presence in massive NSs can be successfully explained with the theory of parametrized hydrostatic equilibrium conditions. The results of this work also satisfy the constraints on $R_{1.4}$ and $R_{1.6}$ from the gravitational wave (GW170817) detection of binary NS merger. The constraint on baryonic mass from PSR J0737-3039 is also satisfied with the solutions of the PTOV equations for all the $\mathrm{\Delta }$ masses considered.

This paper is aimed to set up a thin-shell gravastar model and address its physically accepted features in the background of noncommutative geometry. For this purpose, we have considered the cylindrically symmetric interior metric matched with suitable noncommutative exterior geometry using Israel boundary conditions. The stability of this thin-shell as well as thermodynamical stability is then explored under linear perturbations around the throat. We have found the stable regions near the horizon with some specific values of the involved parameters.