International Journal of Modern Physics D

Compact objects occupy a pivotal role in the exploration of Nature. The interest spans from the role of compact objects in astrophysics to their detection through various methods (gravitational waves interferometry, microlensing, imaging). While the existence of compact objects made of fermions (neutron stars and white dwarfs) has been assessed, a parallel search for localized solitons made of bosons is ongoing, stemming from Wheeler’s original proposal of electromagnetic “geons”. Boson fields can clump up and form compact objects such as boson stars (for complex scalar fields), oscillons and oscillatons (for real scalar fields), or Proca stars (for massive vector fields), which can show up in searches as black hole mimickers, dark matter sources, and a variety of other phenomena. I review some crucial properties of these bosonic systems, including recent progress in the field.

Binary-driven hypernova (BdHN) models long gamma-ray burst (GRB) as occurring in the binary systems involving a carbon–oxygen core (CO${}_{\mathrm{\text{core}}}$) and a companion neutron star (NS) or a black hole (BH). This model, first proposed in 2012, succeeds and improves upon the fireshell model and the induced gravitational collapse (IGC) paradigm. After nearly a decade of development, the BdHN model has reached a nearly complete structure, giving explanation to all the observables of long bursts into its theoretical framework, and has given a refined classification of long GRB according to the original properties of the progenitors. In this paper, we present a summary of the BdHN model and the physical processes at work in each of the envisaged Episodes during its occurrence and lifetime, duly contextualized in the framework of GRB observations.

Eddington–Finkelstein metric is obtained from the Schwarzschild metric by a change of the time variable. It is well known that a test mass falling into a black hole does not reach the event horizon for any finite value of the Schwarzschild time variable $t$. By contrast, we show that the event horizon is reached for a finite value of the Eddington–Finkelstein time variable $t^{\prime }$. Then we study in Eddington–Finkelstein time the fate of a massive particle traversing an Einstein–Rosen bridge and obtain a different conclusion than recent proposals in the literature: we show that the particle reaches the wormhole throat for a finite value $t_1^{\prime }$ of the time marker $t^{\prime }$, and continues its trajectory across the throat for $t^{\prime }>t_1^{\prime }$. Such a behavior does not make sense in Schwarzschild time since it would amount to continuing the trajectory of the particle “beyond the end of time.”

In this work, we present all the possible solutions for a static cylindrical symmetric spacetime in the Einstein-Aether (EA) theory. As far as we know, this is the first work in the literature that considers cylindrically symmetric solutions in the theory of EA. One of these solutions is the generalization in EA theory of the Levi-Civita (LC) spacetime in General Relativity (GR) theory. We have shown that this generalized LC solution has unusual geodesic properties, depending on the parameter $c_{14}$ of the aether field. The circular geodesics are the same of the GR theory, no matter the values of $c_{14}$. However, the radial and $z$-direction geodesics are allowed only for certain values of $\sigma$ and $c_{14}$. The $z$-direction geodesics are restricted to an interval of $\sigma$ different from those predicted by the GR and the radial geodesics show that the motion is confined between the origin and a maximum radius. The latter is not affected by the aether field but the velocity and acceleration of the test particles are besides, for $0\le \sigma <1/2$, when the cylindrical symmetry is preserved, this spacetime is singular at the axis $r=0$, although for $\sigma >1/2$ exists interval of $c_{14}$ where the spacetime is not singular, which is completely different from that one obtained with the GR theory, where the axis $r=0$ is always singular.

This paper aims to put constraints on the parameters of the Scalar Field Dark Matter (SFDM) model, when dark matter is described by a free real scalar field filling the whole universe, plus a cosmological constant term. By using a compilation of 51 $H(z)$ data and 1048 Supernovae data from Panteon, a mass range for the scalar field was obtained, $7.8\times 10^{-35}\le m\le 10^{-17}$eV, in good agreement with light mass axion particles. Also, we have obtained $H_0=69.5_{-2.1}^{+2.0}{\mathrm{\text{km s}}}^{-1}{\mathrm{\text{Mpc}}}^{-1}$, and the present dark matter density parameter ${\mathrm{\Omega }}_{\varphi }=0.230_{-0.031}^{+0.033}$ at $2\sigma$ confidence level. These results are in good agreement to standard model of cosmology, showing that SFDM model is viable in describing the dark matter content of the universe.

In this paper, we study a new class of holographic heat engines via charged AdS black hole solutions of Einstein gravity coupled with logarithmic nonlinear $U(1)$ gauge theory. So, logarithmic $U(1)$ AdS black holes with a horizon of positive, zero and negative constant curvatures are considered as a working substance of a holographic heat engine and the corrections to the usual Maxwell field are controlled by nonlinearity parameter $\beta$. The efficiency of an ideal cycle ($\eta$), consisting of a sequence of isobaric $\to$ isochoric $\to$ isobaric $\to$ isochoric processes, is computed using the exact efficiency formula. It is shown that $\eta /{\eta }_C$, with ${\eta }_C$ the Carnot efficiency (the maximum efficiency available between two fixed temperatures), decreases as we move from the strong coupling regime ($\beta \to 0$) to the weak coupling domain ($\beta \to \infty$). We also obtain analytic relations for the efficiency in the weak and strong coupling regimes in both low and high temperature limits. The efficiency for planar and hyperbolic logarithmic $U(1)$ AdS black holes is computed and it is observed that efficiency versus $\beta$ behaves in the same qualitative manner as the spherical black holes.

We study the acceleration of charged particles by Reissner Nordström black hole by taking into account the term appearing in the formula of the center of mass energy due to charge of the particle. We consider that the particle is radially falling towards the black hole, i.e., $L=0$. It is found that the center of mass energy is infinitely large at the outer horizon without any constraint.

The parabolic–hyperbolic form of the constraints is integrated numerically. The applied numerical stencil is fourth-order accurate (in the spatial directions) while “time”-integration is made by using the method of lines with a fourth-order order accurate Runge–Kutta scheme. The proper implementation of the applied numerical method is verified by convergence tests and monitoring the relative and absolute errors is determined by comparing numerically and analytically known solutions of the constraints involving boosted and spinning vacuum single black hole configurations. The main part of our investigations is, however, centered on the construction of initial data for distorted black holes which, in certain cases, have non-negligible gravitational wave content. Remarkably, the applied new method is unprecedented in that it allows to construct initial data for highly boosted and spinning black holes, essentially for the full physical allowed ranges of these parameters. In addition, the use of the evolutionary form of the constraints is free from applying any sort of boundary conditions in the strong field regime.

The Event Horizon Telescope’s image of the M87 black hole provides an exciting opportunity to study black hole physics. Since a black hole’s event horizon absorbs all electromagnetic waves, it is difficult to actively probe the horizon’s existence. However, with the help of a family of extremely compact, horizon-less objects, named “gravastars”, whose external spacetimes are nearly identical to those of black holes, one can test the absence of event horizons: absences of additional features that arise due to the existence of the gravastar, or its surface, can be used as quantitative evidence for black holes. We apply Gralla et al. approach of studying black hole images to study the images of two types of gravastars: transparent ones and reflective ones. In both cases, the transmission of rays through gravastars, or their reflections on their surfaces, leads to more rings in their images. For simple emission models, where the redshifted emissivity of the disk is peaked at a particular radius $r_{\mathrm{\text{peak}}}$, the position of a series of rings can be related in a simple manner to light ray propagation: a ring shows up around impact parameter $b$ whenever rays incident from infinity at $b$ intersects the disk at $r_{\mathrm{\text{peak}}}$. We show that additional rings will appear in the images of transparent and reflective gravastars. In particular, one of the additional rings for the reflective gravastar is due to the prompt reflection of light on the gravastar surface, and appears to be well separated from the others. This can be an intuitive feature, which may be reliably used to constrain the reflectivity of the black hole’s horizon.

We obtain vacuum solutions in the presence of a cosmological constant in the context of the Weyl geometrical scalar–tensor theory. We investigate the limit when $\omega$ goes to infinity and show by working out the solutions that in this limit there are some cases in which the scalar field tends to a constant (with the implicit consequence of the geometry becoming Riemannian), although the solutions do not reduce to the corresponding Einstein solutions. We have also extended a previous result, known in the literature, by showing that in the case of vacuum with cosmological constant the field equations of the Weyl geometrical scalar–tensor theory are formally identical to Brans–Dicke field equations, even though these theories are not physically equivalent.

In this work, we investigate a cosmological model with the tachyon and fermion fields with barotropic equation of state, where pressure $p$, energy density $\rho$ and barotropic index $\gamma$ are related by the relation $p=(\gamma -1)\rho$. We applied the tachyonization method which allows to consider cosmological model with the fermion and the tachyon fields, driven by special potential. In this paper, tachyonization model was defined from the stability analysis and exact solution standard of the tachyon field. Analysis of the solution via statefinder parameters illustrated that our model in fiducial points with deceleration parameter $q=0.5$ and statefinder $r=1$ corresponds to the matter-dominated universe (SCDM) but ends its evolution at a point in the future $(q=-1,r=1)$ which corresponds to the de Sitter expansion. Comparison of the model parameters with the cosmological observation data demonstrates that our proposed cosmological model is stable at barotropic index ${\gamma }_0=0.00744$.

In this paper, we have developed a static charged anisotropic stellar model for a spherically symmetric spacetime geometry. For this, we have considered charged Durgapal V perfect fluid sphere and constructed its anisotropic counterpart by using gravitational decoupling through the minimal geometric deformation (MGD) approach. We have presented physical analysis of the constructed model in details by taking observational data of two different compact stars, namely, SMC X-4 and Vela X-1, into account. All the physical properties such as density, pressure, sound speeds, energy conditions, stability conditions, Tolman–Oppenheimer–Volkoff (TOV) equation, compactness factor, and redshift are found to meet the general criteria required to ensure a well-behaved stellar system.

In this paper, we study a constant-roll inflationary model in the context of brane-world cosmology caused by a quintessence scalar field for warm inflation with a constant dissipative parameter $Q=\mathrm{\Gamma }/3H$. We determine the analytical solution for the Friedmann equation coupled to the equation of motion of the scalar field. The evolution of the primordial scalar and tensor perturbations is also studied using the modified Langevin equation. To check the viability of the model, we use numerical approaches and plot some figures. Our results for the scalar spectral index and the tensor-to-scalar ratio show good consistency with observations.

In this paper, we consider a quantum mechanical system to model the effect of quantum fields on the evolution of the early universe. The system consists of an inverted oscillator bilinearly coupled to a set of harmonic oscillators. We point out that the role of noise may be crucial in the dynamics of the oscillator, which is analyzed using the theory of harmonic oscillators with random frequency. Using this analogy, we argue that due to the fluctuations around its mean value, a positive vacuum energy density would not produce an exponentially expanding but an oscillating universe, in the same fashion that an inverted pendulum is stabilized by random oscillations of the suspension point (stochastic Kapitza pendulum). The results emphasize the relevance of noise in the evolution of the scale factor.

The QCD phase transition exhibits different periods during the evolution time of the universe. We explore the cosmological implications of QCD phase transition in the early universe, which was mainly full of QGP matter, through the Friedmann equations. This is performed by studying the effect of GUP on the MIT bag model. The obtained modified MIT bag model alongside with the Friedmann equations is utilized to study the thermodynamical quantities in terms of the evolution time. The effect of GUP is set by inserting the parameter $\alpha =10^{-2}{\mathrm{\text{GeV}}}^{-1}$. It is found that the GUP effect on the evolution time of the universe makes it shorter, i.e. the time span of each phase takes shorter time to transfer to another phase than their corresponding ones without GUP effect.

The work presented in this paper aims to study the inflationary cosmology of $f(Q)$ gravity where $Q=6H^2$ by two models of Dark Energy (DE), namely Variable Generalized Chaplygin Gas (VGCG) and the Nojiri–Odintsov Holographic Dark Energy. The reconstruction has been carried out in the absence as well as presence of viscosity. The viscous pressure is considered to be $\mathrm{\Pi }=-3H\xi$, where $\xi ={\xi }_0+{\xi }_{1}H+{\xi }_2(Ḣ+H^2)$ and $H$ is the Hubble parameter. In viscous and nonviscous scenarios, the reconstruction has been carried out for power law scale factor and bounce scale factor. For the specific range of cosmic time $t$, the quintessence and quintom behavior are observed for the equation of state parameters. The slow roll parameters have been studied for both the cases in nonviscous scenario and for the case, namely bounce scale factor for VGCG of nonviscous scenario, we have found that there is a scope of exit from inflation. Furthermore, constraints have been deduced to study the presence of singularity. Finally, the $f(Q)$ gravity is reconstructed as a function of cosmic time $t$ and is found to stay at positive level for a range of cosmic time $t$.