International Journal of Modern Physics D

Einstein, Infeld and Hoffmann (EIH) claimed that the field equations of general relativity theory alone imply the equations of motion of neutral matter particles, viewed as point singularities in space-like slices of spacetime; they also claimed that they had generalized their results to charged point singularities. While their analysis falls apart upon closer scrutiny, the key idea merits our attention. This report identifies necessary conditions for a well-defined general-relativistic joint initial value problem of $N$ classical point charges and their electromagnetic and gravitational fields. Among them, in particular, is the requirement that the electromagnetic vacuum law guarantees a finite field energy–momentum of a point charge. This disqualifies the Maxwell(–Lorentz) law used by EIH. On the positive side, if the electromagnetic vacuum law of Bopp, Landé–Thomas and Podolsky (BLTP) is used, and the singularities equipped with a nonzero bare rest mass, then a joint initial value problem can be formulated in the spirit of the EIH proposal, and shown to be locally well-posed — in the special-relativistic zero-$G$ limit. With gravitational coupling (i.e. $G>0$), though, changing Maxwell’s into the BLTP law and assigning a bare rest mass to the singularities is by itself not sufficient to obtain even a merely well-defined joint initial value problem: the gravitational coupling also needs to be changed, conceivably in the manner of Jordan and Brans–Dicke.

We report on recent developments toward a relativistic quantum-mechanical theory of motion for a fixed, finite number of electrons, photons and their anti-particles, as well as its possible generalizations to other particles and interactions.

This paper deals with warm viscous inflation inspired by tachyon field using an important inflationary method known as Hamilton–Jacobi’s Formalism. This method provides a way to solve the equations of inflationary model without using extra approximations other than slow-roll. In this scenario, a general inflationary formalism is developed for tachyon field and the work is restricted to high dissipative regime. A detailed analysis of the model is presented for three different cases of constant as well as variable bulk and dissipative coefficients. In each case, the model parameters are constrained to plot the physically acceptable range of the perturbed parameters. The parametric trajectories proved that the acquired results for all cases are compatible with Planck astrophysical data.

We consider the newly proposed gravitational modifications that go beyond Horndeski’s theory, named as theories with extended nonminimal derivative couplings. By these modifications, the coefficient functions depend on the scalar field and its kinetic energy. These theories become ghost-free in cosmological background. We consider the flat FRW universe and explore the equation-of-state parameter, ${\omega }_{\text{tot}}$–${\omega }_{\text{tot}}^{\prime }$ plane and the squared speed of sound. The equation-of-state parameter exhibits phantom behavior of the universe, ${\omega }_{\text{tot}}$–${\omega }_{\text{tot}}^{\prime }$ plane represents the freezing region of the universe while the squared speed of sound denotes the stability of the model for the specific choice of constant parameters. Also, we investigate the validity of generalized second law of thermodynamics on the Hubble horizon taking into account the Bekenstein, power-law, Renyi and logarithmic corrections to the horizon entropy.

A five-dimensional braneworld cosmological model in general scalar–tensor action that is comprised of various Horndeski Lagrangians is considered. The Friedmann equations in the case of strongly and weakly coupled ${{ℒ}}_5$ Horndeski Lagrangians have been obtained. The strongly coupled ${{ℒ}}_5$ model produces the Cardassian term ${\rho }^n$ with $n=\pm 1/2$, which can serve as an alternative explanation for the accelerated expansion phase of the universe. Furthermore, the latest combined observational facts from BAO, CMB, SNIa, $f_{{\sigma }_8}$ and $H_0$ value observation suggest that the $n=-1/2$ term lies quite close to the constrained value. On the other hand, the weakly coupled ${{ℒ}}_5$ case has several new correction terms which are omitted in the braneworld Einstein–Hilbert model, e.g. the cubic ${\rho }^3$ and the dark radiation–matter interaction term $\chi a^{-4}\rho$. Furthermore, this model provides a cosmological constant constructed from the bulk scalar field, requires no brane tension and supports the big bang nucleosynthesis (BBN) constraint naturally.

We propose a thermal interpretation of the Schwinger effect for charged spinless scalars and spin-1/2 fermions in an extremal and near-extremal Reissner–Nordström (RN) black hole. The emission of charges has the distribution with an effective temperature determined by the Davies–Unruh temperature for accelerating charges by the electric field and the scalar curvature of ${AdS}_2$ from the near-horizon geometry ${AdS}_2\times S^2$. We find a charge bound for the extremal micro-black hole to remain stable against the Schwinger emission in analogy with the Breitenlohner–Freedman bound for the $AdS$ space. In the in–out formalism, we find the one-loop QED effective action consistent with the vacuum persistence and interpret the vacuum persistence as the leading Schwinger effect and the effect of a charged vacuum of the Coulomb field.

In this paper, we study relativistic dynamics of charged particles corotating with prescribed trajectories, having the shape of dipolar magnetic field lines. In particular, we consider the role of the drag force caused by the photon field the forming of equilibrium positions of the charged particles. Alongside a single particle approach, we also study behavior of ensemble of particles in the context of stable positions. As we have shown, the together they create surfaces where particles are at stable equilibrium positions. In this paper, we examine these shapes and study parameters they depend on. It has been found that under certain conditions, there are two distinct surfaces with stable equilibrium positions.

The thermodynamic stability of a thin-shell wormhole in a Schwarzschild bulk is considered. From the first law, entropy function is found which satisfies the local intrinsic stability conditions. Heat capacity emerges as a well-defined regular function justifying the stability of a Schwarzschild thin-shell wormhole. Our method applies only to static thin-shell wormholes and in this sense it may be considered as supplementary to the classical method of stability. The scope of applications of the method is not limited by the Schwarzschild wormhole.

Few statements in cosmology can be made without assuming a cosmological model within which to interpret data. Statements about cosmic acceleration are no exception to this rule, and the inferred positive volume acceleration of our universe often quoted in the literature is valid in the context of the standard Friedmann–Lemaître–Robertson–Walker (FLRW) class of spacetimes. Using the Joint Light-curve Analysis (JLA) catalogue of supernovae Type Ia (SNIa), we examine the fit of a class of exact scaling solutions with dynamical spatial curvature formulated in the framework of a scalar averaging scheme for relativistic inhomogeneous spacetimes. In these models, global volume acceleration may emerge as a result of the nonlocal variance between expansion rates of clusters and voids, the latter gaining volume dominance in the late-epoch universe. We find best-fit parameters for a scaling model of backreaction that are reasonably consistent with previously found constraints from SNIa, CMB, and baryon acoustic oscillations data. The quality of fit of the scaling solutions is indistinguishable from that of the ΛCDM model and the timescape cosmology from an Akaike Information Criterion (AIC) perspective. This indicates that a broad class of models can account for the $z<\sim 1$ expansion history.

The relativistic mean field (RMF) theory is used to examine the effect of the meson-$\mathrm{\Xi }$ coupling constants on the properties of the neutron star (NS) PSR J0348+0432 when the hyperon potential of $\mathrm{\Xi }$ is fixed. It is found that the greater the $x_{\sigma \mathrm{\Xi }}$ and $x_{\omega \mathrm{\Xi }}$, the stronger the repulsive interaction. The potential field strengths of mesons $\omega$ and $\rho$ and the chemical potentials of neutron n and electron e increase, whereas the potential field strengths of mesons $\sigma$ decrease with the increase of the meson-$\mathrm{\Xi }$ coupling constants. It is also found that the relative particle number density of neutron ${\rho }_n/\rho$ and hyperon $\mathrm{\Lambda }$ in the NS PSR J0348+0432 increase, whereas that of hyperon ${\mathrm{\Xi }}^{-}$ decrease as the meson-$\mathrm{\Xi }$ coupling constants increase. In our calculations, hyperon ${\mathrm{\Xi }}^0$, ${\mathrm{\Sigma }}^{-}$, ${\mathrm{\Sigma }}^0$ and ${\mathrm{\Sigma }}^{+}$ all do not appear. Our results show that the meson-$\mathrm{\Xi }$ coupling constants cannot be uniquely determined as the hyperon-potential of $\mathrm{\Xi }$ is determined. Therefore, the related properties of NS cannot be uniquely determined only by the determined hyperon potential of $\mathrm{\Xi }$.

Unusual and multicore extensive air showers are detected in some experiments. Study of these kinds of showers is an important issue in nuclear collisions at high energies and need to be interpreted by hadronic interaction models. In the work, taking into account results Sleptsova et al. of which emphasized shower size variations at energies above $3\times 10^{18}$eV, and also by using of the auger experimental data, some characteristics of multicore events are investigated. Intense nuclear variations are seen in energy range of $10^{18}$–$10^{19}$eV, and nuclear distinctive properties of multicore showers relative to normal showers are shown.

Tsallis entropy has been widely applied to analyze the gravitational and cosmological setups. We discuss the dark energy (DE) model by its cosmological consequences using Tsallis holographic entropy in the framework of modified Brans–Dicke (BD) gravity. We consider the Hubble horizon as infrared cutoff to study the nature of DE that is responsible for current cosmic acceleration. We focus on flat FRW universe in interacting and noninteracting scenarios between DE and dark matter (DM). In this framework, we discuss the cosmological parameters like equation of state parameter, deceleration parameter, Om-diagnostic, squared speed of sound and planes like evolving equation of state parameter and statefinders. We discuss graphical presentation of these parameters and planes. We compare the results with observation data to check the consistency of results.

We investigate bouncing scenario in the modified $f(T)$ gravity, $T$ being the torsion scalar. Attention is attached to the reconstruction of $f(T)$ able to describe type IV singular bouncing evolution, where we adopt as assumption that the bouncing and type IV singularity points coincide. In the context of the reconstructed $f(T)$ model, we calculate the Hubble slow-roll parameters in order to determine the dynamical evolution of the cosmological system under study. The results show that the Hubble slow-roll parameters become singular at the type IV singularity indicating a dynamical instability. Moreover, we perform the stability analysis of the $f(T)$ gravity solution where, according to the obtained result, the type IV singularity point is a saddle point in agreement with the bounce scenario.

The dark matter admixed neutron stars (DANSs) are studied using the two-fluid TOV equations separately, in which the normal matter (NM) and dark matter (DM) are simulated by the relativistic mean field theory and self-interacting fermionic model, respectively. A universal relationship $M_D^{max}=(0.269\frac{m_f}{m_I}+0.627){(\frac{1\mathrm{\text{GeV}}}{m_f})}^2{\mathrm{\text{M}}}_{\odot }$ is suggested, where $M_D^{max}$ is the maximum mass of DM existing in DANSs, $m_f$ is the particle mass of DM ranging from 5GeV to 1TeV, $m_I$ is the interaction mass scale with the value 300GeV (0.1GeV) for weak (strong) interaction DM model. This simple formula connects directly the microcosmic nature of DM particle with its macrocosmic mass existing in DANSs. Meanwhile, such a formula exhibits that the existence of NM has little effect on $M_D^{max}$. It is found that the ratio of radius of DM in DANSs over $M_D^{max}$ is a constant with the value about 12$\frac{\mathrm{\text{km}}}{{\mathrm{\text{M}}}_{\odot }}$ (7$\frac{\mathrm{\text{km}}}{{\mathrm{\text{M}}}_{\odot }}$) for weak (strong) interaction DM cases. According to the calculated results, only for the strong interaction DM cases with $m_f=5$ to $10$GeV and central energy density ${𝜀}_D>10^3$MeV/fm³, DM has obvious effect on the mass of compact star. Compared with the energy density of DM in the Milky Way galaxy, $\sim 10^{-36}$MeV/fm³, the existence of DM might hardly affect the mass of compact stars in the Milky Way galaxy.

In this paper, we study some relevant cosmological features of a Dark Energy (DE) model with Granda–Oliveros cut-off, which is just a specific case of Nojiri–Odintsov holographic DE [S. Nojiri and S. D. Odintsov, Gen. Relativ. Gravit. 38 (2006) 1285] unifying phantom inflation with late-time acceleration, in the framework of Chameleon Brans–Dicke (BD) cosmology. Choosing a particular ansatz for some of the quantities involved, we derive the expressions of some important cosmological quantities, like the Equation of State (EoS) parameter of DE $w_D$, the effective EoS parameter $w_{\mathrm{\text{eff}}}$, the pressure of DE $p_D$ and the deceleration parameter $q$. Moreover, we study the behavior of statefinder parameters $r$ and $s$, of the cosmographic parameters $j$, $s_{\mathrm{\text{cosmo}}}$, $l$ and $m$ and of the squared speed of the sound $v_s^2$ for both case corresponding to noninteracting and interacting Dark sectors. We also plot the quantities we have derived and we calculate their values for $t\to 0$ (i.e. for the beginning of the universe history), for $t\to \infty$ (i.e. for far future) and for the present time, indicated with $t_0$. The EoS parameters have been tested against various observational values available in the literature.