International Journal of Modern Physics D

We study the quantum magnetic collapse of a partially bosonized $npe$-gas and obtain that this type of collapse might be one of the mechanisms behind matter expulsion out of compact objects. We check also that this gas might form a stable stream of matter whose collimation is due to its strong self-generated magnetic field. Possible astrophysical applications of these results, in particular related to jet formation and its maintenance, are discussed.

We describe the evolution of the early and late universe from thermodynamic considerations, using the generalized nonextensive Tsallis entropy with a variable exponent. A new element in our analysis is the inclusion of a bulk viscosity in the description of the cosmic fluid. Using the generalized Friedmann equation, a description of the early and the late universe is obtained.

Static (not stationary) solutions of the Einstein–Klein–Gordon (EKG) equations including matter are obtained for real scalar fields. The scalar field interaction with matter is considered. The introduced coupling allows the existence of static solutions in contraposition with the case of the simpler EKG equations for real scalar fields and gravity. Surprisingly, when the considered matter is a photon-like gas, it turns out that the gravitational field intensity at large radial distances becomes nearly a constant, exerting an approximately fixed force to small bodies at any distance. The effect is clearly related with the massless character of the photon-like field. It is also argued that the gravitational field can generate a bounding attraction, that could avoid the unlimited increase in mass with the radius of the obtained here solution. This phenomenon, if verified, may furnish a possible mechanism for explaining how the increasing gravitational potential associated to dark matter, finally decays at large distances from the galaxies. A method for evaluating these photon bounding effects is just formulated in order to be further investigated.

It is believed that primordial black holes (PBHs), if they exist, can serve as a powerful tool to probe the early stage of the cosmic history. Essentially, in the radiation-dominated universe, PBHs could form by the gravitational collapse of overdense primordial perturbations produced during inflation. In this picture, one important ingredient is the threshold of density contrast, which defines the onset of PBH formation. In the literature, most of the estimations of threshold, no matter numerically or analytically, are implemented in the framework of general relativity (GR). In this paper, by performing analytic estimations, we point out that the threshold for PBH formation depends on the gravitational theory under consideration. In GR, given a fixed equation of state, the analytic estimations adopted in this paper give a constant value of the formation threshold. If the theory is characterized by additional mass scales other than the Planck mass, the estimated threshold of density contrast may depend on the energy scale of the universe at the time of PBH formation. In this paper, we consider the Eddington-inspired-Born–Infeld gravity as an example. We find that the threshold would be enhanced if the Born–Infeld coupling constant is positive, and would be suppressed for a negative coupling constant. Also, we show explicitly that the threshold depends on the energy scale of the universe at the PBH formation time. This conclusion is expected to be valid for any gravitational theory characterized by additional mass scales, suggesting the possibility of testing gravitational theories with PBHs.

We revisit an emergent gravity scenario in $(4$$+$$1)$ dimensions underlying a propagating geometric torsion ${{\mathscr{H}}}_3$ with a renewed interest. We show that a pair-symmetric $4$th-order curvature tensor is sourced by Neveu–Schwarz (NS) two-form in a $U(1)$ gauge theoretic formulation. Interestingly, the new spacetime curvature governs a torsion-free geometry sourced by an NS form and shares the properties of the Riemann tensor. On the other hand, a completely anti-symmetric $4$th-order tensor in the formulation is shown to incorporate a dynamical geometric torsion and is argued to be identified with a nonperturbative correction. The four-form turns out to be $U(1)$ gauge invariant underlying an onshell NS form. We show that an emergent gravity theory may elegantly be described with an axionic scalar presumably signifying a quintessence coupling to the Riemann-type geometries. The curvatures are appropriately worked out to obtain a $d=12$ emergent form theory. Investigation reveals that a pair of ${(M\bar{M})}_{10}$-brane is created across an event horizon. We show that an emergent $M$ theory in a decoupling limit identifies with the bosonic sector of $N=1$, Supergravity in $d=11$.

Reheating is a process by which the inflaton’s energy density transfers to conventional matter after cosmic inflation. Currently, there is no cosmic observational evidence to directly detect the reheating era, but it may impose additional constraints on inflationary models. Depending upon the model, e-folding number during reheating $N_{\mathrm{\text{re}}}$ and the final reheating temperature $T_{\mathrm{\text{re}}}$, as well as its effective equation of state parameter ${\omega }_{\mathrm{\text{re}}}$, may be directly linked to the inflation observables such as the scalar spectral index $n_s$ and the tensor-to-scalar ratio $r$. By restricting the values of the effective equation of state parameter observationally, one can derive more stringent limits on inflationary models than those obtained from other routes. In this paper, we are interested to consider the reheating era in an inflation model with a nonminimal derivative coupling of the scalar field to impose some severe constraints on the parameter space of the model in the light of Planck 2018 data. We study the reheating final temperature and e-folds number in terms of the scalar spectral index and ${\omega }_{\mathrm{\text{re}}}$ within a numerical analysis on the model’s parameter space. To realize a viable range of the reheating equation of state parameter in this nonminimal derivative inflation model, we obtain some observationally acceptable subspaces in the $N_{\mathrm{\text{re}}}-{\omega }_{\mathrm{\text{re}}}$ phase plane. To this end, we consider some sort of polynomial potentials to obtain some constraints on the model’s parameter space which corresponds to viable values of the scalar spectral index and tensor-to-scalar ratio released by Planck 2018 TT+TE+EE+LowE observational data. Finally, we compare the obtained constraints in this nonminimal set-up with those derived from a single, minimally coupled scalar field inflation model to reveal the physics of the reheating in the context of nonminimal derivative inflation model.

In this paper, we use complexity equals action proposal and investigate the recently introduced ‘mutual complexity’ [M. Alishahiha, K. Babaei Velni and M. R. Mohammadi Mozaffar, Phys. Rev. D 99 (2019) 126016, https://doi.org/10.1103/PhysRevD.99.126016, arXiv:1809.06031 [hep-th]], in the hyperscaling violating backgrounds. For two subregions in order to define holographic complexity, we find the finite bulk action inside the subregions which is followed by introducing the proper counter terms. We show that for two subregions, the mutual complexity is subadditive. Moreover, for three subregions, we define holographic ‘tripartite complexity’ and show that this new quantity is superadditive.

The Palatini $f(R,T)$ gravity theory is considered. The standard Einstein–Hilbert action is replaced by an arbitrary function of the Ricci scalar $R$ and of the trace $T$ of the energy-momentum tensor. In the Palatini approach, the Ricci scalar is a function of the metric and the connection. These two quantities, metric and connection, are taken as independent variables. Then, it is examined whether Palatini $f(R,T)$ gravity theory allows solutions in which lead to violation of causality. The Gödel and Gödel-type spacetimes are considered. In addition, a critical radius, which permits to examine limits for violation of causality, is calculated. It is shown that, for different matter contents, noncausal solutions can be avoided in this Palatini gravitational theory.