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In this work, we investigate the existence of neutron stars (NS) in the framework of f (T , T) gravity, where T is the torsion tensor and T is the trace of the energy–momentum tensor. The hydrostatic equilibrium equations are obtained, however, with p and ρ quantities passed on by effective quantities p ¯ and ρ ¯ , whose mass–radius diagrams are obtained using modern equations of state (EoS) of nuclear matter derived from relativistic mean field models and compared with the ones computed by the Tolman–Oppenheimer–Volkoff (TOV) equations. Substantial changes in the mass–radius profiles of NS are obtained even for small changes in the free parameter of this modified theory. The results indicate that the use of f (T , T) gravity in the study of NS provides good results for the masses and radii of some important astrophysical objects, as, for example, the NS of low-mass X-ray binary in NGC 6397, the millisecond pulsar PSR J0740+6620 and the GW170817 event. In addition, radii results inferred from the Lead Radius Experiment (PREX-2) can also be described for certain parameter values. [ABSTRACT FROM AUTHOR]

We consider the effect of a thermal bath on quantum correlations induced by the gravitational interaction in the weak field limit between two massive cat states, called gravitational cat (gravcat) states. The main goal of this paper is to provide a good understanding of the effects of temperature and several parameters in the entanglement (measured by the concurrence) and quantum coherence (measured by the l 1 -norm that is defined from the minimal distance between the quantum state and the set of incoherent states) which are derived from the thermal quantum density operator. Our results show that the thermal concurrence and l 1 -norm can be significantly optimized by increasing the masses or decreasing the distance between them. We investigate and discuss the behavior of these quantities under temperature variations in different regimes, including some that are expected to be experimentally feasible in the future. In particular, we observe that thermal fluctuations raise non-entangled quantum correlations when entanglement suddenly drops. [ABSTRACT FROM AUTHOR]

In this paper, we present a generalization of Rastall's gravity in which the conservation law of the energy–momentum tensor is altered, and as a result, the trace of the energy–momentum tensor is taken into account together with the Ricci scalar in the expression for the covariant derivative. Afterwards, we obtain the field equations in this theory and solve them by considering a spherically symmetric spacetime. We show that the external solution has two possible classes of solutions with spherical symmetry in the vacuum in generalized Rastall's gravity, and we analyze one of them explicitly. The generalization, in contrast to constant value k = 8 π G in general relativity, has a gravitational parameter k that depends on the Rastall constant α. As an application, we perform a careful analysis of the effects of the theory on neutron stars using realistic equations of state (EoS) as input. Our results show that important differences on the profile of neutron stars are obtained within two representatives EoS. [ABSTRACT FROM AUTHOR]

We study the black hole thermodynamics in the presence of a string cloud matter distribution, considering a work term due to a variable cosmological “constant” in arbitrary dimensions. Then, we explore the criticality of the system and the behavior of the black hole as a heat engine in the context of general relativity and metric f(R) gravity. [ABSTRACT FROM AUTHOR]

The study of greybody factor helps us to understand the quantum nature of the black hole. Gravitational potentials and bounds on the greybody factors for some well known black holes are developed and we investigate the influence of Born–Infeld and massive gravity parameters on them. It is observed that greybody factor bounds depend on the shape of effective potential. For higher values of effective potential, it becomes difficult for the waves to transmit and hence reduces the greybody factor bounds. It is also worthwhile to mention here that charge works as barrier and cosmological constant increases the emission rate of Hawking radiation. It is also observed that the energy of emitted particles increases which results in the enhancement of the greybody factor bound. We also discuss the relationship of transmission probability and reflection probability. • Gravitational potentials and bounds on the greybody factors for some well-known black holes are being developed. • It is observed that greybody factor bounds depend on the shape of effective potential. • It is also observed that the energy of emitted particles increases which results the enhancement of the greybody factor bound. • We also discuss the relationship of transmission probability and reflection probability. [ABSTRACT FROM AUTHOR]