Compact Objects in Alternative Gravities

Abstract

We address neutron stars and black holes in alternative gravities, after recalling their basic properties in General Relativity. Among the plethora of interesting alternative gravities we here focus on an interesting set of scalar-tensor theories. We discuss the phenomenon of spontaneous scalarization, that is, matter induced for neutron stars and curvature induced for black holes. Along with other relevant physical properties, we address the quasi-normal modes of these compact objects. In particular, we consider universal relations of neutron stars to largely reduce the dependence on the equation of state, and we briefly address the shadow of black holes.

$ϕ$-modes of neutron stars in a massless scalar-tensor theory

“…Having now provided the φ-modes and their universal relations for the limiting theories of general relativity and the massless scalar-tensor theory, we should as our next step calculate the φ-modes for finite values of the scalar field mass and extract the corresponding universal relations, as previously done for the fluid modes [44,47,48,49], and the current investigations could serve as a guide in this endeavour. Moreover, the study of the polar modes of neutron stars and their universal relations in alternative theories of gravity has just begun, and numerous interesting alternative gravities are waiting to be explored.…”

“…Therefore early studies of polar modes in such alternative gravity theories have employed the Cowling approximation, where only fluid perturbations are taken into account [40,41]. However, by now also the full set of perturbations has been considered when determining polar quasinormal modes of several alternative theories of gravity with a scalar degree of freedom [42,43,44,45,46,47,48,49]. has previously been investigated.…”

Anisotropic Stellar Systems in f(R)$f(R)$ Connected Static Spacetime

“…Yousaf and his collaborators [44][45][46][47] provided a general platform to analyze less complex astronomical structures within the background of extra curvature ingredients. Blázquez-Salcedo et al [48] analyzed the black hole shadows within a specific quasi-normal backgrounds. After performing some numerical technique, they inferred that extra scalar degrees of freedom allow the system to describe some very interesting astrophysical processes.…”

“…Blázquez‐Salcedo et al. [ 48 ] analyzed the black hole shadows within a specific quasi‐normal backgrounds. After performing some numerical technique, they inferred that extra scalar degrees of freedom allow the system to describe some very interesting astrophysical processes.…”

Universal relations for quasinormal modes of neutron stars in $R^2$ gravity

“…This leads to highly involved sets of perturbation equations, unless the Cowling approximation is used, where the perturbations of the spacetime and the gravitational scalar field are frozen (see e.g., [31,32]). Clearly, the full set of perturbations equations results in a much richer spectrum of QNMs, including scalar monopole and dipole radiation as well as scalar-led quadrupole and higher l modes [33,34,35,36,37,38,39]. Among the alternative theories of gravity, f (R) theories have received much attention [40,41,42].…”