Quasinormal modes of compact objects in alternative theories of gravity

Abstract

We address quasinormal modes of compact objects in several alternative theories of gravity. In particular, we focus on black holes and neutron stars with scalar hair. We consider black holes in dilaton-Einstein-Gauß-Bonnet theory, and in a generalized scalar-Einstein-Gauß-Bonnet theory. In the latter case scalarized black holes arise, and we study the stability of the different branches of solutions. In particular, we discuss how the spectrum of quasinormal modes is changed by the presence of a non-trivial scalar field outside the black hole horizon. We discuss the existence of an (effective) minimum mass in these models, and how the spectrum of modes becomes richer as compared to general relativity, when a scalar field is present. Subsequently we discuss the effect of scalar hair for realistic neutron star models. Here we consider R 2 gravity, scalar-tensor theory, a particular subsector of Horndeski theory with a non-minimal derivative coupling, and again dilatonic-Einstein-Gauß-Bonnet theory. Because of the current lack of knowledge on the internal composition of the neutron stars, we focus on universal relations for the quasinormal modes, that are largely independent of the equations of state and thus the matter content of the stars.

Axial quasinormal modes of neutron stars in R2 gravity

“…The purpose of this letter is to show that this scenario changes fundamentally when dealing with NSs in alternative theories of gravity, that introduce extra degrees of freedom in the form of additional fields, which enrich the spectrum of QNMs even in the absence of matter (for a review see e.g. [33]). As an interesting example, we here investigate NSs in R 2 gravity.…”

Black hole ringdown as a probe for dark energy

“…on solutions of the equation of motion (12). Therefore, at r ∼ r s both contributions to the kinetic mixing are of the same order, but with inequivalent contractions due to their different structure.…”

“…Hairy solutions in sGB theories have been studied mainly in the case when the only other operators present are the Einstein-Hilbert and the standard kinetic term for the scalar, X [7][8][9][10][11][12][13]. In the language of the above Lagrangian, this case amounts to the choices G 2 = X, G 4 = Λ 6 /Λ 6 3 and G 5 = −4 Λ 9 Λ 8 2 M P α log(X), with the remaining functions set to zero.…”

“…2, especially in the region with n < 1. 12 Such exotic models can nevertheless be considered acceptable from the EFT point of view, if one intends to remain agnostic about the UV completion of the theory. Indeed, as discussed around Eq.…”

“…1 As a prototypical example, we will consider below the case of a linear coupling between the scalar and the Gauss-Bonnet operator, whose presence is known to be sufficient to evade the no-hair restrictions [6]. In the literature, such shift-symmetric operator has been widely studied, both analytically and numerically, in the simplest setting in which the only other operator in the Lagrangian for the scalar is the canonical kinetic term [7][8][9][10][11][12][13].…”