Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userSpherical inhomogeneous solutions of Einstein and scalar-tensor gravity: a map of the land
64
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We review spherical and inhomogeneous analytic solutions of the field equations of Einstein and of scalar-tensor gravity, including Brans-Dicke theory, non-minimally (possibly conformally) coupled scalar fields, Horndeski, and beyond Horndeski/DHOST gravity. The zoo includes both static and dynamic solutions, asymptotically flat, and asymptotically Friedmann-Lema^itre-Robertson-Walker ones. We minimize overlap with existing books and reviews and we place emphasis on scalar field spacetimes and on geometries that are general within certain classes. Relations between various solutions, which have largely emerged during the last decade, are pointed out.
rate research
Read More
In this work we study static black holes in the regularized 4D Einstein-Gauss-Bonnet theory of gravity; a shift-symmetric scalar-tensor theory that belongs to the Horndeski class. This theory features a simple black hole solution that can be written in closed form, and which we show is the unique static, spherically-symmetric and asymptotically-flat black hole vacuum solution of the theory. We further show that no asymptotically-flat, time-dependent, spherically-symmetric perturbations to this geometry are allowed, which suggests that it may be the only spherically-symmetric vacuum solution that this theory admits (a result analogous to Birkhoffs theorem). Finally, we consider the thermodynamic properties of these black holes, and find that their final state after evaporation is a remnant with a size determined by the coupling constant of the theory. We speculate that remnants of this kind from primordial black holes could act as dark matter, and we constrain the parameter space for their formation mass, as well as the coupling constant of the theory.
We compute families of spherically symmetric neutron-star models in two-derivative scalar-tensor theories of gravity with a massive scalar field. The numerical approach we present allows us to compute the resulting spacetimes out to infinite radius using a relaxation algorithm on a compactified grid. We discuss the structure of the weakly and strongly scalarized branches of neutron-star models thus obtained and their dependence on the linear and quadratic coupling parameters $alpha_0$, $beta_0$ between the scalar and tensor sectors of the theory, as well as the scalar mass $mu$. For highly negative values of $beta_0$, we encounter configurations resembling a gravitational atom, consisting of a highly compact baryon star surrounded by a scalar cloud. A stability analysis based on binding-energ calculations suggests that these configurations are unstable and we expect them to migrate to models with radially decreasing baryon density {it and} scalar field strength.
This paper provides an extended exploration of the inverse-chirp gravitational-wave signals from stellar collapse in massive scalar-tensor gravity reported in [Phys. Rev. Lett. {bf 119}, 201103]. We systematically explore the parameter space that characterizes the progenitor stars, the equation of state and the scalar-tensor theory of the core collapse events. We identify a remarkably simple and straightforward classification scheme of the resulting collapse events. For any given set of parameters, the collapse leads to one of three end states, a weakly scalarized neutron star, a strongly scalarized neutron star or a black hole, possibly formed in multiple stages. The latter two end states can lead to strong gravitational-wave signals that may be detectable in present continuous-wave searches with ground-based detectors. We identify a very sharp boundary in the parameter space that separates events with strong gravitational-wave emission from those with negligible radiation.
Previously, the Einstein equation has been described as an equation of state, general relativity as the equilibrium state of gravity, and $f({cal R})$ gravity as a non-equilibrium one. We apply Eckarts first order thermodynamics to the effective dissipative fluid describing scalar-tensor gravity. Surprisingly, we obtain simple expressions for the effective heat flux, temperature of gravity, shear and bulk viscosity, and entropy density, plus a generalized Fourier law in a consistent Eckart thermodynamical picture. Well-defined notions of temperature and approach to equilibrium, missing in the current thermodynamics of spacetime scenarios, naturally emerge.
The aim of this paper is to study the stability of soliton-like static solutions via non-linear simulations in the context of a special class of massive tensor-multi-scalar-theories of gravity whose target space metric admits Killing field(s) with a periodic flow. We focused on the case with two scalar fields and maximally symmetric target space metric, as the simplest configuration where solitonic solutions can exist. In the limit of zero curvature of the target space $kappa = 0$ these solutions reduce to the standard boson stars, while for $kappa e 0$ significant deviations can be observed, both qualitative and quantitative. By evolving these solitonic solutions in time, we show that they are stable for low values of the central scalar field $psi_c$ while instability kicks in with the increase of $psi_c$. Specifically, in the stable region, the models oscillate with a characteristic frequency related to the fundamental mode. Such frequency tends to zero with the approach of the unstable models and eventually becomes imaginary when the solitonic solutions lose stability. As expected from the study of the equilibrium models, the change of stability occurs exactly at the maximum mass point, which was checked numerically with a very good accuracy.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
