28 research outputs found
Spontaneous nonlinear scalarization of Kerr black holes
As it became well known in the past years, Einstein-scalar-Gauss-Bonnet
(EsGB) theories evade no-hair theorems and allow for scalarized compact objects
including black holes (BH). The coupling function that defines the theory is
the main character in the process and nature of scalarization. With the right
choice, the theory becomes an extension of general relativity (GR) in the sense
any solution to the GR field equations remains a solution in the EsGB theory,
but it can destabilize if a certain threshold value of the spacetime curvature
is exceeded. Thus BHs can spontaneously scalarized. The most studied driving
mechanism to this phenomenon is a tachyonic instability due to an effective
negative squared mass for the scalar field. However, even when the coupling is
chosen such that this mass is zero, higher order terms with respect to the
scalar field can lead to what is coined nonlinear scalarization. In this paper
we investigate how Kerr BHs spontaneously scalarize by evolving the scalar
field on a fixed background via solving the nonlinear Klein-Gordon equation. We
consider two different coupling functions with higher order terms, one that
yields a non-zero effective mass and another that does not. We sweep through
the Kerr parameter space in its mass and spin and obtain the scalar charge by
the end of the evolution when the field settles in an equilibrium stationary
state. When there is no tachyonic instability present, there is no probe limit
in which the BH scalarizes with zero charge, i.e. there is a gap between bald
and hairy BHs and they only connect when the mass goes to zero together with
the charge
Cosmological models with interacting components and mass-varying neutrinos
A model for a homogeneous and isotropic spatially flat Universe, composed of
baryons, radiation, neutrinos, dark matter and dark energy is analyzed. We
infer that dark energy (considered to behave as a scalar field) interacts with
dark matter (either by the Wetterich model, or by the Anderson and Carroll
model) and with neutrinos by a model proposed by Brookfield et al.. The latter
is understood to have a mass-varying behavior. We show that for a very-softly
varying field, both interacting models for dark matter give the same results.
The models reproduce the expected red-shift performances of the present
behavior of the Universe.Comment: 8 pages, 5 figures, to be published in Gravitation and Cosmolog
Black holes, gravitational waves and fundamental physics: a roadmap
The grand challenges of contemporary fundamental physics—dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem—all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions.
The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature.
The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress. This write-up is an initiative taken within the framework of the European Action on 'Black holes, Gravitational waves and Fundamental Physics'