74 research outputs found
Accretion of dark matter by stars
Searches for dark matter imprints are one of the most active areas of current
research. We focus here on light fields with mass , such as axions and
axion-like candidates. Using perturbative techniques and full-blown nonlinear
Numerical Relativity methods, we show that (i) dark matter can pile up in the
center of stars, leading to configurations and geometries oscillating with
frequency which is a multiple of f= /eV Hz. These
configurations are stable throughout most of the parameter space, and arise out
of credible mechanisms for dark-matter capture. Stars with bosonic cores may
also develop in other theories with effective mass couplings, such as
(massless) scalar-tensor theories. We also show that (ii) collapse of the host
star to a black hole is avoided by efficient gravitational cooling mechanisms.Comment: 5 pages, RevTeX 4. Published in Physical Review Letter
Collapse of self-interacting fields in asymptotically flat spacetimes: do self-interactions render Minkowski spacetime unstable?
The nonlinear instability of anti-de Sitter spacetime has recently been
established with the striking result that generic initial data collapses to
form black holes. This outcome suggests that confined matter generically
collapses, and that collapse can only be halted -- at most -- by nonlinear
bound states. Here we provide evidence that such mechanism can operate even in
asymptotically flat spacetimes, by studying the evolution of the
Einstein-Klein-Gordon system for a self-interacting scalar field. We show that
(i) configurations which do not collapse promptly can do so after successive
reflections off the potential barrier, but (ii) that at intermediate amplitudes
and Compton wavelengths, collapse to black holes is replaced by the appearance
of oscillating soliton stars. Finally, (iii) for very small initial amplitudes,
the field disperses away in a manner consistent with power-law tails of massive
fields. Minkowski is stable against gravitational collapse. Our results provide
one further piece to the rich phenomenology of gravitational collapse and show
the important interplay between bound states, blueshift, dissipation and
confinement effects.Comment: 6 pages, 4 figures. v2: Convergence results added and overall
improvements to the manuscript. Accepted for publication in Phys.Rev.D [Rapid
Communications
On the nonlinear instability of confined geometries
The discovery of a "weakly-turbulent" instability of anti-de Sitter spacetime
supports the idea that confined fluctuations eventually collapse to black holes
and suggests that similar phenomena might be possible in asymptotically-flat
spacetime, for example in the context of spherically symmetric oscillations of
stars or nonradial pulsations of ultracompact objects. Here we present a
detailed study of the evolution of the Einstein-Klein-Gordon system in a
cavity, with different types of deformations of the spectrum, including a mass
term for the scalar and Neumann conditions at the boundary. We provide
numerical evidence that gravitational collapse always occurs, at least for
amplitudes that are three orders of magnitude smaller than Choptuik's critical
value and corresponding to more than reflections before collapse. The
collapse time scales as the inverse square of the initial amplitude in the
small-amplitude limit. In addition, we find that fields with nonresonant
spectrum collapse earlier than in the fully-resonant case, a result that is at
odds with the current understanding of the process. Energy is transferred
through a direct cascade to high frequencies when the spectrum is resonant, but
we observe both direct- and inverse-cascade effects for nonresonant spectra.
Our results indicate that a fully-resonant spectrum might not be a crucial
ingredient of the conjectured turbulent instability and that other mechanisms
might be relevant. We discuss how a definitive answer to this problem is
essentially impossible within the present framework.Comment: 14 pages, 9 figures; v2:Some improvements in convergence results,
accepted for publication in Physical Review
Coalescence of binary neutron stars in a scalar-tensor theory of gravity
We carry out numerical-relativity simulations of coalescing binary neutron
stars in a scalar-tensor theory that admits spontaneous scalarization. We model
neutron stars with realistic equations of state. We choose the free parameters
of the theory taking into account the constraints imposed by the latest
observations of neutron-star-- white-dwarf binaries with pulsar timing. We show
that even within those severe constraints, scalarization can still affect the
evolution of the binary neutron stars not only during the late inspiral, but
also during the merger stage. We also confirm that even when both neutron stars
have quite small scalar charge at large separations, they can be strongly
scalarized dynamically during the final stages of the inspiral. In particular,
we identify the binary parameters for which scalarization occurs either during
the late inspiral or only after the onset of the merger when a remnant,
supramassive or hypermassive neutron star is formed. We also discuss how those
results can impact the extraction of physical information on gravitational
waves once they are detected.Comment: 17 pages, 12 figure
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