571 research outputs found
Binary pulsars as dark-matter probes
During the motion of a binary pulsar around the Galactic center, the pulsar
and its companion experience a wind of dark-matter particles that can affect
the orbital motion through dynamical friction. We show that this effect
produces a characteristic seasonal modulation of the orbit and causes a secular
change of the orbital period whose magnitude can be well within the astonishing
precision of various binary-pulsar observations. Our analysis is valid for
binary systems with orbital period longer than a day. By comparing this effect
with pulsar-timing measurements, it is possible to derive model-independent
upper bounds on the dark-matter density at different distances from the
Galactic center. For example, the precision timing of J1713+0747 imposes
at . The
detection of a binary pulsar at could provide
stringent constraints on dark-matter halo profiles and on growth models of the
central black hole. The Square Kilometer Array can improve current bounds by 2
orders of magnitude, potentially constraining the local density of dark matter
to unprecedented levels.Comment: 8+3 pages, 7 figures. To appear in Phys. Rev. D; v2: matches
published versio
Advanced Methods in Black-Hole Perturbation Theory
Black-hole perturbation theory is a useful tool to investigate issues in
astrophysics, high-energy physics, and fundamental problems in gravity. It is
often complementary to fully-fledged nonlinear evolutions and instrumental to
interpret some results of numerical simulations. Several modern applications
require advanced tools to investigate the linear dynamics of generic small
perturbations around stationary black holes. Here, we present an overview of
these applications and introduce extensions of the standard semianalytical
methods to construct and solve the linearized field equations in curved
spacetime. Current state-of-the-art techniques are pedagogically explained and
exciting open problems are presented.Comment: Lecture notes from the NRHEP spring school held at IST-Lisbon, March
2013. Extra material and notebooks available online at
http://blackholes.ist.utl.pt/nrhep2/. To be published by IJMPA (V. Cardoso,
L. Gualtieri, C. Herdeiro and U. Sperhake, Eds., 2013); v2: references
updated, published versio
Tests for the existence of horizons through gravitational wave echoes
The existence of black holes and of spacetime singularities is a fundamental
issue in science. Despite this, observations supporting their existence are
scarce, and their interpretation unclear. We overview how strong a case for
black holes has been made in the last few decades, and how well observations
adjust to this paradigm. Unsurprisingly, we conclude that observational proof
for black holes is impossible to come by. However, just like Popper's black
swan, alternatives can be ruled out or confirmed to exist with a single
observation. These observations are within reach. In the next few years and
decades, we will enter the era of precision gravitational-wave physics with
more sensitive detectors. Just as accelerators require larger and larger
energies to probe smaller and smaller scales, more sensitive gravitational-wave
detectors will be probing regions closer and closer to the horizon, potentially
reaching Planck scales and beyond. What may be there, lurking?Comment: Published in Nature Astronomy, expanded version with further details
available at arXiv:1707.0302
Analytical template for gravitational-wave echoes: signal characterization and prospects of detection with current and future interferometers
Gravitational-wave echoes in the post-merger ringdown phase are under intense
scrutiny as probes of near-horizon quantum structures and as signatures of
exotic states of matter in ultracompact stars. We present an analytical
template that describes the ringdown and the echo signal for nonspinning
objects in terms of two physical parameters: the reflectivity and the redshift
at the surface of the object. We characterize the properties of the template
and adopt it in a preliminary parameter estimation with current (aLIGO) and
future (Cosmic Explorer, Einstein Telescope, LISA) gravitational-wave
detectors. For fixed signal-to-noise ratio in the post-merger phase, the
constraints on the model parameters depend only mildly on the details of the
detector sensitivity curve, but depend strongly on the reflectivity. Our
analysis suggests that it might be possible to detect or rule out Planckian
corrections at the horizon scale for perfectly-reflecting ultracompact objects
at confidence level with Advanced LIGO/Virgo. On the other hand,
signal-to-noise ratios in the ringdown phase equal to (as
achievable with future interferometers) might allow us to probe near-horizon
quantum structures with reflectivity () at
() level.Comment: v3: 13+4 pages, 11 figures, 4 appendices; matches the PRD version
with a new plot and extended results. v2 (submitted version): 12 pages + 4
appendices; 9 figures. Further discussion and new appendix with template for
localized sources at generic position. Template and waveforms available at
https://www.darkgra.org/gw-echo-catalogue.htm
Tidal capture of a primordial black hole by a neutron star: implications for constraints on dark matter
In a close encounter with a neutron star, a primordial black hole can get
gravitationally captured by depositing a considerable amount of energy into
nonradial stellar modes of very high angular number . If the neutron-star
equation of state is sufficiently stiff, we show that the total energy loss in
the point-particle approximation is formally divergent. Various mechanisms
-including viscosity, finite-size effects and the elasticity of the crust- can
damp high- modes and regularize the total energy loss. Within a short time,
the black hole is trapped inside the star and disrupts it by rapid accretion.
Estimating these effects, we predict that the existence of old neutron stars in
regions where the dark-matter density rho_{DM}>10^2 sigma/(km/s) GeV/cm^3
(where sigma is the dark-matter velocity dispersion) limits the abundance of
primordial black holes in the mass range 10^{17} g < m_{PBH} < 10^{24} g, which
was previously unconstrained. In combination with existing limits, our results
suggest that primordial black holes cannot be the dominant dark matter
constituent.Comment: v3: Changes in the organization of the paper, extended discussion and
new title; results and conclusions unchanged. v4: minor editorial changes,
JCAP accepte
Testing the nature of dark compact objects: a status report
Very compact objects probe extreme gravitational fields and may be the key to
understand outstanding puzzles in fundamental physics. These include the nature
of dark matter, the fate of spacetime singularities, or the loss of unitarity
in Hawking evaporation. The standard astrophysical description of collapsing
objects tells us that massive, dark and compact objects are black holes. Any
observation suggesting otherwise would be an indication of
beyond-the-standard-model physics. Null results strengthen and quantify the
Kerr black hole paradigm. The advent of gravitational-wave astronomy and
precise measurements with very long baseline interferometry allow one to
finally probe into such foundational issues. We overview the physics of exotic
dark compact objects and their observational status, including the
observational evidence for black holes with current and future experiments.Comment: 76 pages + references. Invited review article for Living Reviews in
Relativity. v3: Overall improvements and references added, a few typos
corrected. Version to appear in LR
On gravitational-wave echoes from neutron-star binary coalescences
A tentative detection of gravitational-wave echoes in the post-merger signal
of GW170817 has been recently claimed at significance level. It has
been speculated that the signal might provide evidence for near-horizon quantum
structures in the remnant exotic object. We point out that if the remnant
object is an ultracompact neutron star, echoes are expected for objects with
radius only slightly smaller than that of an ordinary neutron star. The
reported echoes at ~72 Hz are compatible with a toy model of incompressible
star with mass approximately and radius close to the
Buchdahl limit, R~9GM/(4c^2). If confirmed, low-frequency gravitational-wave
echoes would be in tension with all current neutron-star models and would have
dramatic implications for nuclear physics and gravity.Comment: v3: 4 pages, 2 figures, extended discussion, results unchanged.
Version accepted in CQG Letter
Holography of charged dilatonic black branes at finite temperature
We investigate bulk and holographic features of finite-temperature black
brane solutions of 4D anti-de Sitter Einstein-Maxwell-dilaton-gravity (EMDG).
We construct, numerically, black branes endowed with non trivial scalar hairs
for broad classes of EMDG. We consider both exponential and power-law forms for
the coupling functions, as well as several charge configurations: purely
electric, purely magnetic and dyonic solutions. At finite temperature the field
theory holographically dual to these black brane solutions has a rich and
interesting phenomenology reminiscent of electron motion in metals: phase
transitions triggered by nonvanishing VEV of scalar operators, non-monotonic
behavior of the electric conductivities as function of the frequency and of the
temperature, Hall effect and sharp synchrotron resonances of the conductivity
in presence of a magnetic field. Conversely, in the zero temperature limit the
conductivities for these models show a universal behavior. The optical
conductivity has a power-law behavior as a function of the frequency, whereas
the DC conductivity is suppressed at small temperatures.Comment: 29 pages 14 figures, typos corrected, reference adde
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