1,288 research outputs found
Acoustic black holes
We discuss some general aspects of acoustic black holes. We begin by
describing the associated formalism with which acoustic black holes are
established, then we show how to model arbitrary geometries by using a de Laval
nozzle. It is argued that even though the Hawking temperature of these black
holes is too low to be detected, acoustic black holes have interesting
classical properties, some of which are outlined here, that should be explored.Comment: 13 pages, 9 Figures, ReVTeX4. Based on a talk delivered at the Fifth
Meeting on New Worlds in Astroparticle Physics (Faro, Portugal, 8-10 January
2005). Updated references and overall improvemen
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
Testing the black hole "no-hair" hypothesis
Black holes in General Relativity are very simple objects. This property,
that goes under the name of "no-hair," has been refined in the last few decades
and admits several versions. The simplicity of black holes makes them ideal
testbeds of fundamental physics and of General Relativity itself. Here we
discuss the no-hair property of black holes, how it can be measured in the
electromagnetic or gravitational window, and what it can possibly tell us about
our universe.Comment: Commissioned by Classical and Quantum Gravit
Ultralight scalars and resonances in black-hole physics
Ultralight degrees of freedom coupled to matter lead to resonances, which can
be excited when the Compton wavelength of the field equals a dynamical scale in
the problem. For binaries composed of a star orbiting a supermassive black
hole, these resonances lead to a smoking-gun effect: a periastron distance
which {\it stalls}, even in the presence of gravitational-wave dissipation.
This effect, also called a {\it floating orbit}, occurs for generic equatorial
but eccentric orbits and we argue that finite-size effects are not enough to
suppress it.Comment: 10 pages, 5 figure
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
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