3,548 research outputs found
Simulating extreme-mass-ratio systems in full general relativity
We introduce a new method for numerically evolving the full Einstein field
equations in situations where the spacetime is dominated by a known background
solution. The technique leverages the knowledge of the background solution to
subtract off its contribution to the truncation error, thereby more efficiently
achieving a desired level of accuracy. We demonstrate the method by applying it
to the radial infall of a solar-type star into supermassive black holes with
mass ratios . The self-gravity of the star is thus consistently
modeled within the context of general relativity, and the star's interaction
with the black hole computed with moderate computational cost, despite the over
five orders of magnitude difference in gravitational potential (as defined by
the ratio of mass to radius). We compute the tidal deformation of the star
during infall, and the gravitational wave emission, finding the latter is close
to the prediction of the point-particle limit.Comment: 6 pages, 5 figures; added one figure, revised to match PRD RC versio
Ultrarelativistic black hole formation
We study the ultrarelativistic head-on collision of equal mass particles,
modeled as self-gravitating fluid spheres, by numerically solving the coupled
Einstein-hydrodynamic equations. We focus on cases well within the kinetic
energy dominated regime, where between 88-92% ( to 12) of the initial
net energy of the spacetime resides in the translation kinetic energy of the
particles. We find that for sufficiently large boosts, black hole formation
occurs. Moreover, near yet above the threshold of black hole formation, the
collision initially leads to the formation of two distinct apparent horizons
that subsequently merge. We argue that this can be understood in terms of a
focusing effect, where one boosted particle acts as a gravitational lens on the
other and vice versa, and that this is further responsible for the threshold
being lower (by a factor of a few) compared to simple hoop conjecture
estimates. Cases slightly below threshold result in complete disruption of the
model particles. The gravitational radiation emitted when black holes form
reaches luminosities of 0.014 , carrying of the total energy.Comment: 5 pages, 4 figures; revised to match PRL versio
Superradiant Instability and Backreaction of Massive Vector Fields around Kerr Black Holes
We study the growth and saturation of the superradiant instability of a
complex, massive vector (Proca) field as it extracts energy and angular
momentum from a spinning black hole, using numerical solutions of the full
Einstein-Proca equations. We concentrate on a rapidly spinning black hole
() and the dominant azimuthal mode of the Proca field, with real
and imaginary components of the field chosen to yield an axisymmetric
stress-energy tensor and, hence, spacetime. We find that in excess of of
the black hole's mass can be transferred into the field. In all cases studied,
the superradiant instability smoothly saturates when the black hole's horizon
frequency decreases to match the frequency of the Proca cloud that
spontaneously forms around the black hole.Comment: 6 pages, 6 figures; revised to match PRL versio
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Dorsal root ganglion neurons maintained in a 3D culture model exhibit similar electrophysiological properties to fresh explants
Tissue engineered culture models provide a powerful tool for neuroscience research1. They overcome limitations associated with monolayer cultures of neurons and glia by maintaining cells in a more realistic 3D spatial arrangement, and permit continuous monitoring and control of variables that cannot be achieved in animal models. Here we report the development of a system for recording electrophysiological behaviour in neurons in 3D culture
Comparing Fully General Relativistic and Newtonian Calculations of Structure Formation
In the standard approach to studying cosmological structure formation, the
overall expansion of the Universe is assumed to be homogeneous, with the
gravitational effect of inhomogeneities encoded entirely in a Newtonian
potential. A topic of ongoing debate is to what degree this fully captures the
dynamics dictated by general relativity, especially in the era of precision
cosmology. To quantitatively assess this, we directly compare standard N-body
Newtonian calculations to full numerical solutions of the Einstein equations,
for cold matter with various magnitude initial inhomogeneities on scales
comparable to the Hubble horizon. We analyze the differences in the evolution
of density, luminosity distance, and other quantities defined with respect to
fiducial observers. This is carried out by reconstructing the effective
spacetime and matter fields dictated by the Newtonian quantities, and by taking
care to distinguish effects of numerical resolution. We find that the fully
general relativistic and Newtonian calculations show excellent agreement, even
well into the nonlinear regime. They only notably differ in regions where the
weak gravity assumption breaks down, which arise when considering extreme cases
with perturbations exceeding standard values.Comment: 17 pages, 14 figures; revised to match PRD versio
Eccentric mergers of black holes with spinning neutron stars
We study dynamical capture binary black hole-neutron star (BH-NS) mergers
focusing on the effects of the neutron star spin. These events may arise in
dense stellar regions, such as globular clusters, where the majority of neutron
stars are expected to be rapidly rotating. We initialize the BH-NS systems with
positions and velocities corresponding to marginally unbound Newtonian orbits,
and evolve them using general-relativistic hydrodynamical simulations. We find
that even moderate spins can significantly increase the amount of mass in
unbound material. In some of the more extreme cases, there can be up to a third
of a solar mass in unbound matter. Similarly, large amounts of tidally stripped
material can remain bound and eventually accrete onto the BH---as much as a
tenth of a solar mass in some cases. These simulations demonstrate that it is
important to treat neutron star spin in order to make reliable predictions of
the gravitational wave and electromagnetic transient signals accompanying these
sources.Comment: 7 pages, 4 figures; revised to match published versio
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