2,352 research outputs found
Black Hole Superradiance in Dynamical Spacetime
We study the superradiant scattering of gravitational waves by a nearly
extremal black hole (dimensionless spin ) by numerically solving the
full Einstein field equations, thus including backreaction effects. This allows
us to study the dynamics of the black hole as it loses energy and angular
momentum during the scattering process. To explore the nonlinear phase of the
interaction, we consider gravitational wave packets with initial energies up to
of the mass of the black hole. We find that as the incident wave energy
increases, the amplification of the scattered waves, as well as the energy
extraction efficiency from the black hole, is reduced. During the interaction
the apparent horizon geometry undergoes sizable nonaxisymmetric oscillations.
The largest amplitude excitations occur when the peak frequency of the incident
wave packet is above where superradiance occurs, but close to the dominant
quasinormal mode frequency of the black hole.Comment: 5 pages, 4 figures; revised to match PRD versio
Spacetime Dynamics of a Higgs Vacuum Instability During Inflation
A remarkable prediction of the Standard Model is that, in the absence of
corrections lifting the energy density, the Higgs potential becomes negative at
large field values. If the Higgs field samples this part of the potential
during inflation, the negative energy density may locally destabilize the
spacetime. We use numerical simulations of the Einstein equations to study the
evolution of inflation-induced Higgs fluctuations as they grow towards the true
(negative-energy) minimum. These simulations show that forming a single patch
of true vacuum in our past light cone during inflation is incompatible with the
existence of our Universe; the boundary of the true vacuum region grows outward
in a causally disconnected manner from the crunching interior, which forms a
black hole. We also find that these black hole horizons may be arbitrarily
elongated---even forming black strings---in violation of the hoop conjecture.
By extending the numerical solution of the Fokker-Planck equation to the
exponentially suppressed tails of the field distribution at large field values,
we derive a rigorous correlation between a future measurement of the
tensor-to-scalar ratio and the scale at which the Higgs potential must receive
stabilizing corrections in order for the Universe to have survived inflation
until today.Comment: 36 pages, 11 figures; revised to match published versio
Vitamin A supplementation for postpartum women
In areas where vitamin A deficiency (VAD) is a public health concern, the maternal dietary intake of vitamin A may be not sufficient to meet either the maternal nutritional requirements, or those of the breastfed infant, due the low retinol concentrations in breast milk. To evaluate the effects of vitamin A supplementation for postpartum women on maternal and infant health. We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (8 February 2016), LILACS (1982 to December 2015), Web of Science (1945 to December 2015), and the reference lists of retrieved studies. Randomised controlled trials (RCTs) or cluster‐randomised trials that assessed the effects of vitamin A supplementation for postpartum women on maternal and infant health (morbidity, mortality and vitamin A nutritional status). Two review authors independently assessed trials for inclusion, conducted data extraction, assessed risk of bias and checked for accuracy. We assessed the quality of the evidence using the GRADE approach. Fourteen trials of mainly low or unclear risk of bias, enrolling 25,758 women and infant pairs were included. The supplementation schemes included high, single or double doses of vitamin A (200,000 to 400,000 internation units (IU)), or 7.8 mg daily beta‐carotene compared with placebo, no treatment, other (iron); or higher (400,000 IU) versus lower dose (200,000 IU). In all trials, a considerable proportion of infants were at least partially breastfed until six months.
In search of ‘lost’ knowledge and outsourced expertise in flood risk management
This paper examines the parallel discourses of ‘lost’ local flood expertise and the growing use of commercial consultancies to outsource aspects of flood risk work. We critically examine the various claims and counter-claims about lost, local and external expertise in flood management, focusing on the aftermath of the 2007 floods in East Yorkshire, England. Drawing on interviews with consultants, drainage engineers and others, we caution against claims that privilege ‘local’ floods knowledge as ‘good’ and expert knowledge as somehow suspect. This paper urges carefulness in interpreting claims about local knowledge, arguing that it is important always to think instead of hybrid knowledge formations. We conclude by arguing that experiments in the co-production of flood risk knowledge need to be seen as part of a spectrum of ways for producing shared knowledge
GPI-anchored uPAR requires Endo180 for rapid directional sensing during chemotaxis
Urokinase-type plasminogen activator (uPA) and its receptor (uPAR) play an important role in cell guidance and chemotaxis during normal and pathological events. uPAR is GPI-anchored and the mechanism by which it transmits intracellular polarity cues across the plasma membrane during directional sensing has not been elucidated. The constitutively recycling endocytic receptor Endo180 forms a trimolecular complex with uPAR in the presence of uPA, hence its alternate name uPAR-associated protein. Here, we demonstrate that Endo180 is a general promoter of random cell migration and has a more specific function in cell chemotaxis up a uPA gradient. Endo180 expression was demonstrated to enhance uPA-mediated filopodia production and promote rapid activation of Cdc42 and Rac. Expression of a noninternalizing Endo180 mutant revealed that promotion of random cell migration requires receptor endocytosis, whereas the chemotactic response to uPA does not. From these studies, we conclude that Endo180 is a crucial link between uPA–uPAR and setting of the internal cellular compass
Methods and prospects for gravitational wave searches targeting ultralight vector boson clouds around known black holes
Ultralight bosons are predicted in many extensions to the Standard Model and
are popular dark matter candidates. The black hole superradiance mechanism
allows for these particles to be probed using only their gravitational
interaction. In this scenario, an ultralight boson cloud may form spontaneously
around a spinning black hole and extract a non-negligible fraction of the black
hole's mass. These oscillating clouds produce quasi-monochromatic,
long-duration gravitational waves that may be detectable by ground-based or
space-based gravitational wave detectors. We discuss the capability of a new
long-duration signal tracking method, based on a hidden Markov model, to detect
gravitational wave signals generated by ultralight vector boson clouds,
including cases where the signal frequency evolution timescale is much shorter
than that of a typical continuous wave signal. We quantify the detection
horizon distances for vector boson clouds with current- and next-generation
ground-based detectors. We demonstrate that vector clouds hosted by black holes
with mass and spin are within the reach of
current-generation detectors up to a luminosity distance of Gpc. This
search method enables one to target vector boson clouds around remnant black
holes from compact binary mergers detected by gravitational-wave detectors. We
discuss the impact of the sky localization of the merger events and demonstrate
that a typical remnant black hole reasonably well-localized by the current
generation detector network is accessible in a follow-up search.Comment: 21 pages, 12 figure
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