4,774 research outputs found
Modeling dynamical scalarization with a resummed post-Newtonian expansion
Despite stringent constraints set by astrophysical observations, there remain
viable scalar-tensor theories that could be distinguished from general
relativity with gravitational-wave detectors. A promising signal predicted in
these alternative theories is dynamical scalarization, which can dramatically
affect the evolution of neutron-star binaries near merger. Motivated by the
successful treatment of spontaneous scalarization, we develop a formalism that
partially resums the post-Newtonian expansion to capture dynamical
scalarization in a mathematically consistent manner. We calculate the
post-Newtonian order corrections to the equations of motion and scalar mass of
a binary system. Through comparison with quasi-equilibrium configuration
calculations, we verify that this new approximation scheme can accurately
predict the onset and magnitude of dynamical scalarization.Comment: 24 pages, 8 figures; recolored figures, fixed typos, added emai
Structures 2 - The response of a clamped circular plate to impulsive loads
Computer analysis of clamped circular plate response to axisymmetric impulsive loa
Structures i- the response of beams and rings to high-intensity, short-duration loading
Computer programs for determining response of beams and rings to high intensity, short duration loadin
Concentrating minds: how the Greeks designed spaces for public debate
What can we learn from ancient Greece when it comes to designing spaces for political debate? In an article for Theatrum Mundi, Richard Sennett describes how ancient Athenians used amphitheatres and the agora to debate, take decisions and participate in public life. He recalls Aristotle’s notion of how a complex urban society could reconcile differences. Modern societies – where concentrating on a political question has become ever more difficult – need public spaces conducive to focus and deliberation
Constraining nonperturbative strong-field effects in scalar-tensor gravity by combining pulsar timing and laser-interferometer gravitational-wave detectors
Pulsar timing and gravitational-wave (GW) detectors are superb laboratories
to study gravity theories in the strong-field regime. Here we combine those
tools to test the mono-scalar-tensor theory of Damour and Esposito-Far{\`e}se
(DEF), which predicts nonperturbative scalarization phenomena for neutron stars
(NSs). First, applying Markov-chain Monte Carlo techniques, we use the absence
of dipolar radiation in the pulsar-timing observations of five binary systems
composed of a NS and a white dwarf, and eleven equations of state (EOSs) for
NSs, to derive the most stringent constraints on the two free parameters of the
DEF scalar-tensor theory. Since the binary-pulsar bounds depend on the NS mass
and the EOS, we find that current pulsar-timing observations leave
scalarization windows, i.e., regions of parameter space where scalarization can
still be prominent. Then, we investigate if these scalarization windows could
be closed and if pulsar-timing constraints could be improved by
laser-interferometer GW detectors, when spontaneous (or dynamical)
scalarization sets in during the early (or late) stages of a binary NS (BNS)
evolution. For the early inspiral of a BNS carrying constant scalar charge, we
employ a Fisher matrix analysis to show that Advanced LIGO can improve
pulsar-timing constraints for some EOSs, and next-generation detectors, such as
the Cosmic Explorer and Einstein Telescope, will be able to improve those
bounds for all eleven EOSs. Using the late inspiral of a BNS, we estimate that
for some of the EOSs under consideration the onset of dynamical scalarization
can happen early enough to improve the constraints on the DEF parameters
obtained by combining the five binary pulsars. Thus, in the near future the
complementarity of pulsar timing and direct observations of GWs on the ground
will be extremely valuable in probing gravity theories in the strong-field
regime.Comment: 19 pages, 11 figures; accepted by Physical Review
A preliminary investigation into the adjustment to university of first-year students at the University of Cape Town, with particular emphasis on the relative adjustment of black students
Bibliography : leaves 66-72.The change from school to university is a major life transition to which many students experience considerable difficulty in adjusting. This process of adjustment is multidimensional requiring that students develop effective strategies for adapting to a host of new demands (Baker & Siryk, 1989) including those found in the academic, social and emotional spheres of development. Yet, in addition to factors relating to individual developmental or background variables, the interactive effects of student demographics and institutional environment may also influence a student's ability to cope effectively with adjustment to university. This may be the case particularly for students of disadvantaged or minority backgrounds, of which, the literature suggests, black African students in South Africa are a likely instance
Distinguishing Boson Stars from Black Holes and Neutron Stars from Tidal Interactions in Inspiraling Binary Systems
Binary systems containing boson stars---self-gravitating configurations of a
complex scalar field--- can potentially mimic black holes or neutron stars as
gravitational-wave sources. We investigate the extent to which tidal effects in
the gravitational-wave signal can be used to discriminate between these
standard sources and boson stars. We consider spherically symmetric boson stars
within two classes of scalar self-interactions: an
effective-field-theoretically motivated quartic potential and a solitonic
potential constructed to produce very compact stars. We compute the tidal
deformability parameter characterizing the dominant tidal imprint in the
gravitational-wave signals for a large span of the parameter space of each
boson star model. We find that the tidal deformability for boson stars with a
quartic self-interaction is bounded below by and
for those with a solitonic interaction by .
Employing a Fisher matrix analysis, we estimate the precision with which
Advanced LIGO and third-generation detectors can measure these tidal parameters
using the inspiral portion of the signal. We discuss a new strategy to improve
the distinguishability between black holes/neutrons stars and boson stars by
combining deformability measurements of each compact object in a binary system,
thereby eliminating the scaling ambiguities in each boson star model. Our
analysis shows that current-generation detectors can potentially distinguish
boson stars with quartic potentials from black holes, as well as from
neutron-star binaries if they have either a large total mass or a large mass
ratio. Discriminating solitonic boson stars from black holes using only tidal
effects during the inspiral will be difficult with Advanced LIGO, but
third-generation detectors should be able to distinguish between binary black
holes and these binary boson stars.Comment: 18 pages, 8 figures. Submitted to Physical Review
The Effects of Blood Flow Restriction (BFR) on Muscle Activation and Hypertrophy in Patients with Chronic Ankle Instability (CAI): A Critically Appraised Topic
Graduate
Theoretical Proposa
Hairy binary black holes in Einstein-Maxwell-dilaton theory and their effective-one-body description
In General Relativity and many modified theories of gravity, isolated black
holes (BHs) cannot source massless scalar fields. Einstein-Maxwell-dilaton
(EMd) theory is an exception: through couplings both to electromagnetism and
(non-minimally) to gravity, a massless scalar field can be generated by an
electrically charged BH. In this work, we analytically model the dynamics of
binaries comprised of such scalar-charged ("hairy") BHs. While BHs are not
expected to have substantial electric charge within the Standard Model of
particle physics, nearly-extremally charged BHs could occur in models of
minicharged dark matter and dark photons. We begin by studying the test-body
limit for a binary BH in EMd theory, and we argue that only very compact
binaries of nearly-extremally charged BHs can manifest non-perturbative
phenomena similar to those found in certain scalar-tensor theories. Then, we
use the post-Newtonian approximation to study the dynamics of binary BHs with
arbitrary mass ratios. We derive the equations governing the conservative and
dissipative sectors of the dynamics at next-to-leading order, use our results
to compute the Fourier-domain gravitational waveform in the stationary-phase
approximation, and compute the number of useful cycles measurable by the
Advanced LIGO detector. Finally, we construct two effective-one-body (EOB)
Hamiltonians for binary BHs in EMd theory: one that reproduces the exact
test-body limit and another whose construction more closely resembles similar
models in General Relativity, and thus could be more easily integrated into
existing EOB waveform models used in the data analysis of gravitational-wave
events by the LIGO and Virgo collaborations.Comment: 36 pages, 12 figures, updated to match published versio
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