280 research outputs found
On the rate of black hole binary mergers in galactic nuclei due to dynamical hardening
We assess the contribution of dynamical hardening by direct three-body
scattering interactions to the rate of stellar-mass black hole binary (BHB)
mergers in galactic nuclei. We derive an analytic model for the single-binary
encounter rate in a nucleus with spherical and disk components hosting a
super-massive black hole (SMBH). We determine the total number of encounters
needed to harden a BHB to the point that inspiral due to
gravitational wave emission occurs before the next three-body scattering event.
This is done independently for both the spherical and disk components. Using a
Monte Carlo approach, we refine our calculations for to include
gravitational wave emission between scattering events. For astrophysically
plausible models we find that typically 10.
We find two separate regimes for the efficient dynamical hardening of BHBs:
(1) spherical star clusters with high central densities, low velocity
dispersions and no significant Keplerian component; and (2) migration traps in
disks around SMBHs lacking any significant spherical stellar component in the
vicinity of the migration trap, which is expected due to effective orbital
inclination reduction of any spherical population by the disk. We also find a
weak correlation between the ratio of the second-order velocity moment to
velocity dispersion in galactic nuclei and the rate of BHB mergers, where this
ratio is a proxy for the ratio between the rotation- and dispersion-supported
components. Because disks enforce planar interactions that are efficient in
hardening BHBs, particularly in migration traps, they have high merger rates
that can contribute significantly to the rate of BHB mergers detected by the
advanced Laser Interferometer Gravitational-Wave Observatory.Comment: 13 pages, 9 figures, accepted for publication in MNRA
Geons of Galileons
We suggest that galileon theories should have an additional self-coupling of
the fields to the trace of their own energy-momentum tensor. We explore the
classical features of one such model, in flat 4D spacetime, with emphasis on
solutions that are scalar analogues of gravitational geons. We discuss the
stability of these scalar geons, and some of their possible signatures,
including shock fronts.Comment: References added in v
Supersymmetric sound in fluids
We consider the hydrodynamics of supersymmetric fluids. Supersymmetry is
broken spontaneously and the low energy spectrum includes a fermionic massless
mode, the . We use two complementary approaches to describe
the system: First, we construct a generating functional from which we derive
the equations of motion of the fluid and of the phonino propagating through the
fluid. We write the form of the leading corrections in the derivative
expansion, and show that the so called diffusion terms in the supercurrent are
in fact not dissipative. Second, we use an effective field theory approach
which utilizes a non-linear realization of supersymmetry to analyze the
interactions between phoninos and phonons, and demonstrate the conservation of
entropy in ideal fluids. We comment on possible phenomenological consequences
for gravitino physics in the early universe.Comment: Modified introduction and discussion of diffusion terms in the
supercurren
Generalizing Galileons
The Galileons are a set of terms within four-dimensional effective field
theories, obeying symmetries that can be derived from the dynamics of a
3+1-dimensional flat brane embedded in a 5-dimensional Minkowski Bulk. These
theories have some intriguing properties, including freedom from ghosts and a
non-renormalization theorem that hints at possible applications in both
particle physics and cosmology. In this brief review article, we will summarize
our attempts over the last year to extend the Galileon idea in two important
ways. We will discuss the effective field theory construction arising from
co-dimension greater than one flat branes embedded in a flat background - the
multiGalileons - and we will then describe symmetric covariant versions of the
Galileons, more suitable for general cosmological applications. While all these
Galileons can be thought of as interesting four-dimensional field theories in
their own rights, the work described here may also make it easier to embed them
into string theory, with its multiple extra dimensions and more general
gravitational backgrounds.Comment: 16 pages; invited brief review article for a special issue of
Classical and Quantum Gravity. Submitted to CQ
The Worldvolume Action of Kink Solitons in AdS Spacetime
A formalism is presented for computing the higher-order corrections to the
worldvolume action of co-dimension one solitons. By modifying its potential, an
explicit "kink" solution of a real scalar field in AdS spacetime is found. The
formalism is then applied to explicitly compute the kink worldvolume action to
quadratic order in two expansion parameters--associated with the hypersurface
fluctuation length and the radius of AdS spacetime respectively. Two
alternative methods are given for doing this. The results are expressed in
terms of the trace of the extrinsic curvature and the intrinsic scalar
curvature. In addition to conformal Galileon interactions, we find a
non-Galileon term which is never sub-dominant. This method can be extended to
any conformally flat bulk spacetime.Comment: 32 pages, 3 figures, typos corrected and additional comments adde
Galileon Higgs vortices
Vortex solutions are topologically stable field configurations that can play
an important role in condensed matter, field theory, and cosmology. We
investigate vortex configuration in a 2+1 dimensional Abelian Higgs theory
supplemented by higher order derivative self-interactions, related with
Galileons. Our vortex solutions have features that make them qualitatively
different from well-known Abrikosov-Nielsen-Olesen configurations, since the
derivative interactions turn on gauge invariant field profiles that break axial
symmetry. By promoting the system to a 3+1 dimensional string configuration, we
study its gravitational backreaction. Our results are all derived within a
specific, analytically manageable system, and might offer indications for
understanding Galileonic interactions and screening mechanisms around
configurations that are not spherically symmetric, but only at most
cylindrically symmetric.Comment: 26 pages, 8 figure
Making up meanings in a capital city: power, memory and monuments in Berlin
Much contemporary writing on cities focuses on their position within wider global networks, so there is a risk of underplaying the significance of other aspects of the urban experience.This paper explores the particular role of Berlin as capital city in the making of the (new) Berliner Republic and the ways in which it is defined (and defines itself) within that Republic. Berlin is the (and often literally the building) site on which a new Germany is being constructed. The making up of the new Berlin is dominated by attempts to reinterpret and reimagine its history: it is a city of memorials and of deliberate absences; of remembering and forgetting, or trying to forget; of reshaping the past as well as trying to build a new future. The juxtapositions of urban experience, the layering of memories and the attempt to imagine a different future come together to define Berlin as a contemporary capital city
Motor step size and ATP coupling efficiency of the dsDNA translocase EcoR124I
The Type I restriction-modification enzyme EcoR124I is an archetypical helicase-based dsDNA translocase that moves unidirectionally along the 3′–5′ strand of intact duplex DNA. Using a combination of ensemble and single-molecule measurements, we provide estimates of two physicochemical constants that are fundamental to a full description of motor protein activity—the ATP coupling efficiency (the number of ATP consumed per base pair) and the step size (the number of base pairs transported per motor step). Our data indicate that EcoR124I makes small steps along the DNA of 1 bp in length with 1 ATP consumed per step, but with some uncoupling of the ATPase and translocase cycles occurring so that the average number of ATP consumed per base pair slightly exceeds unity. Our observations form a framework for understanding energy coupling in a great many other motors that translocate along dsDNA rather than ssDNA
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