29,636 research outputs found
Strong-field tidal distortions of rotating black holes: Formalism and results for circular, equatorial orbits
Tidal coupling between members of a compact binary system can have an
interesting and important influence on that binary's dynamical inspiral. Tidal
coupling also distorts the binary's members, changing them (at lowest order)
from spheres to ellipsoids. At least in the limit of fluid bodies and Newtonian
gravity, there are simple connections between the geometry of the distorted
ellipsoid and the impact of tides on the orbit's evolution. In this paper, we
develop tools for investigating tidal distortions of rapidly rotating black
holes using techniques that are good for strong-field, fast-motion binary
orbits. We use black hole perturbation theory, so our results assume extreme
mass ratios. We develop tools to compute the distortion to a black hole's
curvature for any spin parameter, and for tidal fields arising from any bound
orbit, in the frequency domain. We also develop tools to visualize the
horizon's distortion for black hole spin (leaving the more
complicated case to a future analysis). We then study how a
Kerr black hole's event horizon is distorted by a small body in a circular,
equatorial orbit. We find that the connection between the geometry of tidal
distortion and the orbit's evolution is not as simple as in the Newtonian
limit.Comment: 37 pages, 8 figures. Accepted for publication to Physical Review D.
This version corrects a number of typographical errors found when reviewing
the page proof
Hemiparasitic plant impacts animal and plant communities across four trophic levels
1.Understanding the impact of species on community structure is a fundamental question in ecology. There is a growing body of evidence that suggests that both sub-dominant species and parasites can have a disproportionately large impact.
2.Here we report the impacts of an organism that is both subdominant and parasitic, the hemiparasite Rhinanthus minor. Whilst the impact of parasitic angiosperms on their hosts and, to a lesser degree, co-existing plant species, have been well characterized, much less is known about their impacts on higher trophic levels.
3.We experimentally manipulated field densities of the hemiparasite Rhinanthus minor in a species rich grassland, comparing the plant and invertebrate communities in plots where it was removed, at natural densities or at enhanced densities.
4.Plots with natural and enhanced densities of R. minor had lower plant biomass than plots without the hemiparasite, but enhanced densities almost doubled the abundance of invertebrates within the plots across all trophic levels, with effects evident in herbivores, predators and detritivores.
5.The hemiparasite R. minor, despite being a sub-dominant and transient component within plant communities that it inhabits, has profound effects on four different trophic levels. These effects persist beyond the life of the hemiparasite,
emphasizing its role as a keystone species in grassland communitie
Millisecond accuracy video display using OpenGL under Linux
To measure people’s reaction times to the nearest millisecond, it is necessary to know exactly when
a stimulus is displayed. This article describes how to display stimuli with millisecond accuracy on a
normal CRT monitor, using a PC running Linux. A simple C program is presented to illustrate how this
may be done within X Windows using the OpenGL rendering system. A test of this system is reported
that demonstrates that stimuli may be consistently displayed with millisecond accuracy. An algorithm
is presented that allows the exact time of stimulus presentation to be deduced, even if there are relatively
large errors in measuring the display time
Natural Chaotic Inflation in Supergravity
We propose a chaotic inflation model in supergravity. In the model the
K\"ahler potential has a Nambu-Goldstone-like shift symmetry of the inflaton
chiral multiplet which ensures the flatness of the inflaton potential beyond
the Planck scale. We show that the chaotic inflation naturally takes place by
introducing a small breaking term of the shift symmetry in the superpotential.
This may open a new branch of model building for inflationary universe in the
framework of supergravity.Comment: Some typos are corrected. To appear in Phys. Rev. Let
Cosmological perturbation theory and conserved quantities in the large-scale limit
The linear cosmological perturbation theory of an almost homogeneous and
isotropic perfect fluid universe is reconsidered and formally simplified by
introducing new covariant and gauge-invariant variables with physical
interpretations on hypersurfaces of constant expansion, constant curvature or
constant energy density. The existence of conserved perturbation quantities on
scales larger than the Hubble scale is discussed. The quantity which is
conserved on large scales in a flat background universe may be expressed in
terms of the fractional, spatial gradient of the energy density on constant
expansion hypersurfaces or, alternatively, with the help of expansion or
curvature perturbation variables on hypersurfaces of constant energy density.
For nonvanishing background curvature the perturbation dynamics is most
suitably described in terms of energy density perturbations on hypersurfaces of
constant curvature.Comment: 12 pages, Revtex, to appear in Class.Quantum Gra
A New delta N Formalism for Multi-Component Inflation
The delta N formula that relates the final curvature perturbation on comoving
slices to the inflaton perturbation on flat slices after horizon crossing is a
powerful and intuitive tool to compute the curvature perturbation spectrum from
inflation. However, it is customarily assumed further that the conventional
slow-roll condition is satisfied, and satisfied by all components, during
horizon crossing. In this paper, we develop a new delta N formalism for
multi-component inflation that can be applied in the most general situations.
This allows us to generalize the idea of general slow-roll inflation to the
multi-component case, in particular only applying the general slow-roll
condition to the relevant component. We compute the power spectrum of the
curvature perturbation in multi-component general slow-roll inflation, and find
that under quite general conditions it is invertible.Comment: 24 pages, no figur
Inflationary models with a flat potential enforced by non-abelian discrete gauge symmetries
Non-abelian discrete gauge symmetries can provide the inflaton with a flat
potential even when one takes into account gravitational strength effects. The
discreteness of the symmetries also provide special field values where
inflation can end via a hybrid type mechanism. An interesting feature of this
method is that it can naturally lead to extremely flat potentials and so, in
principle, to inflation at unusually low energy scales. Two examples of
effective field theories with this mechanism are given, one with a hybrid exit
and one with a mutated hybrid exit. They include an explicit example in which
the single field consistency condition is violated.Comment: 24 pages, uses revtex.sty, submitted to PRD (Nov. 1999) Final version
to appear in PRD. Background information on supergravity expande
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