2,217 research outputs found
Tidal Tails Test the Equivalence Principle in the Dark Sector
Satellite galaxies currently undergoing tidal disruption offer a unique
opportunity to constrain an effective violation of the equivalence principle in
the dark sector. Theories in which cold dark matter (CDM) couples to a light
scalar field naturally lead to a long-range force between dark matter
particles. An inverse-square-law force of this kind would manifest itself as a
violation of the equivalence principle in the dynamics of CDM compared to
baryons in the form of gas or stars. In a previous paper, we showed that an
attractive force would displace stars outwards from the bottom of the
satellite's gravitational potential well, leading to a higher fraction of stars
being disrupted from the tidal bulge further from the Galactic center. Since
stars disrupted from the far (near) side of the satellite go on to form the
trailing (leading) tidal stream, an attractive dark-matter force will produce a
relative enhancement of the trailing stream compared to the leading stream.
This distinctive signature of a dark-matter force might be detected through
detailed observations of the tidal tails of a disrupting satellite, such as
those recently performed by the Two-Micron All-Sky Survey (2MASS) and Sloan
Digital Sky Survey (SDSS) on the Sagittarius (Sgr) dwarf galaxy. Here we show
that this signature is robust to changes in our models for both the satellite
and Milky Way, suggesting that we might hope to search for a dark-matter force
in the tidal features of other recently discovered satellite galaxies in
addition to the Sgr dwarf.Comment: 29 pages, 13 figures, final version published in PR
Self-force approach for radiation reaction
We overview the recently proposed mode-sum regularization prescription (MSRP)
for the calculation of the local radiation-reaction forces, which are crucial
for the orbital evolution of binaries. We then describe some new results which
were obtained using MSRP, and discuss their importance for gravitational-wave
astronomy.Comment: Talk given at the 3rd Edoardo Amaldi Conference on Gravitational
Waves, 12-16 July, 199
Evidence for a gravitational Myers effect
An indication for the existence of a collective Myers solution in the
non-abelian D0-brane Born-Infeld action is the presence of a tachyonic mode in
fluctuations around the standard diagonal background. We show that this
computation for non-abelian D0-branes in curved space has the geometric
interpretation of computing the eigenvalues of the geodesic deviation operator
for U(N)-valued coordinates. On general grounds one therefore expects a
geometric Myers effect in regions of sufficiently negative curvature. We
confirm this by explicit computations for non-abelian D0-branes on a sphere and
a hyperboloid. For the former the diagonal solution is stable, but not so for
the latter. We conclude by showing that near the horizon of a Schwarzschild
black hole one also finds a tachyonic mode in the fluctuation spectrum,
signaling the possibility of a near-horizon gravitationally induced Myers
effect.Comment: LaTeX, 23 page
A Numerical Perspective on Hartree-Fock-Bogoliubov Theory
The method of choice for describing attractive quantum systems is
Hartree-Fock-Bogoliubov (HFB) theory. This is a nonlinear model which allows
for the description of pairing effects, the main explanation for the
superconductivity of certain materials at very low temperature. This paper is
the first study of Hartree-Fock-Bogoliubov theory from the point of view of
numerical analysis. We start by discussing its proper discretization and then
analyze the convergence of the simple fixed point (Roothaan) algorithm.
Following works by Canc\`es, Le Bris and Levitt for electrons in atoms and
molecules, we show that this algorithm either converges to a solution of the
equation, or oscillates between two states, none of them being a solution to
the HFB equations. We also adapt the Optimal Damping Algorithm of Canc\`es and
Le Bris to the HFB setting and we analyze it. The last part of the paper is
devoted to numerical experiments. We consider a purely gravitational system and
numerically discover that pairing always occurs. We then examine a simplified
model for nucleons, with an effective interaction similar to what is often used
in nuclear physics. In both cases we discuss the importance of using a damping
algorithm
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