86,427 research outputs found
The Milky Way Galaxy as a Strong Gravitational Lens
We study the gravitational lensing effects of spiral galaxies by taking a
model of the Milky Way and computing its lensing properties. The model is
composed of a spherical Hernquist bulge, a Miyamoto-Nagai disc and an
isothermal halo. As a strong lens, a spiral galaxy like the Milky Way can give
rise to four different imaging geometries. They are (i) three images on one
side of the galaxy centre (`disc triplets'), (ii) three images with one close
to the centre (`core triplets'), (iii) five images and (iv) seven images.
Neglecting magnification bias, we show that the core triplets, disc triplets
and fivefold imaging are roughly equally likely. Even though our models contain
edge-on discs, their image multiplicities are not dominated by disc triplets.
The halo has a small effect on the caustic structure, the time delays and
brightnesses of the images. The Milky Way model has a maximum disc (i.e., the
halo is not dynamically important in the inner parts). Strong lensing by nearly
edge-on disc galaxies breaks the degeneracy between the relative contribution
of the disc and halo to the overall rotation curve. If a spiral galaxy has a
sub-maximum disc, then the astroid caustic shrinks dramatically in size, whilst
the radial caustic shrinks more modestly. This causes changes in the relative
likelihood of the image geometries, specifically (i) core triplets are now 9/2
times more likely than disc triplets, (ii) the cross section for threefold
imaging is reduced by a factor of 2/3, whilst (iii) the cross section for
fivefold imaging is reduced by 1/2. Although multiple imaging is less likely
(the cross sections are smaller), the average total magnification is greater.Comment: MNRAS, in pres
Rules for transition rates in nonequilibrium steady states
Just as transition rates in a canonical ensemble must respect the principle
of detailed balance, constraints exist on transition rates in driven steady
states. I derive those constraints, by maximum information-entropy inference,
and apply them to the steady states of driven diffusion and a sheared lattice
fluid. The resulting ensemble can potentially explain nonequilibrium phase
behaviour and, for steady shear, gives rise to stress-mediated long-range
interactions.Comment: 4 pages. To appear in Physical Review Letter
Radiation Induced Fermion Resonance
The Dirac equation is solved for two novel terms which describe the
interaction energy between the half integral spin of a fermion and the
classical, circularly polarized, electromagnetic field. A simple experiment is
suggested to test the new terms and the existence of radiation induced fermion
resonance.Comment: latex, 4 pages, no figure
Criticality and Condensation in a Non-Conserving Zero Range Process
The Zero-Range Process, in which particles hop between sites on a lattice
under conserving dynamics, is a prototypical model for studying real-space
condensation. Within this model the system is critical only at the transition
point. Here we consider a non-conserving Zero-Range Process which is shown to
exhibit generic critical phases which exist in a range of creation and
annihilation parameters. The model also exhibits phases characterised by
mesocondensates each of which contains a subextensive number of particles. A
detailed phase diagram, delineating the various phases, is derived.Comment: 15 pages, 4 figure, published versi
An exactly solvable dissipative transport model
We introduce a class of one-dimensional lattice models in which a quantity,
that may be thought of as an energy, is either transported from one site to a
neighbouring one, or locally dissipated. Transport is controlled by a
continuous bias parameter q, which allows us to study symmetric as well as
asymmetric cases. We derive sufficient conditions for the factorization of the
N-body stationary distribution and give an explicit solution for the latter,
before briefly discussing physically relevant situations.Comment: 7 pages, 1 figure, submitted to J. Phys.
Diffusion and rheology in a model of glassy materials
We study self-diffusion within a simple hopping model for glassy materials.
(The model is Bouchaud's model of glasses [J.-P. Bouchaud, J. Physique I 2,
1705 (1992)], as extended to describe rheological properties [P. Sollich, F.
Lequeux, P. Hebraud and M.E. Cates, Phys. Rev. Lett. 78, 2020 (1997)].) We
investigate the breakdown, near the glass transition, of the (generalized)
Stokes-Einstein relation between self-diffusion of a tracer particle and the
(frequency-dependent) viscosity of the system as a whole. This stems from the
presence of a broad distribution of relaxation times of which different moments
control diffusion and rheology. We also investigate the effect of flow
(oscillatory shear) on self-diffusion and show that this causes a finite
diffusivity in the temperature regime below the glass transition (where this
was previously zero). At higher temperatures the diffusivity is enhanced by a
power law frequency dependence that also characterises the rheological
response. The relevance of these findings to soft glassy materials (foams,
emulsions etc.) as well as to conventional glass-forming liquids is discussed.Comment: 39 page (double spaced), 2 figure
Condensation Transition in Polydisperse Hard Rods
We study a mass transport model, where spherical particles diffusing on a
ring can stochastically exchange volume , with the constraint of a fixed
total volume , being the total number of particles. The
particles, referred to as -spheres, have a linear size that behaves as
and our model thus represents a gas of polydisperse hard rods with
variable diameters . We show that our model admits a factorized
steady state distribution which provides the size distribution that minimizes
the free energy of a polydisperse hard rod system, under the constraints of
fixed and . Complementary approaches (explicit construction of the
steady state distribution on the one hand ; density functional theory on the
other hand) completely and consistently specify the behaviour of the system. A
real space condensation transition is shown to take place for : beyond a
critical density a macroscopic aggregate is formed and coexists with a critical
fluid phase. Our work establishes the bridge between stochastic mass transport
approaches and the optimal polydispersity of hard sphere fluids studied in
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