1,645 research outputs found
Persistence of Randomly Coupled Fluctuating Interfaces
We study the persistence properties in a simple model of two coupled
interfaces characterized by heights h_1 and h_2 respectively, each growing over
a d-dimensional substrate. The first interface evolves independently of the
second and can correspond to any generic growing interface, e.g., of the
Edwards-Wilkinson or of the Kardar-Parisi-Zhang variety. The evolution of h_2,
however, is coupled to h_1 via a quenched random velocity field. In the limit
d\to 0, our model reduces to the Matheron-de Marsily model in two dimensions.
For d=1, our model describes a Rouse polymer chain in two dimensions advected
by a transverse velocity field. We show analytically that after a long waiting
time t_0\to \infty, the stochastic process h_2, at a fixed point in space but
as a function of time, becomes a fractional Brownian motion with a Hurst
exponent, H_2=1-\beta_1/2, where \beta_1 is the growth exponent characterizing
the first interface. The associated persistence exponent is shown to be
\theta_s^2=1-H_2=\beta_1/2. These analytical results are verified by numerical
simulations.Comment: 15 pages, 3 .eps figures include
The Cosmic Lens All-Sky Survey parent population - I. Sample selection and number counts
We present the selection of the Jodrell Bank Flat-spectrum (JBF) radio source
sample, which is designed to reduce the uncertainties in the Cosmic Lens
All-Sky Survey (CLASS) gravitational lensing statistics arising from the lack
of knowledge about the parent population luminosity function. From observations
at 4.86 GHz with the Very Large Array, we have selected a sample of 117
flat-spectrum radio sources with flux densities greater than 5 mJy. These
sources were selected in a similar manner to the CLASS complete sample and are
therefore representative of the parent population at low flux densities. The
vast majority (~90 per cent) of the JBF sample are found to be compact on the
arcsecond scales probed here and show little evidence of any extended radio jet
emission. Using the JBF and CLASS complete samples we find the differential
number counts slope of the parent population above and below the CLASS 30 mJy
flux density limit to be -2.07+/-0.02 and -1.96+/-0.12, respectively.Comment: 10 pages, 4 figures, accepted for publication in MNRA
The Casimir Effect for Parallel Plates Revisited
The Casimir effect for a massless scalar field with Dirichlet and periodic
boundary conditions (b.c.) on infinite parallel plates is revisited in the
local quantum field theory (lqft) framework introduced by B.Kay. The model
displays a number of more realistic features than the ones he treated. In
addition to local observables, as the energy density, we propose to consider
intensive variables, such as the energy per unit area , as
fundamental observables. Adopting this view, lqft rejects Dirichlet (the same
result may be proved for Neumann or mixed) b.c., and accepts periodic b.c.: in
the former case diverges, in the latter it is finite, as is shown by
an expression for the local energy density obtained from lqft through the use
of the Poisson summation formula. Another way to see this uses methods from the
Euler summation formula: in the proof of regularization independence of the
energy per unit area, a regularization-dependent surface term arises upon use
of Dirichlet b.c. but not periodic b.c.. For the conformally invariant scalar
quantum field, this surface term is absent, due to the condition of zero trace
of the energy momentum tensor, as remarked by B.De Witt. The latter property
does not hold in tha application to the dark energy problem in Cosmology, in
which we argue that periodic b.c. might play a distinguished role.Comment: 25 pages, no figures, late
LOFAR discovery of a 700-kpc remnant radio galaxy at low redshift
Remnant radio galaxies represent the final dying phase of radio galaxy
evolution, in which the jets are no longer active. Due to their rarity in flux
limited samples and the difficulty of identification, this dying phase remains
poorly understood and the luminosity evolution largely unconstrained. Here we
present the discovery and detailed analysis of a large (700 kpc) remnant radio
galaxy with a low surface brightness that has been identified in LOFAR images
at 150 MHz. By combining LOFAR data with new follow-up Westerbork observations
and archival data at higher frequencies, we investigated the source morphology
and spectral properties from 116 to 4850 MHz. By modelling the radio spectrum
we probed characteristic timescales of the radio activity. The source has a
relatively smooth, diffuse, amorphous appearance together with a very weak
central compact core which is associated with the host galaxy located at
z=0.051. From our ageing and morphological analysis it is clear that the
nuclear engine is currently switched off or, at most, active at a very low
power state. The host galaxy is currently interacting with another galaxy
located at a projected separation of 15 kpc and a radial velocity offset of 300
km/s. This interaction may have played a role in the triggering and/or shut
down of the radio jets. The spectral shape of this remnant radio galaxy differs
from the majority of the previously identified remnant sources, which show
steep or curved spectra at low to intermediate frequencies. In light of this
finding and in preparation for new-generation deep low-frequency surveys, we
discuss the selection criteria to be used to select representative samples of
these sources.Comment: 10 pages, 6 figures, 5 tables, A&A accepte
The trace of the heat kernel on a compact hyperbolic 3-orbifold
The heat coefficients related to the Laplace-Beltrami operator defined on the
hyperbolic compact manifold H^3/\Ga are evaluated in the case in which the
discrete group \Ga contains elliptic and hyperbolic elements. It is shown
that while hyperbolic elements give only exponentially vanishing corrections to
the trace of the heat kernel, elliptic elements modify all coefficients of the
asymptotic expansion, but the Weyl term, which remains unchanged. Some physical
consequences are briefly discussed in the examples.Comment: 11 page
Gauge fixing and equivariant cohomology
The supersymmetric model developed by Witten to study the equivariant
cohomology of a manifold with an isometric circle action is derived from the
BRST quantization of a simple classical model. The gauge-fixing process is
carefully analysed, and demonstrates that different choices of gauge-fixing
fermion can lead to different quantum theories.Comment: 18 pages LaTe
Survival of a Diffusing Particle in a Transverse Shear Flow: A First-Passage Problem with Continuously Varying Persistence Exponent
We consider a particle diffusing in the y-direction, dy/dt=\eta(t), subject
to a transverse shear flow in the x-direction, dx/dt=f(y), where x \ge 0 and
x=0 is an absorbing boundary. We treat the class of models defined by f(y) =
\pm v_{\pm}(\pm y)^\alpha where the upper (lower) sign refers to y>0 (y<0). We
show that the particle survives with probability Q(t) \sim t^{-\theta} with
\theta = 1/4, independent of \alpha, if v_{+}=v_{-}. If v_{+} \ne v_{-},
however, we show that \theta depends on both \alpha and the ratio v_{+}/v_{-},
and we determine this dependence.Comment: 4 page
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