652 research outputs found
CMB anisotropies from acausal scaling seeds
We investigate models where structure formation is initiated by scaling
seeds: We consider rapidly expanding relativistic shells of energy and show
that they can fit current CMB and large scale structure data if they expand
with super-luminal velocities. These acausally expanding shells provide a
viable alternative to inflation for cosmological structure formation with the
same minimal number of parameters to characterize the initial fluctuations.
Causally expanding shells alone cannot fit present data. Hybrid models where
causal shells and inflation are mixed also provide good fits.Comment: 9 pages,13 figures, revised version accepted for publication in PR
Birefringent Gravitational Waves and the Consistency Check of Inflation
In this work we show that the gravitational Chern-Simons term, aside from
being a key ingredient in inflationary baryogenesis, modifies super-horizon
gravitational waves produced during inflation. We compute the super-Hubble
gravitational power spectrum in the slow-roll approximation and show that its
overall amplitude is modified while its spectral index remains unchanged (at
leading order in the slow-roll parameters). Then, we calculate the correction
to the tensor to scalar ratio, T/S. We find a correction of T/S which is
dependent on (more precisely quadratic in ), the parameter
characterizing the amplitude of the Chern-Simons terms. In a stringy embedding
of the leptogenesis mechanism, is the ratio between the Planck scale
and the fundamental string scale. Thus, in principle, we provide a direct probe
of leptogenesis due to stringy dynamics in the Cosmic Microwave Background
(CMB). However, we demonstrate that the corresponding correction of T/S is in
fact very small and not observable in the regime where our calculations are
valid. To obtain a sizable effect, we argue that a non-linear calculation is
necessary.Comment: 9 pages, 1 figure, RevTe
Ballistic aggregation: a solvable model of irreversible many particles dynamics
The adhesive dynamics of a one-dimensional aggregating gas of point particles
is rigorously described. The infinite hierarchy of kinetic equations for the
distributions of clusters of nearest neighbours is shown to be equivalent to a
system of two coupled equations for a large class of initial conditions. The
solution to these nonlinear equations is found by a direct construction of the
relevant probability distributions in the limit of a continuous initial mass
distribution. We show that those limiting distributions are identical to those
of the statistics of shocks in the Burgers turbulence. The analysis relies on a
mapping on a Brownian motion problem with parabolic constraints.Comment: 23 pages, 6 figures include
Self Similar Solutions of the Evolution Equation of a Scalar Field in an Expanding Geometry
We consider the functional Schrodinger equation for a self interacting scalar
field in an expanding geometry. By performing a time dependent scale
transformation on the argument of the field we derive a functional Schrodinger
equation whose hamiltonian is time independent but involves a time-odd term
associated to a constraint on the expansion current. We study the mean field
approximation to this equation and generalize in this case, for interacting
fields, the solutions worked out by Bunch and Davies for free fields.Comment: 8 pages, Latex, IPNO/TH 94-3
A New Approach to Lower Dimensional Gauge Theories
We apply the method of differential renormalization to two and three
dimensional abelian gauge theories. The method is especially well suited for
these theories as the problems of defining the antisymmetric tensor are avoided
and the calculus involved is impressively simple. The topological and dynamical
photon masses are obtained.Comment: 8 pag. in Late
A complete criterion for separability detection
Using new results on the separability properties of bosonic systems, we
provide a new complete criterion for separability. This criterion aims at
characterizing the set of separable states from the inside by means of a
sequence of efficiently solvable semidefinite programs. We apply this method to
derive arbitrarily good approximations to the optimal measure-and-prepare
strategy in generic state estimation problems. Finally, we report its
performance in combination with the criterion developed by Doherty et al. [1]
for the calculation of the entanglement robustness of a relevant family of
quantum states whose separability properties were unknown
Exact Green's Function of the reversible diffusion-influenced reaction for an isolated pair in 2D
We derive an exact Green's function of the diffusion equation for a pair of
spherical interacting particles in 2D subject to a back-reaction boundary
condition.Comment: 6 pages, 1 Figur
Geometrically Consistent Approach to Stochastic DBI Inflation
Stochastic effects during inflation can be addressed by averaging the quantum
inflaton field over Hubble-patch sized domains. The averaged field then obeys a
Langevin-type equation into which short-scale fluctuations enter as a noise
term. We solve the Langevin equation for a inflaton field with Dirac Born
Infeld (DBI) kinetic term perturbatively in the noise and use the result to
determine the field value's Probability Density Function (PDF). In this
calculation, both the shape of the potential and the warp factor are arbitrary
functions, and the PDF is obtained with and without volume effects due to the
finite size of the averaging domain. DBI kinetic terms typically arise in
string-inspired inflationary scenarios in which the scalar field is associated
with some distance within the (compact) extra dimensions. The inflaton's
accessible range of field values therefore is limited because of the extra
dimensions' finite size. We argue that in a consistent stochastic approach the
distance-inflaton's PDF must vanish for geometrically forbidden field values.
We propose to implement these extra-dimensional spatial restrictions into the
PDF by installing absorbing (or reflecting) walls at the respective boundaries
in field space. As a toy model, we consider a DBI inflaton between two
absorbing walls and use the method of images to determine its most general PDF.
The resulting PDF is studied in detail for the example of a quartic warp factor
and a chaotic inflaton potential. The presence of the walls is shown to affect
the inflaton trajectory for a given set of parameters.Comment: 20 pages, 3 figure
Fractional photon-assisted tunneling in an optical superlattice: large contribution to particle transfer
Fractional photon-assisted tunneling is investigated both analytically and
numerically for few interacting ultra-cold atoms in the double-wells of an
optical superlattice. This can be realized experimentally by adding periodic
shaking to an existing experimental setup [Phys. Rev. Lett. 101, 090404
(2008)]. Photon-assisted tunneling is visible in the particle transfer between
the wells of the individual double wells. In order to understand the physics of
the photon-assisted tunneling, an effective model based on the rotating wave
approximation is introduced. The validity of this effective approach is tested
for wide parameter ranges which are accessible to experiments in double-well
lattices. The effective model goes well beyond previous perturbation theory
approaches and is useful to investigate in particular the fractional
photon-assisted tunneling resonances. Analytic results on the level of the
experimentally realizable two-particle quantum dynamics show very good
agreement with the numerical solution of the time-dependent Schr\"odinger
equation. Far from being a small effect, both the one-half-photon and the
one-third-photon resonance are shown to have large effects on the particle
transfer.Comment: 9 pages, 11 png-figure
One-dimensional infinite component vector spin glass with long-range interactions
We investigate zero and finite temperature properties of the one-dimensional
spin-glass model for vector spins in the limit of an infinite number m of spin
components where the interactions decay with a power, \sigma, of the distance.
A diluted version of this model is also studied, but found to deviate
significantly from the fully connected model. At zero temperature, defect
energies are determined from the difference in ground-state energies between
systems with periodic and antiperiodic boundary conditions to determine the
dependence of the defect-energy exponent \theta on \sigma. A good fit to this
dependence is \theta =3/4-\sigma. This implies that the upper critical value of
\sigma is 3/4, corresponding to the lower critical dimension in the
d-dimensional short-range version of the model. For finite temperatures the
large m saddle-point equations are solved self-consistently which gives access
to the correlation function, the order parameter and the spin-glass
susceptibility. Special attention is paid to the different forms of finite-size
scaling effects below and above the lower critical value, \sigma =5/8, which
corresponds to the upper critical dimension 8 of the hypercubic short-range
model.Comment: 27 pages, 27 figures, 4 table
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