2,210 research outputs found
Analytic Lyapunov exponents in a classical nonlinear field equation
It is shown that the nonlinear wave equation , which is the continuum limit of
the Fermi-Pasta-Ulam (FPU) beta model, has a positive Lyapunov exponent
lambda_1, whose analytic energy dependence is given. The result (a first
example for field equations) is achieved by evaluating the lattice-spacing
dependence of lambda_1 for the FPU model within the framework of a Riemannian
description of Hamiltonian chaos. We also discuss a difficulty of the
statistical mechanical treatment of this classical field system, which is
absent in the dynamical description.Comment: 4 pages, 1 figur
Lyapunov exponents from geodesic spread in configuration space
The exact form of the Jacobi -- Levi-Civita (JLC) equation for geodesic
spread is here explicitly worked out at arbitrary dimension for the
configuration space manifold M_E = {q in R^N | V(q) < E} of a standard
Hamiltonian system, equipped with the Jacobi (or kinetic energy) metric g_J. As
the Hamiltonian flow corresponds to a geodesic flow on (M_E,g_J), the JLC
equation can be used to study the degree of instability of the Hamiltonian
flow. It is found that the solutions of the JLC equation are closely resembling
the solutions of the standard tangent dynamics equation which is used to
compute Lyapunov exponents. Therefore the instability exponents obtained
through the JLC equation are in perfect quantitative agreement with usual
Lyapunov exponents. This work completes a previous investigation that was
limited only to two-degrees of freedom systems.Comment: REVTEX file, 10 pages, 2 figure
On the origin of Phase Transitions in the absence of Symmetry-Breaking
In this paper we investigate the Hamiltonian dynamics of a lattice gauge
model in three spatial dimension. Our model Hamiltonian is defined on the basis
of a continuum version of a duality transformation of a three dimensional Ising
model. The system so obtained undergoes a thermodynamic phase transition in the
absence of symmetry-breaking. Besides the well known use of quantities like the
Wilson loop we show how else the phase transition in such a kind of models can
be detected. It is found that the first order phase transition undergone by
this model is characterised according to an Ehrenfest-like classification of
phase transitions applied to the configurational entropy. On the basis of the
topological theory of phase transitions, it is discussed why the seemingly
divergent behaviour of the third derivative of configurational entropy can be
considered as the "shadow" of some suitable topological transition of certain
submanifolds of configuration space.Comment: 31 pages, 9 figure
Estimating the mass density of neutral gas at
We use the relationships between galactic HI mass and B-band luminosity
determined by Rao & Briggs to recalculate the mass density of neutral gas at
the present epoch based on more recent measures of the galaxy luminosity
function than were available to those authors. We find in good agreement with the original Rao & Briggs
value, suggesting that this quantity is now reasonably secure. We then show
that, if the scaling between H I mass and B-band luminosity has remained
approximately constant since , the evolution of the luminosity function
found by the Canada-France redshift survey translates to an increase of
by a factor of at . A similar value is
obtained quite independently from consideration of the luminosity function of
Mg II absorbers at . By combining these new estimates with data from
damped \lya systems at higher redshift, it is possible to assemble a rough
sketch of the evolution of over the last 90% of the age of the
universe. The consumption of H I gas with time is in broad agreement with
models of chemical evolution which include the effects of dust, although more
extensive samples of damped \lya systems at low and intermediate redshift are
required for a quantitative assessment of the dust bias.Comment: LaTeX file, 11 pages, 1 figure, accepted MNRAS pink page
Near-pristine gas at high redshifts: a window on early nucleosynthesis
It has now become recognised that damped Lyman alpha systems play an
important role in helping us unravel the origin of chemical elements. In this
presentation, we describe the main results of a recently completed survey of
the most metal-poor DLAs, aimed at complementing and extending studies of the
oldest stars in the Galaxy. The survey has clarified a number of lingering
issues concerning the abundances of C, N, O in the low metallicity regime, has
revealed the existence of DLA analogues to Carbon-enhanced metal-poor stars,
and is providing some of the most precise measures of the primordial abundance
of Deuterium.Comment: 11 pages, 7 Figures. Invited presentation at the XII International
Symposium on Nuclei in the Cosmos, Cairns, Australia, 5-10 August 2012. To
appear in Proceedings of Scienc
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