19 research outputs found
Static spectroscopy of a dense superfluid
Dense Bose superfluids, as HeII, differ from dilute ones by the existence of
a roton minimum in their excitation spectrum. It is known that this roton
minimum is qualitatively responsible for density oscillations close to any
singularity, such as vortex cores, or close to solid boundaries. We show that
the period of these oscillations, and their exponential decrease with the
distance to the singularity, are fully determined by the position and the width
of the roton minimum. Only an overall amplitude factor and a phase shift are
shown to depend on the details of the interaction potential. Reciprocally, it
allows for determining the characteristics of this roton minimum from static
"observations" of a disturbed ground state, in cases where the dynamics is not
easily accessible. We focus on the vortex example. Our analysis further shows
why the energy of these oscillations is negligible compared to the kinetic
energy, which limits their influence on the vortex dynamics, except for high
curvatures.Comment: 14 pages, 4 figures, extended version, published in J. Low Temp. Phy
Particle velocity in noncommutative space-time
We investigate a particle velocity in the -Minkowski space-time,
which is one of the realization of a noncommutative space-time. We emphasize
that arrival time analyses by high-energy -rays or neutrinos, which
have been considered as powerful tools to restrict the violation of Lorentz
invariance, are not effective to detect space-time noncommutativity. In
contrast with these examples, we point out a possibility that {\it low-energy
massive particles} play an important role to detect it.Comment: 16 pages, corrected some mistake
Singularity-free cosmological solutions in quadratic gravity
We study a general field theory of a scalar field coupled to gravity through
a quadratic Gauss-Bonnet term . The coupling function has
the form , where is a positive integer. In the absence of
the Gauss-Bonnet term, the cosmological solutions for an empty universe and a
universe dominated by the energy-momentum tensor of a scalar field are always
characterized by the occurrence of a true cosmological singularity. By
employing analytical and numerical methods, we show that, in the presence of
the quadratic Gauss-Bonnet term, for the dual case of even , the set of
solutions of the classical equations of motion in a curved FRW background
includes singularity-free cosmological solutions. The singular solutions are
shown to be confined in a part of the phase space of the theory allowing the
non-singular solutions to fill the rest of the space. We conjecture that the
same theory with a general coupling function that satisfies certain criteria
may lead to non-singular cosmological solutions.Comment: Latex, 25 pages, 6 figures, some explanatory sentences and Comments
added, version to appear in Physical Review