20,722 research outputs found
The effects of forcing and dissipation on phase transitions in thin granular layers
Recent experimental and computational studies of vibrated thin layers of
identical spheres have shown transitions to ordered phases similar to those
seen in equilibrium systems. Motivated by these results, we carry out
simulations of hard inelastic spheres forced by homogenous white noise. We find
a transition to an ordered state of the same symmetry as that seen in the
experiments, but the clear phase separation observed in the vibrated system is
absent. Simulations of purely elastic spheres also show no evidence for phase
separation. We show that the energy injection in the vibrated system is
dramatically different in the different phases, and suggest that this creates
an effective surface tension not present in the equilibrium or randomly forced
systems. We do find, however, that inelasticity suppresses the onset of the
ordered phase with random forcing, as is observed in the vibrating system, and
that the amount of the suppression is proportional to the degree of
inelasticity. The suppression depends on the details of the energy injection
mechanism, but is completely eliminated when inelastic collisions are replaced
by uniform system-wide energy dissipation.Comment: 10 pages, 5 figure
Semiclassical geons as solitonic black hole remnants
We find that the end state of black hole evaporation could be represented by
non-singular and without event horizon stable solitonic remnants with masses of
the order the Planck scale and up to 16 units of charge. Though these objects
are locally indistinguishable from spherically symmetric, massive electric (or
magnetic) charges, they turn out to be sourceless geons containing a wormhole
generated by the electromagnetic field. Our results are obtained by
interpreting semiclassical corrections to Einstein's theory in the first-order
(Palatini) formalism, which yields second-order equations and avoids the
instabilities of the usual (metric) formulation of quadratic gravity. We also
discuss the potential relevance of these solutions for primordial black holes
and the dark matter problem.Comment: 9 pages, 1 figur
A Review on the Cosmology of the de Sitter Horndeski Models
We review the most general scalar-tensor cosmological models with up to
second-order derivatives in the field equations that have a fixed spatially
flat de Sitter critical point independent of the material content or vacuum
energy. This subclass of the Horndeski Lagrangian is capable of dynamically
adjusting any value of the vacuum energy of the matter fields at the critical
point. We present the cosmological evolution of the linear models and the
non-linear models with shift symmetry. We come to the conclusion that the shift
symmetric non-linear models can deliver a viable background compatible with
current observations.Comment: 7 pages, 2 figures. Proceedings accepted for publication in
"Universe", Special Issue "Varying Constants and Fundamental Cosmology" for
the VARCOSMOFUN16 in Szczecin, Poland, 12-17 September, 201
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