55 research outputs found
Examples with minimal number of brake orbits and homoclinics in annular potential regions
We use a geometric construction to exhibit examples of autonomous Lagrangian
systems admitting exactly two homoclinics emanating from a nondegenerate
maximum of the potential energy and reaching a regular level of the potential
having the same value of the maximum point. Similarly, we show examples of
Hamiltonian systems that admit exactly two brake orbits in an annular potential
region connecting the two connected components of the boundary of the potential
well. These examples show that the estimates proven in [R. Giamb\`o, F.
Giannoni, P. Piccione, Arch. Ration. Mech. Anal. 200, (2011) 691-724] are
sharp.Comment: 12 pages, 1 figur
Qualitative analysis of collapsing isotropic fluid spacetimes
The structure of the Einstein field equations describing the gravitational
collapse of spherically symmetric isotropic fluids is analyzed here for general
equations of state. A suitable system of coordinates is constructed which
allows us, under a hypothesis of Taylor-expandability with respect to one of
the coordinates, to approach the problem of the nature of the final state
without knowing explicitely the metric. The method is applied to investigate
the singularities of linear barotropic perfect fluids solutions and to a family
of accelerating fluids.Comment: 16 pages; to appear on Class. Quantum Gra
Expanding universes in the conformal frame of gravity
The late time evolution of Friedmann-Robertson-Walker (FRW) models with a
perfect fluid matter source is studied in the conformal frame of
gravity. We assume that the corresponding scalar field, nonminimally coupled to
matter, has an arbitrary non-negative potential function . We prove
that equilibria corresponding to non-negative local minima for are
asymptotically stable. We investigate all cases where one of the matter
components eventually dominates. The results are valid for a large class of
non-negative potentials without any particular assumptions about the behavior
of the potential at infinity. In particular for a nondegenerate minimum of the
potential with zero critical value we show that if , the parameter of
the equation of state is larger than one, then there is a transfer of energy
from the fluid to the scalar field and the later eventually dominates.Comment: Talk given in "The Invisible Universe", June 29 - July 3, 2009, Pari
Genericity of nondegenerate geodesics with general boundary conditions
Let M be a possibly noncompact manifold. We prove, generically in the
C^k-topology (k=2,...,\infty), that semi-Riemannian metrics of a given index on
M do not possess any degenerate geodesics satisfying suitable boundary
conditions. This extends a result of Biliotti, Javaloyes and Piccione for
geodesics with fixed endpoints to the case where endpoints lie on a compact
submanifold P of the product MxM that satisfies an admissibility condition.
Such condition holds, for example, when P is transversal to the diagonal of
MxM. Further aspects of these boundary conditions are discussed and general
conditions under which metrics without degenerate geodesics are C^k-generic are
given.Comment: LaTeX2e, 21 pages, no figure
Effective field description of the Anton-Schmidt cosmic fluid
The effective theory of the Anton-Schmidt cosmic fluid within the Debye
approximation is investigated. In this picture, the universe is modeled out by
means of a medium without cosmological constant. In particular, the
Anton-Schmidt representation of matter describes the pressure of crystalline
solids under deformations imposed by isotropic stresses. The approach scheme is
related to the fact that the universe deforms under the action of the cosmic
expansion itself. Thus, we frame the dark energy term as a function of scalar
fields and obtain the corresponding dark energy potential .
Different epochs of the universe evolution are investigated in terms of the
evolution of . We show how the Anton-Schmidt equation of state is
capable of describing both late and early epochs of cosmic evolution. Finally,
numerical bounds on the Anton-Schmidt model with are derived through a
Markov Chain Monte Carlo analysis on the combination of data coming from type
Ia Supernovae, observations of Hubble parameter and baryon acoustic
oscillations. Statistical comparison with the CDM model is performed
by the AIC and BIC selection criteria. Results are in excellent agreement with
the low-redshift data. A further generalization of the model is presented to
satisfy the theoretical predictions at early-stage cosmology.Comment: 13 pages, Accepted for publication in Phys. Rev.
De Sitter-like configurations with asymptotic quintessence environment
We examine a spherically-symmetric class of spacetimes carrying vacuum
energy, while considering the influence of an external dark energy environment
represented by a non-dynamical quintessence field. Our investigation focuses on
a specific set of solutions affected by this field, leading to distinct kinds
of spacetime deformations, resulting in regular, singular, and wormhole
solutions. We thoroughly discuss the underlying physics associated with each
case and demonstrate that more complex deformations are prone to instability.
Ultimately, we find that our results lead to an \emph{isotropic de Sitter-like
solution} that behaves as a quintessence fluid. To achieve this, we investigate
the nature of the corresponding fluid, showing that it cannot provide the sound
speed equal to a constant equation of state near the center. Consequently, we
reinterpret the fluid as a slow-roll quintessence by investigating its behavior
in asymptotic regimes. Further, we explore the potential implications of
violating the isotropy condition on the pressures and we finally compare our
findings with the de Sitter and Hayward solutions, highlighting both the
advantages and disadvantages of our scenarios.Comment: 14 pages, 4 figures, 1 tabl
Global variational methods on smooth nonholonomic constraints
AbstractWe develop a variational theory for critical points of integral functionals in a space of curves on a manifold M, between a fixed point and a one-dimensional submanifold of M, and satisfying a nonholonomic constraint equation φ=0, where φ is a C2 function defined on TM×R.We obtain existence, regularity and multiplicity results, writing the integro-differential equations satisfied by critical points. Moreover, we present some results concerning a sort of exponential map relative to the integro-differential equations and some examples
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