882 research outputs found
Dark energy in motion
Recent large-scale peculiar velocity surveys suggest that large matter
volumes could be moving with appreciable velocity with respect to the CMB rest
frame. If confirmed, such results could conflict with the Cosmological
Principle according to which the matter and CMB rest frames should converge on
very large scales. In this work we explore the possibility that such large
scale bulk flows are due, not to the motion of matter with respect to the CMB,
but to the flow of dark energy with respect to matter. Indeed, when dark energy
is moving, the usual definition of the CMB rest frame as that in which the CMB
dipole vanishes is not appropriate. We find instead that the dipole vanishes
for observers at rest with respect to the cosmic center of mass, i.e. in motion
with respect to the background radiation.Comment: 9 pages, 1 figure. Essay selected for "Honorable Mention" in the 2006
Awards for Essays on Gravitation (Gravity Research Foundation
Black holes in modified gravity theories
In the context of gravity theories, the issue of finding static and
spherically symmetric black hole solutions is addressed. Two approaches to
study the existence of such solutions are considered: first, constant curvature
solutions, and second, the general case (without imposing constant curvature)
is also studied. Performing a perturbative expansion around the
Einstein-Hilbert action, it is found that only solutions of the
Schwarzschild-(Anti-) de Sitter type are present (up to second order in
perturbations) and the explicit expressions for these solutions are provided in
terms of the function. Finally we consider the thermodynamics of black
holes in Anti-de Sitter space-time and study their local and global stability.Comment: 4 pages, 2 figures. Contribution to the proceedings of Spanish
Relativity Meeting 2009, Bilbao, Spain, 7-11 September 200
Cosmology with moving dark energy and the CMB quadrupole
We study the consequences of a homogeneous dark energy fluid having a
non-vanishing velocity with respect to the matter and radiation large-scale
rest frames. We consider homogeneous anisotropic cosmological models with four
fluids (baryons, radiation, dark matter and dark energy) whose velocities can
differ from each other. Performing a perturbative calculation up to second
order in the velocities, we obtain the contribution of the anisotropies
generated by the fluids motion to the CMB quadrupole and compare with
observations. We also consider the exact problem for arbitrary velocities and
solve the corresponding equations numerically for different dark energy models.
We find that models whose equation of state is initially stiffer than
radiation, as for instance some tracking models, are unstable against velocity
perturbations, thus spoiling the late-time predictions for the energy
densities. In the case of scaling models, the contributions to the quadrupole
can be non-negligible for a wide range of initial conditions. We also consider
fluids moving at the speed of light (null fluids) with positive energy and show
that, without assuming any particular equation of state, they generically act
as a cosmological constant at late times. We find the parameter region for
which the models considered could be compatible with the measured (low)
quadrupole.Comment: 23 pages, 6 figures. Confidence intervals calculated from WMAP data,
new references and comments included. Final version to appear in PR
On the dark energy rest frame and the CMB
Dark energy is usually parametrized as a perfect fluid with negative pressure
and a certain equation of state. Besides, it is supposed to interact very
weakly with the rest of the components of the universe and, as a consequence,
there is no reason to expect it to have the same large-scale rest frame as
matter and radiation. Thus, apart from its equation of state and its energy
density one should also consider its velocity as a free parameter
to be determined by observations. This velocity defines a cosmological
preferred frame, so the universe becomes anisotropic and, therefore, the CMB
temperature fluctuations will be affected, modifying mainly the dipole and the
quadrupole.Comment: 4 pages. Contribution to the proceedings of Spanish Relativity
Meeting 2008, Salamanca, Spain, 15-19 September 200
Equivalence between Kaluza Klein modes of gravitinos and goldstinos in brane induced supersymmetry breaking
We identify the goldstino fields that give mass to the Kaluza Klein modes of
five dimensional supergravity, when supersymmetry breaking is induced by brane
effects. We then proof the four dimensional Equivalence Theorem that, in
renormalizable gauges, allows for the replacement of Kaluza Klein modes of
helicity gravitinos in terms of goldstinos. Finally we identify the
five dimensional renormalizable gauge fixing that leads to the Equivalence
Theorem.Comment: Final version published in JHEP. Typo corrected in eq. 2.
Cosmological density perturbations in modified gravity theories
In the context of f(R) theories of gravity, we study the cosmological
evolution of scalar perturbations by using a completely general procedure. We
find that the exact fourth-order differential equation for the matter density
perturbations in the longitudinal gauge, reduces to a second-order equation for
sub-Hubble modes. This simplification is compared with the standard
(quasi-static) equation used in the literature. We show that for general f(R)
functions the quasi-static approximation is not justified. However for those
f(R) adequately describing the present phase of accelerated expansion and
satisfying local gravity tests, it does give a correct description for the
evolution of perturbations.Comment: 4 pages, 2 figures. Contribution to the proceedings of Spanish
Relativity Meeting 2008, Salamanca, Spain, 15-19 September 200
Viability of vector-tensor theories of gravity
We present a detailed study of the viability of general vector-tensor
theories of gravity in the presence of an arbitrary temporal background vector
field. We find that there are six different classes of theories which are
indistinguishable from General Relativity by means of local gravity
experiments. We study the propagation speeds of scalar, vector and tensor
perturbations and obtain the conditions for classical stability of those
models. We compute the energy density of the different modes and find the
conditions for the absence of ghosts in the quantum theory. We conclude that
the only theories which can pass all the viability conditions for arbitrary
values of the background vector field are not only those of the pure Maxwell
type, but also Maxwell theories supplemented with a (Lorentz type) gauge fixing
term.Comment: 13 pages, 2 figures, 1 table. Final version to appear in JCA
Trastornos de la conducta en el niño y repercusión en la atención odontopediátrica. El Trastorno por Déficit de Atención con Hiperactividad (TDAH)
El trastorno por déficit de atención e hiperactividad es un trastorno del desarrollo que presenta un patrón persistente de desatención, hiperactividad e impulsividad (TDAH). Es una enfermedad crónica, frecuente en la infancia y que vamos a encontrar en pacientes rutinarios. es imprescindible realizar una completa historia clínica para conocer a lo que nos enfetamos y saber manejar al paciente afecto de esta patología. Son bien conocidas las numerosas manifestaicones bucodentales que sufren los pacientes con TDAH, poe ello es necesrio incluirlos dentro de un programa preventivo individualizado de alto riesgo para evitar que se agraven dichas patologías
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