1,340 research outputs found
On Einstein clusters as galactic dark matter halos
We consider global and gravitational lensing properties of the recently
suggested Einstein clusters of WIMPs as galactic dark matter halos. Being
tangential pressure dominated, Einstein clusters are strongly anisotropic
systems which can describe any galactic rotation curve by specifying the
anisotropy. Due to this property, Einstein clusters may be considered as dark
matter candidates. We analyse the stability of the Einstein clusters against
both radial and non-radial pulsations, and we show that the Einstein clusters
are dynamically stable. With the use of the Buchdahl type inequalities for
anisotropic bodies, we derive upper limits on the velocity of the particles
defining the cluster. These limits are consistent with those obtained from
stability considerations. The study of light deflection shows that the
gravitational lensing effect is slightly smaller for the Einstein clusters, as
compared to the singular isothermal density sphere model for dark matter.
Therefore lensing observations may discriminate, at least in principle, between
Einstein cluster and other dark matter models.Comment: MNRAS LaTeX, 7 pages, accepted by MNRAS; reference adde
Is dark matter an extra-dimensional effect?
We investigate the possibility that the observed behavior of test particles
outside galaxies, which is usually explained by assuming the presence of dark
matter, is the result of the dynamical evolution of particles in higher
dimensional space-times. Hence, dark matter may be a direct consequence of the
presence of an extra force, generated by the presence of extra-dimensions,
which modifies the dynamic law of motion, but does not change the intrinsic
properties of the particles, like, for example, the mass (inertia). We discuss
in some detail several possible particular forms for the extra force, and the
acceleration law of the particles is derived. Therefore, the constancy of the
galactic rotation curves may be considered as an empirical evidence for the
existence of the extra dimensions.Comment: 11 pages, no figures, accepted for publication in MPLA; references
adde
Teleparallel Theories of Gravity: Illuminating a Fully Invariant Approach
Teleparallel gravity and its popular generalization gravity can be
formulated as fully invariant (under both coordinate transformations and local
Lorentz transformations) theories of gravity. Several misconceptions about
teleparallel gravity and its generalizations can be found in the literature,
especially regarding their local Lorentz invariance. We describe how these
misunderstandings may have arisen and attempt to clarify the situation. In
particular, the central point of confusion in the literature appears to be
related to the inertial spin connection in teleparallel gravity models. While
inertial spin connections are commonplace in special relativity, and not
something inherent to teleparallel gravity, the role of the inertial spin
connection in removing the spurious inertial effects within a given frame of
reference is emphasized here. The careful consideration of the inertial spin
connection leads to the construction of a fully invariant theory of
teleparallel gravity and its generalizations. Indeed, it is the nature of the
spin connection that differentiates the relationship between what have been
called good tetrads and bad tetrads and clearly shows that, in principle, any
tetrad can be utilized. The field equations for the fully invariant formulation
of teleparallel gravity and its generalizations are presented and a number of
examples using different assumptions on the frame and spin connection are
displayed to illustrate the covariant procedure. Various modified teleparallel
gravity models are also briefly reviewed.Comment: v2: 72 pages, revised version, references added, matches published
versio
Spherically Symmetric Solutions on a Non-Trivial Frame in f(T) Theories of Gravity
A new solution with constant torsion is derived using the field equations of
f(T). Asymptotic forms of energy density, radial and transversal pressures are
shown to meet the standard energy conditions, i.e., weak and null energy
conditions according to some restrictions on T0, f(T0) and fT(T0). Other
solutions are obtained for vanishing radial pressure and for specific choices
of f(T). The physics relevant to the resulting models is discussed.Comment: 6 pages, 4 figures, published versio
Angular size test on the expansion of the Universe
Assuming the standard cosmological model as correct, the average linear size
of galaxies with the same luminosity is six times smaller at z=3.2 than at z=0,
and their average angular size for a given luminosity is approximately
proportional to 1/z. Neither the hypothesis that galaxies which formed earlier
have much higher densities nor their luminosity evolution, mergers ratio, or
massive outflows due to a quasar feedback mechanism are enough to justify such
a strong size evolution. Also, at high redshift, the intrinsic ultraviolet
surface brightness would be prohibitively high with this evolution, and the
velocity dispersion much higher than observed. We explore here another
possibility to overcome this problem by considering different cosmological
scenarios that might make the observed angular sizes compatible with a weaker
evolution.
One of the models explored, a very simple phenomenological extrapolation of
the linear Hubble law in a Euclidean static universe, fits the angular size vs.
redshift dependence quite well, which is also approximately proportional to 1/z
with this cosmological model. There are no free parameters derived ad hoc,
although the error bars allow a slight size/luminosity evolution. The type Ia
supernovae Hubble diagram can also be explained in terms of this model with no
ad hoc fitted parameter.
WARNING: I do not argue here that the true Universe is static. My intention
is just to discuss which theoretical models provide a better fit to the data of
observational cosmology.Comment: 44 pages, accepted to be published in Int. J. Mod. Phys.
Palatini formulation of modified gravity with a nonminimal curvature-matter coupling
We derive the field equations and the equations of motion for massive test
particles in modified theories of gravity with an arbitrary coupling between
geometry and matter by using the Palatini formalism. We show that the
independent connection can be expressed as the Levi-Civita connection of an
auxiliary, matter Lagrangian dependent metric, which is related with the
physical metric by means of a conformal transformation. Similarly to the metric
case, the field equations impose the non-conservation of the energy-momentum
tensor. We derive the explicit form of the equations of motion for massive test
particles in the case of a perfect fluid, and the expression of the extra-force
is obtained in terms of the matter-geometry coupling functions and of their
derivatives. Generally, the motion is non-geodesic, and the extra force is
orthogonal to the four-velocity.Comment: 7 pages, no figures; v2, revised and corrected version; new Section
adde
Conformal Invariance in Einstein-Cartan-Weyl space
We consider conformally invariant form of the actions in Einstein, Weyl,
Einstein-Cartan and Einstein-Cartan-Weyl space in general dimensions() and
investigate the relations among them. In Weyl space, the observational
consistency condition for the vector field determining non-metricity of the
connection can be obtained from the equation of motion. In Einstein-Cartan
space a similar role is played by the vector part of the torsion tensor. We
consider the case where the trace part of the torsion is the Kalb-Ramond type
of field. In this case, we express conformally invariant action in terms of two
scalar fields of conformal weight -1, which can be cast into some interesting
form. We discuss some applications of the result.Comment: 10 pages, version to appear MPL
Dark Interactions and Cosmological Fine-Tuning
Cosmological models involving an interaction between dark matter and dark
energy have been proposed in order to solve the so-called coincidence problem.
Different forms of coupling have been studied, but there have been claims that
observational data seem to narrow (some of) them down to something annoyingly
close to the CDM model, thus greatly reducing their ability to deal
with the problem in the first place. The smallness problem of the initial
energy density of dark energy has also been a target of cosmological models in
recent years. Making use of a moderately general coupling scheme, this paper
aims to unite these different approaches and shed some light as to whether this
class of models has any true perspective in suppressing the aforementioned
issues that plague our current understanding of the universe, in a quantitative
and unambiguous way.Comment: 13 pages, 9 figures, accepted for publication in JCAP. Minor
corrections, one figure replaced, references adde
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