38 research outputs found
On virialization with dark energy
We review the inclusion of dark energy into the formalism of spherical
collapse, and the virialization of a two-component system, made of matter and
dark energy. We compare two approaches in previous studies. The first assumes
that only the matter component virializes, e.g. as in the case of a classic
cosmological constant. The second approach allows the full system to virialize
as a whole. We show that the two approaches give fundamentally different
results for the final state of the system. This might be a signature
discriminating between the classic cosmological constant which cannot virialize
and a dynamical dark energy mimicking a cosmological constant. This signature
is independent of the measured value of the equation of state. An additional
issue which we address is energy non-conservation of the system, which
originates from the homogeneity assumption for the dark energy. We propose a
way to take this energy loss into account.Comment: 15 pages, 5 figures. Accepted for publication in JCA
Ellipsoidal configurations in the de Sitter spacetime
The cosmological constant modifies certain properties of large
astrophysical rotating configurations with ellipsoidal geometries, provided the
objects are not too compact. Assuming an equilibrium configuration and so using
the tensor virial equation with we explore several equilibrium
properties of homogeneous rotating ellipsoids. One shows that the bifurcation
point, which in the oblate case distinguishes the Maclaurin ellipsoid from the
Jacobi ellipsoid, is sensitive to the cosmological constant. Adding to that,
the cosmological constant allows triaxial configurations of equilibrium
rotating the minor axis as solutions of the virial equations. The significance
of the result lies in the fact that minor axis rotation is indeed found in
nature. Being impossible for the oblate case, it is permissible for prolate
geometries, with zero and positive. For the triaxial case, however,
an equilibrium solution is found only for non-zero positive . Finally,
we solve the tensor virial equation for the angular velocity and display
special effects of the cosmological constant there.Comment: 15 pages, 11 figures, published in Class. Quant. Grav. References
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Static Configurations of Dark Energy and Dark Matter
We study static configurations of dark matter coupled to a scalar field
responsible for the dark energy of the Universe. The dark matter is modelled as
a Fermi gas within the Thomas-Fermi approximation. The mass of the dark matter
particles is a function of the scalar field. We analyze the profile of the dark
matter halos in galaxies. In this case our framework is equivalent to the model
of the isothermal sphere. In the presence of a scalar field, the velocity of a
massive object orbiting the galaxy is not of the order of the typical velocity
of the dark matter particles, as in the conventional picture. Instead, it is
reduced by a factor that quantifies the dependence of the dark matter mass on
the scalar field. This has implications for dark matter searches. We derive new
solutions of the Einstein equations which describe compact objects composed of
dark matter. Depending on the scale of the dark matter mass, the size of these
objects can vary between microscopic scales and cosmological distances. We
determine the mass to radius relation and discuss the similarities with
conventional neutron stars and exotic astrophysical objects.Comment: 23 pages, 3 figures, minor additions to the tex
Cluster number counts dependence on dark energy inhomogeneities and coupling to dark matter
Cluster number counts can be used to test dark energy models. We investigate
dark energy candidates which are coupled to dark matter. We analyze the cluster
number counts dependence on the amount of dark matter coupled to dark energy.
Further more, we study how dark energy inhomogeneities affect cluster
abundances. It is shown that increasing the coupling reduces significantly the
cluster number counts, and that dark energy inhomogeneities increases cluster
abundances. Wiggles in cluster number counts are shown to be a specific
signature of coupled dark energy models. Future observations will possibly
detect such oscillations and discriminate among the different dark energy
models.Comment: 9 pages, 8 figures. Further extensions on section on discriminating
models with future surveys. Accepted for publication in Mon. Not. Roy. Astro.
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Effects of the interaction between dark energy and dark matter on cosmological parameters
We examine the effects of possible phenomenological interactions between dark
energy and dark matter on cosmological parameters and their efficiency in
solving the coincidence problem. We work with two simple parameterizations of
the dynamical dark energy equation of state and the constant dark energy
equation of state. Using observational data coming from the new 182 Gold type
Ia supernova samples, the shift parameter of the Cosmic Microwave Background
given by the three-year Wilkinson Microwave Anisotropy Probe observations, and
the baryon acoustic oscillation measurement from the Sloan Digital Sky Survey,
we perform a statistical joint analysis of different forms of phenomenological
interactions between dark energy and dark matter.Comment: revised version, accepted for publication in JCA