5 research outputs found
ISW effect in Unified Dark Matter Scalar Field Cosmologies: an analytical approach
We perform an analytical study of the Integrated Sachs-Wolfe (ISW) effect
within the framework of Unified Dark Matter models based on a scalar field
which aim at a unified description of dark energy and dark matter. Computing
the temperature power spectrum of the Cosmic Microwave Background anisotropies
we are able to isolate those contributions that can potentially lead to strong
deviations from the usual ISW effect occurring in a CDM universe. This
helps to highlight the crucial role played by the sound speed in the Unified
Dark Matter models. Our treatment is completely general in that all the results
depend only on the speed of sound of the dark component and thus it can be
applied to a variety of unified models, including those which are not described
by a scalar field but relies on a single dark fluid.Comment: 15 pages, LateX file; one comment after Eq.(36) and formula (44)
added in order to underline procedure and main results. Accepted for
publication in JCAP; some typos correcte
Halos of Unified Dark Matter Scalar Field
We investigate the static and spherically symmetric solutions of Einstein's
equations for a scalar field with non-canonical kinetic term, assumed to
provide both the dark matter and dark energy components of the Universe. In
particular, we give a prescription to obtain solutions (dark halos) whose
rotation curve v_c(r) is in good agreement with observational data. We show
that there exist suitable scalar field Lagrangians that allow to describe the
cosmological background evolution and the static solutions with a single dark
fluid.Comment: 19 pages LaTeX file; minor corrections made affecting Eqs.(52)-(56
Large-scale instability in interacting dark energy and dark matter fluids
If dark energy interacts with dark matter, this gives a new approach to the
coincidence problem. But interacting dark energy models can suffer from
pathologies. We consider the case where the dark energy is modelled as a fluid
with constant equation of state parameter w. Non-interacting constant-w models
are well behaved in the background and in the perturbed universe. But the
combination of constant w and a simple interaction with dark matter leads to an
instability in the dark sector perturbations at early times: the curvature
perturbation blows up on super-Hubble scales. Our results underline how
important it is to carefully analyze the relativistic perturbations when
considering models of coupled dark energy. The instability that we find has
been missed in some previous work where the perturbations were not consistently
treated. The unstable mode dominates even if adiabatic initial conditions are
used. The instability also arises regardless of how weak the coupling is. This
non-adiabatic instability is different from previously discovered adiabatic
instabilities on small scales in the strong-coupling regime.Comment: 15 pages, 5 figures. New reference; published versio
Large-scale instability in interacting dark energy and dark matter fluids
If dark energy interacts with dark matter, this gives a new approach to the
coincidence problem. But interacting dark energy models can suffer from
pathologies. We consider the case where the dark energy is modelled as a fluid
with constant equation of state parameter w. Non-interacting constant-w models
are well behaved in the background and in the perturbed universe. But the
combination of constant w and a simple interaction with dark matter leads to an
instability in the dark sector perturbations at early times: the curvature
perturbation blows up on super-Hubble scales. Our results underline how
important it is to carefully analyze the relativistic perturbations when
considering models of coupled dark energy. The instability that we find has
been missed in some previous work where the perturbations were not consistently
treated. The unstable mode dominates even if adiabatic initial conditions are
used. The instability also arises regardless of how weak the coupling is. This
non-adiabatic instability is different from previously discovered adiabatic
instabilities on small scales in the strong-coupling regime.Comment: 15 pages, 5 figures. New reference; published versio