64 research outputs found
Viscous Cold Dark Matter in agreement with observations
We discuss bulk viscous cosmological models. Since the bulk viscous pressure
is negative, viable viscous cosmological scenarios with late time accelerated
expansion can in principle be constructed. After discussing some alternative
models based on bulk viscous effects we will focus on a model very similar to
the standard CDM. We argue that a {\rm v}CDM model, where we
assign a very small (albeit perceptible) bulk viscosity to dark matter is in
agreement with available cosmological observations. Hence, we work with the
concept of viscous Cold Dark Matter ({\rm v}CDM). At the level of the
perturbations, the growth of {\rm v}CDM structures is slightly suppressed when
compared with the standard CDM ones. Having in mind that the small scale
problems of the CDM model are related to an excess of clustering, our
proposal seems to indicate a possible direction for solving the serious
drawbacks of the CDM paradigm within the standard cosmological model.Comment: 16 pages, 2 figures, to the proceedings of "49th Winter School of
Theoretical Physics Cosmology and non-equilibrium statistical mechanics",
L{\ka}dek-Zdr\'oj, Poland, February 10-16, 201
Cosmology with matter diffusion
We construct a viable cosmological model based on velocity diffusion of
matter particles. In order to ensure the conservation of the total
energy-momentum tensor in the presence of diffusion, we include a cosmological
scalar field which we identify with the dark energy component of the
Universe. The model is characterized by only one new degree of freedom, the
diffusion parameter . The standard CDM model can be recovered
by setting . If diffusion takes place () the dynamics of
the matter and of the dark energy fields are coupled. We argue that the
existence of a diffusion mechanism in the Universe can serve as a theoretical
motivation for interacting models. We constrain the background dynamics of the
diffusion model with Supernovae, H(z) and BAO data. We also perform a
perturbative analysis of this model in order to understand structure formation
in the Universe. We calculate the impact of diffusion both on the CMB spectrum,
with particular attention to the integrated Sachs-Wolfe signal, and on the
matter power spectrum . The latter analysis places strong constraints on
the magnitude of the diffusion mechanism but does not rule out the model.Comment: 20 pages, 8 figures, accepted for publication in JCA
Non-linear clustering during the BEC dark matter phase transition
Spherical collapse of the Bose-Einstein Condensate (BEC) dark matter model is
studied in the Thomas Fermi approximation. The evolution of the overdensity of
the collapsed region and its expansion rate are calculated for two scenarios.
We consider the case of a sharp phase transition (which happens when the
critical temperature is reached) from the normal dark matter state to the
condensate one and the case of a smooth first order phase transition where
there is a continuous conversion of "normal" dark matter to the BEC phase. We
present numerical results for the physics of the collapse for a wide range of
the model's space parameter, i.e. the mass of the scalar particle
and the scattering length . We show the dependence of the transition
redshift on and . Since small scales collapse earlier and
eventually before the BEC phase transition the evolution of collapsing halos in
this limit is indeed the same in both the CDM and the BEC models. Differences
are expected to appear only on the largest astrophysical scales. However, we
argue that the BEC model is almost indistinguishable from the usual dark matter
scenario concerning the evolution of nonlinear perturbations above typical
clusters scales, i.e., . This provides an analytical
confirmation for recent results from cosmological numerical simulations [H.-Y.
Schive {\it et al.}, Nature Physics, {\bf10}, 496 (2014)].Comment: 11 pages. Final version to appear in EPJ
Limits on the anomalous speed of gravitational waves from binary pulsars
A large class of modified theories of gravity used as models for dark energy
predict a propagation speed for gravitational waves which can differ from the
speed of light. This difference of propagations speeds for photons and
gravitons has an impact in the emission of gravitational waves by binary
systems. Thus, we revisit the usual quadrupolar emission of binary system for
an arbitrary propagation speed of gravitational waves and obtain the
corresponding period decay formula. We then use timing data from the
Hulse-Taylor binary pulsar and obtain that the speed of gravitational waves can
only differ from the speed of light at the percentage level. This bound places
tight constraints on dark energy models featuring an anomalous propagations
speed for the gravitational waves.Comment: 6 pages, 1 figure, Prepared for the IWARA2016 proceeding
Piercing the Vainshtein screen with anomalous gravitational wave speed: Constraints on modified gravity from binary pulsars
By using observations of the Hulse-Taylor pulsar we constrain the
gravitational wave (GW) speed to the level of . We apply this result
to scalar-tensor theories that generalize Galileon 4 and 5 models, which
display anomalous propagation speed and coupling to matter for GWs. We argue
that this effect survives conventional screening due to the persistence of a
scalar field gradient inside virialized overdensities, which effectively
"pierces" the Vainshtein screening. In specific branches of solutions, our
result allows to directly constrain the cosmological couplings in the effective
field theory of dark energy formalism.Comment: v3: typos corrected. Final version to appear on PR
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