5,416 research outputs found
Astrophysical Configurations with Background Cosmology: Probing Dark Energy at Astrophysical Scales
We explore the effects of a positive cosmological constant on astrophysical
and cosmological configurations described by a polytropic equation of state. We
derive the conditions for equilibrium and stability of such configurations and
consider some astrophysical examples where our analysis may be relevant. We
show that in the presence of the cosmological constant the isothermal sphere is
not a viable astrophysical model since the density in this model does not go
asymptotically to zero. The cosmological constant implies that, for polytropic
index smaller than five, the central density has to exceed a certain minimal
value in terms of the vacuum density in order to guarantee the existence of a
finite size object. We examine such configurations together with effects of
in other exotic possibilities, such as neutrino and boson stars, and
we compare our results to N-body simulations. The astrophysical properties and
configurations found in this article are specific features resulting from the
existence of a dark energy component. Hence, if found in nature would be an
independent probe of a cosmological constant, complementary to other
observations.Comment: 23 pages, 11 figures, 2 tables. Reference added. Mon. Not. Roy.
Astro. Soc in prin
Symmetron with a non-minimal kinetic term
We investigate the compatibility of the Symmetron with dark energy by
introducing a non-minimal kinetic term associated with the Symmetron. In this
new model, the effect of the friction term appearing in the equation of motion
of the Symmetron field becomes more pronounced due to the non-minimal kinetic
term appearing in the action and, under specific conditions after symmetry
breaking, the universe experiences an accelerating phase which, in spite of the
large effective mass of the scalar field, lasts as long as the Hubble time
.Comment: 12 pages, 4 figures, to appear in JCA
Cosmic voids in modified gravity scenarios
Modified gravity (MG) theories aim to reproduce the observed acceleration of
the Universe by reducing the dark sector while simultaneously recovering
General Relativity (GR) within dense environments. Void studies appear to be a
suitable scenario to search for imprints of alternative gravity models on
cosmological scales. Voids cover an interesting range of density scales where
screening mechanisms fade out, which reaches from a density contrast close to their centers to close to their
boundaries. We present an analysis of the level of distinction between GR and
two modified gravity theories, the Hu-Sawicki and the symmetron theory.
This study relies on the abundance, linear bias, and density profile of voids
detected in n-body cosmological simulations. We define voids as connected
regions made up of the union of spheres with a {\it \textup{mean}} density
given by , but disconnected from any
other voids. We find that the height of void walls is considerably affected by
the gravitational theory, such that it increases for stronger gravity
modifications. Finally, we show that at the level of dark matter n-body
simulations, our constraints allow us to distinguish between GR and MG models
with and . Differences of best-fit values for
MG parameters that are derived independently from multiple void probes may
indicate an incorrect MG model. This serves as an important consistency check.Comment: 15 pages, 12 figure
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