1,179 research outputs found
MPC Joins The Dark Side
status: publishe
Polynomial Palatini cosmology -- dynamical system approach
We investigate cosmological dynamics based on gravity in the Palatini
formulation. In this study we use the dynamical system methods. We show that
the evolution of the Friedmann equation reduces to the form of the piece-wise
smooth dynamical system. This system is is reduced to a 2D dynamical system of
the Newtonian type. We demonstrate how the trajectories can be sewn to
guarantee extendibility of the metric similarly as `Milne-like' FLRW
spacetimes are -extendible. We point out that importance of dynamical
system of Newtonian type with non-smooth right-hand sides in the context of
Palatini cosmology. In this framework we can investigate singularities which
appear in the past and future of the cosmic evolution. We consider cosmological
systems in both Einstein and Jordan frames. We show that at each frame the
topological structures of phase space are different.Comment: RevTeX 4-1, 30 pages, 19 figure
Solutions to the tethered galaxy problem in an expanding universe and the observation of receding blueshifted objects
We use the dynamics of a galaxy, set up initially at a constant proper
distance from an observer, to derive and illustrate two counter-intuitive
general relativistic results. Although the galaxy does gradually join the
expansion of the universe (Hubble flow), it does not necessarily recede from
us. In particular, in the currently favored cosmological model, which includes
a cosmological constant, the galaxy recedes from the observer as it joins the
Hubble flow, but in the previously favored cold dark matter model, the galaxy
approaches, passes through the observer, and joins the Hubble flow on the
opposite side of the sky. We show that this behavior is consistent with the
general relativistic idea that space is expanding and is determined by the
acceleration of the expansion of the universe -- not a force or drag associated
with the expansion itself. We also show that objects at a constant proper
distance will have a nonzero redshift; receding galaxies can be blueshifted and
approaching galaxies can be redshifted.Comment: 8 pages including 6 figures, to appear in Am. J. Phys., 2003.
Reference added in postscrip
Cosmic voids in coupled dark energy cosmologies: the impact of halo bias
In this work we analyse the properties of cosmic voids in standard and
coupled dark energy cosmologies. Using large numerical simulations, we
investigate the effects produced by the dark energy coupling on three
statistics: the filling factor, the size distribution and the stacked profiles
of cosmic voids. We find that the bias of the tracers of the density field used
to identify the voids strongly influences the properties of the void
catalogues, and, consequently, the possibility of using the identified voids as
a probe to distinguish coupled dark energy models from the standard CDM cosmology. In fact, on one hand coupled dark energy models are
characterised by an excess of large voids in the cold dark matter distribution
as compared to the reference standard cosmology, due to their higher
normalisation of linear perturbations at low redshifts. Specifically, these
models present an excess of large voids with Mpc h^{-1},
at , respectively. On the other hand, we do not find any
significant difference in the properties of the void detected in the
distribution of collapsed dark matter halos. These results imply that the
tracer bias has a significant impact on the possibility of using cosmic void
catalogues to probe cosmology.Comment: 11 pages, 8 figures and 2 tables. Submitted to MNRA
Cosmic Acceleration Driven by Mirage Inhomogeneities
A cosmological model based on an inhomogeneous D3-brane moving in an AdS_5 X
S_5 bulk is introduced. Although there is no special points in the bulk, the
brane Universe has a center and is isotropic around it. The model has an
accelerating expansion and its effective cosmological constant is inversely
proportional to the distance from the center, giving a possible geometrical
origin for the smallness of a present-day cosmological constant. Besides, if
our model is considered as an alternative of early time acceleration, it is
shown that the early stage accelerating phase ends in a dust dominated FRW
homogeneous Universe. Mirage-driven acceleration thus provides a dark matter
component for the brane Universe final state. We finally show that the model
fulfills the current constraints on inhomogeneities.Comment: 14 pages, 1 figure, IOP style. v2, changed style, minor corrections,
references added, version accepted in Class. Quant. Gra
Cosmic reionization in a dynamic quintessence cosmology
In this paper we investigate the effects that a dynamic dark energy component
dominant in the universe at late epochs has on reionization. We follow the
evolution of HII regions with the analytic approach of Furlanetto and Oh (2005)
in two different universes for which we assume the Peebles and Ratra (2003) and
Brax and Martin (2000) quintessence models and we compare our results to the
LCDM scenario. We show that, for a fixed ionization efficiency, at the same
cosmological epoch the topology of bubbles is dominated by high-mass objects
and the characteristic size of the ionized regions is slightly smaller than in
the LCDM model, especially at the latest stages of reionization, due to the
higher recombination efficiency. As a consequence, the bubbles' `epoch of
overlap' happens earlier than in LCDM. Finally, we show how the different
evolution of the HII regions affects the transmission of the high-z QSO
spectra, reducing the Lyman flux absorption at small optical depths.Comment: 10 pages, minor changes to match the version accepted for publication
by MNRA
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