4,150 research outputs found
Galaxy clusters and microwave background anisotropy
Previous estimates of the microwave background anisotropies produced by
freely falling spherical clusters are discussed. These estimates are based on
the Swiss-Cheese and Tolman-Bondi models. It is proved that these models give
only upper limits to the anisotropies produced by the observed galaxy clusters.
By using spherically symmetric codes including pressureless matter and a hot
baryonic gas, new upper limits are obtained. The contributions of the hot gas
and the pressureless component to the total anisotropy are compared. The
effects produced by the pressure are proved to be negligible; hence,
estimations of the cluster anisotropies based on N-body simulations are
hereafter justified. After the phenomenon of violent relaxation, any realistic
rich cluster can only produce small anisotropies with amplitudes of order
. During the rapid process of violent relaxation, the anisotropies
produced by nonlinear clusters are expected to range in the interval
. The angular scales of these anisotropies are discussed.Comment: 31 pages, 3 postscript figures, accepted MNRA
Learning from observations of the microwave background at small angular scales
In this paper, we focus our attention on the following question: How well can
we recover the power spectrum of the cosmic microwave background from the maps
of a given experiment?. Each experiment is described by a a pixelization scale,
a beam size, a noise level and a sky coverage. We use accurate numerical
simulations of the microwave sky and a cold dark matter model for structure
formation in the universe. Angular scales smaller than those of previous
simulations are included. The spectrum obtained from the simulated maps is
appropriately compared with the theoretical one. Relative deviations between
these spectra are estimated. Various contributions to these deviations are
analyzed. The method used for spectra comparisons is discussed.Comment: 15 pages (LATEX), 2 postcript figures, accepted in Ap
Cyclic, ekpyrotic and little rip universe in modified gravity
We propose the reconstruction of gravity in such a way that
corresponding theory admits cyclic and ekpyrotic universe solutions. The number
of explicit examples of such model is found. The comparison with the
reconstructed scalar-tensor theory is made. We also present gravity
which provides the little rip evolution and gives the realistic gravitational
alternative for CDM cosmology. The time for little rip dissolution of
bound structures in such theory is estimated. We demonstrate that transformed
little rip solution becomes qualitatively different cosmological
solution with Big Bang type singularity in Einstein frame.Comment: LaTeX 11 pages, no figure, typos correcte
Is exponential gravity a viable description for the whole cosmological history?
Here we analysed a particular type of gravity, the so-called
exponential gravity which includes an exponential function of the Ricci scalar
in the action. Such term represents a correction to the usual Hilbert-Einstein
action. By using Supernovae Ia, Barionic Acoustic Oscillations, Cosmic
Microwave Background and data, the free parameters of the model are well
constrained. The results show that such corrections to General Relativity
become important at cosmological scales and at late-times, providing an
alternative to the dark energy problem. In addition, the fits do not determine
any significant difference statistically with respect to the CDM
model. Finally, such model is extended to include the inflationary epoch in the
same gravitational Lagrangian. As shown in the paper, the additional terms can
reproduce the inflationary epoch and satisfy the constraints from Planck data.Comment: 20 pages, 6 figures, analysis extended, version published in EPJ
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