## Posts Tagged ‘**DARK ENERGY**’

## Largest cosmic structures ‘too big’ for theories

Space is festooned with vast “hyperclusters” of galaxies, a new cosmic map suggests. It could mean that gravity or dark energy – or perhaps something completely unknown – is behaving very strangely indeed.

We know that the universe was smooth just after its birth. Measurements of the cosmic microwave background radiation (CMB), the light emitted 370,000 light years after the big bang, reveal only very slight variations in density from place to place. Gravity then took hold and amplified these variations into today’s galaxies and galaxy clusters, which in turn are arranged into big strings and knots called superclusters, with relatively empty voids in between.

On even larger scales, though, cosmological models say that the expansion of the universe should trump the clumping effect of gravity. That means there should be very little structure on scales larger than a few hundred million light years across….. Read the rest of this entry »

## Dark matter without dark energy

**Newtonian Perturbations on Models with Matter Creation**

Creation of Cold Dark Matter (CCDM) can macroscopically be described by a negative pressure, and, therefore, the mechanism is capable to accelerate the Universe, without the need of an additional dark energy component. In this framework we discuss the evolution of perturbations by considering a Neo-Newtonian approach where, unlike in the standard Newtonian cosmology, the ﬂuid pressure is taken into account even in the homogeneous and isotropic background equations (Lima, Zanchin and Brandenberger, MNRAS 291, L1, 1997). The evolution of the density contrast is calculated in the linear approximation and compared to the one predicted by the ΛCDM model. The diﬀerence between the CCDM and ΛCDM predictions at the perturbative level is quantiﬁed by using three diﬀerent statistical methods, namely: a simple χ^2 -analysis in the relevant space parameter, a Bayesian statistical inference, and, ﬁnally, a Kolmogorov-Smirnov test. We ﬁnd that under

certain circumstances the CCDM scenario analysed here predicts an overall dynamics (including Hubble ﬂow and matter ﬂuctuation ﬁeld) which fully recovers that of the traditional cosmic concordance model. Our basic conclusion is that such a reduction of the dark sector provides a viable alternative description to the accelerating ΛCDM cosmology…. Read more: http://arxiv.org/PS_cache/arxiv/pdf/1105/1105.1027v1.pdf