* Astronomy

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RE: Dark Energy

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Clocking galaxy clusters to gauge dark energy

The universe's oldest light and its largest objects could provide a new way to study dark energy, the mysterious entity believed to be pushing the universe apart at an ever-faster rate.
The discovery of the universe's accelerating expansion earned three physicists a Nobel prize last year, but no one knows its source.
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Title: Investigate the interaction between dark matter and dark energy
Authors: Jianbo Lu, Yabo Wu, Yongyi Jin, Yan Wang

In this paper we investigate the interaction between dark matter and dark energy by considering two different interacting scenarios, i.e. the cases of constant interaction function and variable interaction function. By fitting the current observational data to constrain the interacting models, it is found that the interacting strength is non-vanishing, but weak for the case of constant interaction function, and the interaction is not obvious for the case of variable interaction function. In addition, for seeing the influence from interaction we also investigate the evolutions of interaction function, effective state parameter for dark energy and energy density of dark matter. At last some geometrical quantities in the interacting scenarios are discussed.

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Title: Dark Energy and Fate of the Universe
Authors: Xiao-Dong Li, Shuang Wang, Qing-Guo Huang, Xin Zhang, Miao Li

We explore the ultimate fate of the Universe by using a divergence-free parameterisation for dark energy w(z)=w_0+w_a({\ln (2+z)\over 1+z}-\ln2). Unlike the CPL parameterisation, this parameterisation has well behaved, bounded behaviour for both high redshifts and negative redshifts, and thus can genuinely cover many theoretical dark energy models. After constraining the parameter space of this parameterisation by using the current cosmological observations, we find that, at the 95.4% confidence level, our Universe can still exist at least 16.7 Gyr before it ends in a big rip. Moreover, for the phantom energy dominated Universe, we find that a gravitationally bound system will be destroyed at a time t \simeq P\sqrt{2|1+3w(-1)|}/[6\pi |1+w(-1)|], where P is the period of a circular orbit around this system, before the big rip.

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Supernovae, Dark Energy, and the Accelerating Universe

Dr. Saul Perlmutter presents his findings and methods for how he helped the Energy Department to receive another Nobel Prize.

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Title: "Dark energy" in the Local Void
Authors: M. Villata

The unexpected discovery of the accelerated cosmic expansion in 1998 has filled the Universe with the embarrassing presence of an unidentified "dark energy", or cosmological constant, devoid of any physical meaning. While this standard cosmology seems to work well at the global level, improved knowledge of the kinematics and other properties of our extragalactic neighborhood indicates the need for a better theory. We investigate whether the recently suggested repulsive-gravity scenario can account for some of the features that are unexplained by the standard model. Through simple dynamical considerations, we find that the Local Void could host an amount of antimatter (~5 x 10^{15} solar masses) roughly equivalent to the mass of a typical supercluster, thus restoring the matter-antimatter symmetry. The antigravity field produced by this "dark repulsor" can explain the anomalous motion of the Local Sheet away from the Local Void, as well as several other properties of nearby galaxies that seem to require void evacuation and structure formation much faster than expected from the standard model. At the global cosmological level, gravitational repulsion from antimatter hidden in voids can provide more than enough potential energy to drive both the cosmic expansion and its acceleration, with no need for an initial "explosion" and dark energy. Moreover, the discrete distribution of these dark repulsors, in contrast to the uniformly permeating dark energy, can also explain dark flows and other recently observed excessive inhomogeneities and anisotropies of the Universe.

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Clearest Picture Yet of Dark Matter Points the Way to Better Understanding of Dark Energy

Two teams of physicists at the U.S. Department of Energy's Fermilab and Lawrence Berkeley National Laboratory (Berkeley Lab) have independently made the largest direct measurements of the invisible scaffolding of the universe, building maps of dark matter using new methods that, in turn, will remove key hurdles for understanding dark energy with ground-based telescopes.
The teams' measurements look for tiny distortions in the images of distant galaxies, called "cosmic shear," caused by the gravitational influence of massive, invisible dark matter structures in the foreground. Accurately mapping out these dark-matter structures and their evolution over time is likely to be the most sensitive of the few tools available to physicists in their ongoing effort to understand the mysterious space-stretching effects of dark energy.
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Variable dark energy could explain old galaxy clusters

Does dark energy change over time? An alternative model of the as yet undetected entity that is thought to be accelerating the universe's expansion could explain some puzzling observations of galaxy clusters. But it will have to jump many more hurdles to compete with the simplest and so far most successful model of the elusive entity.
That model, called the cosmological constant, holds that there is a certain amount of repulsive energy in every cubic centimetre of space, and that amount stays the same over time. As the universe expands, more space exists, and so the expansion accelerates.
Now Edoardo Carlesi of the Autonomous University in Madrid, Spain, and his colleagues have simulated a universe where the amount of repulsive energy per unit of volume changes with time.
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Title: One explanation for the acceleration of the universe's expansion
Authors: Dong-Biao Kang

We have observed the acceleration of the expansion of the universe. To explain this phenomenon, we usually introduce the dark energy (DE) which has a negative pressure or we need to modify the Einstein's equation to produce a term which is equivalent to the dark energy. Are there other possibilities? Combining our previous works of statistical mechanics of self-gravitating system with the derivation of van der waals equation, we propose a different matter's equation of state (EoS) in this paper. Then we find that if the matter's density is low enough, its pressure can be negative, which means that it is the matter that drives the expansion's acceleration. So here we will not need to add the DE to the universe. Our results also predict that the universe finally tends to be dominated by an approximate constant energy density, but its value can be smaller than DE. The data of Supernova can not differentiate our model from the standard model, but they may indicate some deviations from LCDM.

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Glow around black holes could light up dark energy

There's a new way to measure the accelerating expansion of the primordial universe - and it may just reveal what dark energy is
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