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TOPIC: Dark Energy
Blobrana


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RE: Dark Energy
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Megamasers to unravel dark energy
Extremely accurate measurements of 'megamasers' - powerful natural amplifiers of radio waves that operate similar to the way lasers amplify light - will extend the cosmic distance scale and help astronomers understand the nature of dark energy.
Masers (an acronym for Microwave Amplification by Stimulated Emission of Radiation) are sources that amplify or generate microwaves. They phenomenon occurs naturally in molecular clouds, stellar or planetary atmospheres and the accretion disk where matter falls into black holes.
The Megamaser Cosmology Project is an ambitious new attempt to measure the distance to masers in far-off galaxies. Astrophysicist James Braatz, from the U.S. National Radio Astronomy Observatory in Charlottesville, Virginia, reported on how the project is progressing at a meeting of the American Astronomical Society in Pasadena, California on Tuesday.

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Blobrana


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Title: Did Dark Energy Suddenly Emerge At Redshift z ~0.331?
Authors: Qing-Guo Huang, Miao Li, Xiao-Dong Li, Shuang Wang
(Version v3)

In this work, we explore the cosmological consequences of the recently released Constitution sample of 397 Type Ia supernovae (SNIa). By revisiting the Chevallier-Polarski-Linder (CPL) parameterisation, we find that, for fitting the Constitution set alone, the behaviour of dark energy (DE) significantly deviate from the cosmological constant \Lambda, where the equation of state (EOS) w and the energy density
ho_\Lambda of DE will rapidly decrease along with the increase of redshift z. Inspired by this clue, we separate the redshifts into different bins, and discuss the models of a constant w or a constant
ho_\Lambda in each bin, respectively. It is found that for fitting the Constitution set alone, w and
ho_\Lambda will also rapidly decrease along with the increase of z, which is consistent with the result of CPL model. Moreover, a step function model in which DE(dark energy) did not exist in the past and suddenly emerged at redshift z ~0.331 presents a significant improvement (\Delta \chiČ=-4.361) over the CPL parameterisation, and performs better than other DE models. We also plot the error bars of DE density of this model, and find that this model deviates from the cosmological constant \Lambda at 68.3% confidence level (CL); this may arise from some biasing systematic errors in the handling of SNIa data, or more interestingly from the nature of DE itself. In addition, for models with same number of redshift bins, a piecewise constant
ho_\Lambda model always performs better than a piecewise constant w model; this shows the advantage of using
ho_\Lambda, instead of w, to probe the variation of DE.

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Blobrana


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Posts: 131433
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RE: Cosmological Constant
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Title: Proposal for the origin of the cosmological constant
Authors: R. Dale, J. A. Morales-Lladosa, D. Saez

We work in the framework of a simple vector-tensor theory. The parameterised post-Newtonian approximation of this theory is identical to that of general relativity. Our attention is focused on cosmology. In an homogeneous isotropic universe, it is proved that the energy density,
ho_{A}, of the vector field A, and its pressure, p_{A}, do not depend on time, and also that the equation of state is
ho_{A} = -p_{A}. This means that, in the theory under consideration, there is a cosmological constant, which is not vacuum energy, but the dark energy of the cosmic vector field A, whose evolution is classical.

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Blobrana


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Title: The cosmological constant from the Veneziano ghost which solves the U(1) problem in QCD
Authors: Federico R. Urban, Ariel R. Zhitnitsky

We suggest that the solution to the cosmological vacuum energy puzzle is linked to the infrared sector of the effective theory of gravity interacting with standard model fields, with QCD fields specifically. We work in the framework of low energy quantum gravity as an effective field theory. In particular, we compute the vacuum energy in terms of QCD parameters and the Hubble constant H such that the vacuum energy is \epsilon_{vac} ~ H \cdot  m_q\la\bar{q}q
a /m_{\eta'} ~ (3.6\cdot 10^{-3} {eV})^4, which is amazingly close to the observed value today. The QCD ghost (responsible for the solution of the U(1)_A problem) plays a crucial role in the computation of the vacuum energy, because the ghost properties at very large but finite distances slightly deviate (as ~ H / \Lqcd ) from their infinite volume Minkowski values. Another important prediction of this framework states that the vacuum energy owes its existence to the asymmetry of the cosmos. Indeed, this effect is a direct consequence of the embedding of our Universe on a non-trivial manifold such as a torus with (slightly) different linear sizes. Such a violation of cosmological isotropy is apparently indeed supported by WMAP, and will be confirmed (or ruled out) by future PLANCK data.

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Blobrana


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Title: On the Nature of the Cosmological Constant Problem
Authors: M. D. Maia, A. J. S.Capistrano, E. M. Monte
(Version v2)

General relativity postulates the Minkowski space-time to be the standard flat geometry against which we compare all curved space-times and the gravitational ground state where particles, quantum fields and their vacuum states are primarily conceived. On the other hand, experimental evidences show that there exists a non-zero cosmological constant, which implies in a deSitter space-time, not compatible with the assumed Minkowski structure. Such inconsistency is shown to be a consequence of the lack of a application independent curvature standard in Riemann's geometry, leading eventually to the cosmological constant problem in general relativity.
We show how the curvature standard in Riemann's geometry can be fixed by Nash's theorem on locally embedded Riemannian geometries, which imply in the existence of extra dimensions. The resulting gravitational theory is more general than general relativity, similar to brane-world gravity, but where the propagation of the gravitational field along the extra dimensions is a mathematical necessity, rather than being a a postulate. After a brief introduction to Nash's theorem, we show that the vacuum energy density must remain confined to four-dimensional space-times, but the cosmological constant resulting from the contracted Bianchi identity is a gravitational contribution which propagates in the extra dimensions. Therefore, the comparison between the vacuum energy and the cosmological constant in general relativity ceases to be. Instead, the geometrical fix provided by Nash's theorem suggests that the vacuum energy density contributes to the perturbations of the gravitational field.

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Blobrana


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RE: Dark Energy
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Dark Energy and the Runaway Universe

World-renowned astronomer and prize-winning professor of astronomy at the University of California, Berkeley, Alex Filippenko, explores some of the mysteries of the universe at a special lecture at the Santa Barbara Museum of Natural History. Filippenko discusses observations of very distant exploding starts called super-novae that provide intriguing evidence that the expansion of the universe is now speeding up. Over the largest scales of space, the universe seems to be dominated by a repulsive "dark energy" of unknown origin, stretching the very fabric of space itself faster and faster with time.


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Blobrana


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Title: Geometry of Dark Energy
Authors: M. D. Maia, A. J. S. Capistrano, J. S. Alcaniz, E. M. Monte

The acceleration of the universe is described as a dynamical effect of the extrinsic curvature of space-time. By extending previous results, the extrinsic curvature is regarded as an independent spin-2 field, determined by a set of non-linear equations similar to Einstein's equations. In this framework, we investigate some cosmological consequences of this class of scenarios and test its observational viability by performing a statistical analysis with current type Ia Supernova data.

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Blobrana


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Accelerating universe
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Title: Accelerating universe from warped extra dimensions
Authors: Ishwaree P Neupane

Accelerating universe or the existence of a small and positive cosmological constant is probably the most pressing obstacle as well as opportunity to significantly improving the models of four-dimensional cosmology from fundamental theories of gravity, including string theory. In seeking to resolve this problem, one naturally wonders if the real world can somehow be interpreted as an inflating de Sitter brane embedded in a higher-dimensional spacetime described by warped geometry. In this scenario, the four-dimensional cosmological constant may be uniquely determined in terms of two length scales: one is a scale associated with the size of extra dimensions and the other is a scale associated with the expansion rate of our universe.

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Blobrana


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RE: Dark Energy
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The existence of dark energy was revealed 11 years ago by two teams, one led by the Australian astronomer Brian Schmidt, who discovered the expansion of the universe was accelerating.
Australian astronomers are also now leading the way in throwing light on this dark side of the cosmos.
The director of the Anglo-Australian Observatory, Matthew Colless, said his team was measuring the position of 200,000 distant galaxies, and seeking tiny "wiggles" in their distribution.

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Blobrana


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Australian astronomers test dark energy


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