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TOPIC: Dark Energy
Blobrana


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Title: Cosmic Inhomogeneities and the Average Cosmological Dynamics
Authors: Aseem Paranjape, T. P. Singh
(Version v2)

If one assumes that the dynamics of the expanding Universe is described by general relativity, then the observed cosmic acceleration implies that the Universe is dominated by a 'dark energy', which has negative pressure. In principle however, it might be possible to explain the acceleration without invoking dark energy. This is because while the Universe is homogeneous on sufficiently large scales, it is inhomogeneous on smaller scales. Although the gravitational dynamics of the inhomogeneous Universe is described by Einstein's equations, the dynamics of the homogeneous Universe obtained by averaging out the inhomogeneities, obeys modified Einstein equations. Could these modifications by themselves explain the acceleration? We answer this question in the negative. Starting from realistic initial conditions, and using a well-defined averaging scheme for a simple but generic model of nonlinear inhomogeneity, we show that the modifications have a negligible influence on the average cosmological dynamics.

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Blobrana


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Title: Can the Existence of Dark Energy Be Directly Detected?
Authors: Martin L. Perl
(Version v2)

The majority of astronomers and physicists accept the reality of dark energy and also believe that it can only be studied indirectly through observation of the motions of stars and galaxies. In this paper I open the experimental question of whether it is possible to directly detect dark energy through the presence of dark energy density. Two thirds of this paper outlines the major aspects of dark energy density as now comprehended by the astronomical and physics community. The final third summarizes various proposals for direct detection of dark energy density or its possible effects. At this time I do not have a fruitful answer to the question: Can the Existence of Dark Energy Be Directly Detected?

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Blobrana


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If the notion of dark energy sounds improbable, get ready for an even more outlandish suggestion.
Earth may be trapped in an abnormal bubble of space-time that is particularly void of matter. Scientists say this condition could account for the apparent acceleration of the universe's expansion, for which dark energy currently is the leading explanation.

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Blobrana


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Title: First Cosmological Constraints on Dark Energy from the Radial Baryon Acoustic Scale
Authors: Enrique Gaztanaga, Ramon Miquel, Eusebio Sanchez

We discuss cosmological constraints arising from the first measurement of the radial (line-of-sight) baryon acoustic oscillations (BAO) scale in the large scale structure traced by the galaxy distribution. This differs from earlier BAO measurements, which provided a measurement which was either averaged over all directions or just in the transverse direction. We argue here that the radial measurements are practically independent from previous measurements, even when they come from similar galaxy samples. The two radial BAO measurements give two new adimensional estimations of the acoustic scale at z = 0.24 and z = 0.43. Here we use these measurements to derive new and independent constraints on the amount of dark energy and its equation of state for a flat universe, without any other assumptions on the cosmological model. We find an excellent agreement with the /\CDM cosmological constant model with w = -1.14 ± 0.33 (assumed constant) and Omega_m = 0.24 ± 0.05. If we drop the assumption of flatness and combine these new measurements with previous CMB and supernova measurements, we find w = -0.930 ± 0.054, Omega_m = 0.249 ± 0.013 and Omega_k = 0.0062 ± 0.0058, again in good agreement with a flat /\CDM cosmology. As a by-product, we obtain a measurement of the Hubble parameter h = 0.66 ±0.02.

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Blobrana


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Astronomers say they have detected the clearest evidence yet for the mysterious dark energy that stretches and flattens out the universe.
The research, led by astrophysicist Dr István Szapudi of the Institute for Astronomy at the University of Hawaii in Honolulu, is published online on the arXiv.org physics website ahead of publication in Astrophysical Journal Letters.

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Blobrana


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A team of astronomers at the University of Hawaii Institute for Astronomy (IfA) led by Dr. István Szapudi has found direct evidence for the existence of dark energy. Dark energy works against the tendency of gravity to pull galaxies together and so causes the universes expansion to speed up. The nature of dark energy (what precisely it is, and why it exists) is one of the biggest puzzles of modern science.

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Blobrana


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Mapping the peculiar velocities of stars
All things dark are all the rage is cosmology at the moment. There is dark mattera type of matter that only weakly interacts with light. And dark energythe label used to denote the observed increase in the rate of expansion of the universe. Our knowledge of what dark matter is and what dark energy denotes is woefully inadequate, opening up a theoretician's paradise. There are all sorts of models out there and, in the case of dark energy, they all have to fit one data point, making it kind of trivial to obtain a good result. In the meantime, astronomers are scrabbling aroundin, yes, the darkfiguring out how to obtain more precise measurements of the increasing acceleration of the universe.

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Blobrana


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Dark Energy Survey (DES) camera
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UK astronomers, as part of an international team, have reached a milestone in the construction of one of the largest ever cameras to detect the mysterious Dark Energy component of the Universe. The pieces of glass for the five unique lenses of the camera have been shipped from the US to France to be shaped and polished into their final form. The largest of the five lenses is one metre in diameter, making it one of the largest in the world.
Each milestone in the completion of this sophisticated camera brings us closer to detecting the mysterious and invisible matter that cosmologists estimate makes up around three quarters of our Universe and is driving its accelerating expansion. Observations suggest that roughly 4% of the Universe is made up from ordinary matter and 22% from Dark Matter; this leaves 74% unaccounted for - the so-called Dark Energy.

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Blobrana


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This fall, NASA and the Department of Energy plan to invite proposals for a $600 million satellite mission devoted to dark energy. But some scientists fear that might not be enough. When astronomers and physicists gathered at the Space Telescope Science Institute recently to take stock of the revolution, their despair of getting to the bottom of the dark energy mystery anytime soon, if ever, was palpable, even as they anticipate a flood of new data from the sky in coming years. When it came time for one physicist to discuss new ideas about dark energy, he showed a blank screen.

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Blobrana


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In a bid to solve the dark energy question once and for all, István Szapudi at the University of Hawaii in Honolulu turned to the cosmic microwave background - the radiation left behind by the big bang.   Detailed maps of the CMB show hot and cold spots that reflect variations in the density of the early universe.
When dark energy was proposed, astronomers realised that it should create additional temperature bumps on the map.
This extra dark energy effect is generated because the temperature of a photon zipping across the universe can be changed depending on whether it has passed through a region of dense matter or a sparser region.

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