* Astronomy

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RE: Dark Energy

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Title: Van der Waals quintessence stars
Authors: Francisco S. N. Lobo
(revised v2)

The van der Waals quintessence equation of state is an interesting scenario for describing the late universe, and seems to provide a solution to the puzzle of dark energy, without the presence of exotic fluids or modifications of the Friedmann equations. In this work, the construction of inhomogeneous compact spheres supported by a van der Waals equation of state is explored. These relativistic stellar configurations shall be denoted as van der Waals quintessence stars. Despite of the fact that, in a cosmological context, the van der Waals fluid is considered homogeneous, inhomogeneities may arise through gravitational instabilities. Thus, these solutions may possibly originate from density fluctuations in the cosmological background. Two specific classes of solutions, namely, gravastars and traversable wormholes are analysed. Exact solutions are found, and their respective characteristics and physical properties are further explored.

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Title: Scrutinising Exotic Cosmological Models Using ESSENCE Supernova Data Combined with Other Cosmological Probes
Authors: T. M. Davis, E. Mortsell, J. Sollerman, A. C. Becker, S. Blondin, P. Challis, A. Clocchiatti, A. V. Filippenko, R. J. Foley, P. M. Garnavich, S. Jha, K. Krisciunas, R. P. Kirshner, B. Leibundgut, W. Li, T. Matheson, G. Miknaitis, G. Pignata, A. Rest, A. G. Riess, B. P. Schmidt, R. C. Smith, J. Spyromilio, C. W. Stubbs, N. B. Suntzeff, J. L. Tonry, W. M. Wood-Vasey

The first cosmological results from the ESSENCE supernova survey (Wood-Vasey et al. 2007) are extended to a wider range of cosmological models including dynamical dark energy and non-standard cosmological models. We fold in a greater number of external data sets such as the recent Higher-z release of high-redshift supernovae (Riess et al. 2007) as well as several complementary cosmological probes. Model comparison statistics such as the Bayesian and Akaike information criteria are applied to gauge the worth of models. These statistics favour models that give a good fit with fewer parameters.
Based on this analysis, the preferred cosmological model is the flat cosmological constant model, where the expansion history of the universe can be adequately described with only one free parameter describing the energy content of the universe. Amongst the more exotic models that provide good fits to the data, we note a preference for models whose best-fit parameters reduce them to the cosmological constant model.

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Dark Energy may be vacuum

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RE: Dark Energy

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Title: Comments on "Bigger Rip with no dark energy"
Authors: A. Kwang-Hua Chu
(revised v2)

We make corrections on the paper by Frampton and Takahashi {{\it Astropart. Phys.} {\bf 22} (2004) 307}. Our focus is especially upon Eqs. (12-15) therein.

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The mysterious substance known as dark energy has been fuelling the expansion of the universe for at least nine billion years, according to astronomers in the US. Adam Riess of Johns Hopkins University and colleagues made the discovery by using the Hubble Space Telescope to study ancient exploding stars. They have also concluded that dark energy appears to be related to the "cosmological constant" first proposed – and then retracted -- by Albert Einstein.

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Previous Hubble observations of the most distant supernovae known revealed that the early universe was dominated by matter whose gravity was slowing down the universe's expansion rate, like a ball rolling up a slight incline. The observations also confirmed that the expansion rate of the cosmos began speeding up about five to six billion years ago. That is when astronomers believe that dark energy's repulsive force overtook gravity's attractive grip.
The latest results are based on an analysis of the 24 most distant supernovae known, most found within the last two years.
By measuring the universe's relative size over time, astrophysicists have tracked the universe's growth spurts, much as a parent may witness the growth spurts of a child by tracking changes in height on a doorframe. Distant supernovae provide the doorframe markings read by Hubble.

"After we subtract the gravity from the known matter in the universe, we can see the dark energy pushing to get out" - Lou Strolger, astronomer and Hubble team member at Western Kentucky University, Bowling Green, Kentucky.

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Dark energy - the mysterious force that is speeding up the expansion of the Universe - has been a part of space for at least nine billion years.
That is the conclusion of astronomers who presented results from a three-year study using the Hubble Space Telescope.
The finding may rule out some competing theories that predict the strength of dark energy changes over time.
The findings are consistent with the idea of dark energy behaving like Albert Einstein's cosmological constant. The cosmological constant describes the idea that there is a density and pressure associated with "empty" space.
In this scenario, dark energy never changes; it has the same properties across the age of the Universe.

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Scientists using NASA's Hubble Space Telescope have discovered that dark energy is not a new constituent of space, but rather has been present for most of the universe's history. Dark energy is a mysterious repulsive force that causes the universe to expand at an increasing rate. Investigators used Hubble to find that dark energy was already boosting the expansion rate of the universe as long as nine billion years ago. This picture of dark energy is consistent with Albert Einstein's prediction of nearly a century ago that a repulsive form of gravity emanates from empty space. Data from Hubble provides supporting evidence to help astrophysicists to understand the nature of dark energy. This will allow them to begin ruling out some competing explanations that predict that the strength of dark energy changes over time.

Credit NASA

Researchers also have found that the class of ancient exploding stars, or supernovae, used to measure the expansion of space today look remarkably similar to those that exploded nine billion years ago and are just now being seen by Hubble. This important finding gives additional credibility to the use of these supernovae for tracking the cosmic expansion over most of the universe's lifetime. Supernovae provide reliable measurements because their intrinsic brightness is well understood. They are therefore reliable distance markers, allowing astronomers to determine how far away they are from Earth. These snapshots, taken by Hubble reveal five supernovae and their host galaxies. The arrows in the top row of images point to the supernovae. The bottom row shows the host galaxies before or after the stars exploded. The supernovae exploded between 3.5 and 10 billion years ago.

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