* Astronomy

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RE: Dark Energy

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Dark-energy fingerprints found in ancient radiation

Only cat burglars can match the stealth of dark energy, credited with speeding up the universe's expansion over time, but now its fingerprints have been glimpsed in the universe's oldest radiation.
The strongest evidence for dark energy comes from supernovae, which suggest the universe is expanding faster now than in the past. But the force should also change the extent to which the cosmic microwave background (CMB), relic radiation from the big bang, is warped, or "lensed", by the gravity from distant galaxies and dark matter.
That's because the accelerating expansion of the universe should prevent the growth of very massive structures
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Title: The Little Rip
Authors: Paul H. Frampton, Kevin J. Ludwick, Robert J. Scherrer

We examine models in which the dark energy density increases with time (so that the equation-of-state parameter w satisfies w < -1), but w approaches -1 asymptotically, such that there is no future singularity. We refine previous calculations to determine the conditions necessary to produce this evolution. Such models can display arbitrarily rapid expansion in the near future, leading to the destruction of all bound structures (a "little rip"). We determine observational constraints on these models and calculate the point at which the disintegration of bound structures occurs. For the same present-day value of w, a big rip with constant w disintegrates bound structures earlier than a little rip.

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New method 'confirms dark energy'

First results from a major astronomical survey have confirmed the existence of mysterious dark energy using a cutting-edge technique.
Dark energy makes up some 74% of the Universe and its existence explains why the Universe appears to be expanding at an accelerating rate.
The finding was based on studies of more than 200,000 galaxies.
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NASA'S Galaxy Evolution Explorer Finds Dark Energy Repulsive

A five-year survey of 200,000 galaxies, stretching back seven billion years in cosmic time, has led to one of the best independent confirmations that dark energy is driving our universe apart at accelerating speeds.
The survey used data from NASA's space-based Galaxy Evolution Explorer and the Anglo-Australian Telescope on Siding Spring Mountain in Australia.
The findings offer new support for the favoured theory of how dark energy works - as a constant force, uniformly affecting the universe and propelling its runaway expansion. They contradict an alternate theory, where gravity, not dark energy, is the force pushing space apart. According to this alternate theory, with which the new survey results are not consistent, Albert Einstein's concept of gravity is wrong, and gravity becomes repulsive instead of attractive when acting at great distances.
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Dark energy is real

A survey of more than 200,000 galaxies led by Australian astronomers has shown that 'dark energy' is real and not a mistake in Einstein's theory of gravity.
The finding is conveyed in two papers led by Dr Chris Blake from Swinburne University's Centre for Astrophysics and Supercomputing, which will be published in the Monthly Notices of the Royal Astronomical Society.
Using the Anglo-Australian Telescope, 26 astronomers contributed to the 'WiggleZ Dark Energy Survey' which mapped the distribution of galaxies over an unprecedented volume of the Universe. Because light takes so long to reach Earth, it was the equivalent of looking seven billion years back in time - more than half way back to the Big Bang.
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Title: Early dark energy from zero-point quantum fluctuations
Authors: Michele Maggiore, Lukas Hollenstein, Maud Jaccard, Ermis Mitsou

We examine a cosmological model where the dark energy density has the form rho_{DE}(t)=rho_X(t)+rho_Z(t), where rho_X is the dominant component of dark energy and rho_Z(t) is an extra contribution proportional to H²(t). This form of rho_Z(t) follows from the recent suggestion that the contribution of zero-point fluctuations of quantum fields to the cosmic expansion is obtained subtracting from the zero-point energy density computed in a FRW space-time the value computed in Minkowski space. Assuming an energy conservation of the form d rho_X/dt+3H(1+w_X) rho_X+d rho_Z/dt+3H(1+w_Z) rho_Z=0 we find that, for w_Z\neq w_X, the total dark energy density rho_{DE}(t) always remains a finite fraction of the critical density at any time, thus providing a simple and appealing solution to the coincidence problem. We compare this model to CMB, supernovae and BAO data.

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Title: Dark Energy
Authors: Miao Li, Xiao-Dong Li, Shuang Wang, Yi Wang

We review the problem of dark energy, including a survey of phenomenological models and some aspects of data fitting.

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Title: What do we really know about Dark Energy?
Authors: Ruth Durrer

In this paper I discuss what we truly know about dark energy. I shall argue that up to date our single indication for the existence of dark energy comes from distance measurements and their relation to redshift. Supernovae, CMB anisotropies and observations of baryon acoustic oscillations, they all simply tell us that the observed distance to a given redshift is larger than the one expected from a Friedmann Lemaitre universe with matter only and the locally measured Hubble parameter.

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NASA's Hubble Rules Out One Alternative to Dark Energy

Astronomers using NASA's Hubble Space Telescope have ruled out an alternate theory on the nature of dark energy after recalculating the expansion rate of the universe to unprecedented accuracy. The universe appears to be expanding at an ever-increasing rate, and one explanation is that the universe is filled with a dark energy that works in the opposite way of gravity. One alternative to that hypothesis is that an enormous bubble of relatively empty space eight billion light-years across surrounds our galactic neighbourhood. If we lived near the center of this void, observations of galaxies being pushed away from each other at accelerating speeds would be an illusion. This hypothesis has been invalidated because astronomers have refined their current understanding of the universe's present expansion rate to an uncertainty of just 3.3 percent. The new measurement reduces the error margin by 30 percent over Hubble's previous best measurement in 2009. The results are reported in the April 1 issue of The Astrophysical Journal.
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