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TOPIC: Dark Energy
Blobrana


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RE: Dark Energy
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Less than 100 years ago scientists didn't know if the universe was coming or going, literally. It even fooled the great mind of Albert Einstein. He assumed the universe must be static. But to keep the universe from collapsing under gravity like a house of cards, Einstein hypothesized there was a repulsive force at work, called the cosmological constant, that counterbalanced gravity's tug. Along came Edwin Hubble in 1923 who found that galaxies were receding from us at a proportional rate, called the Hubble constant, which meant the universe was uniformly expanding, so there was no need to shore it up with any mysterious force from deep space. In measuring how this expansion was expected to slow down over time, 11 years ago, two studies, one led by Adam Riess of the Space Telescope Science Institute and the Johns Hopkins University and Brian Schmidt of Mount Stromlo Observatory, and the other by Saul Perlmutter of Lawrence Berkeley National Laboratory, independently discovered dark energy, which seems to behave like Einstein's cosmological constant.
To better characterise dark energy, Riess used Hubble Space Telescope's crisp view (combined with 2003 data from NASA's Wilkinson Microwave Anisotropy Probe, WMAP) to refine the value of the universe's expansion rate to a precision of three percent. That's a big step from 20 years ago when astronomers' estimates for the Hubble constant disagreed by a factor of two. This new value implies that dark energy really is a steady push on the universe as Einstein imagined, rather than something more effervescent (like the early inflationary universe) that changes markedly over time.

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Blobrana


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Dark Energy and the Runaway Universe (mirrored)



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Blobrana


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After billions of years of runaway expansion, is the universe starting to slow down? A new analysis of nearby supernovae suggests space might not be expanding as quickly as it once was, a tantalising hint that the source of dark energy may be more exotic than we thought.
For more than a decade, astrophysicists have grappled with evidence of a baffling force that seems to be pushing the universe apart at an ever-increasing rate. Exactly what constitutes the dark energy responsible for this cosmic speed-up is unknown.

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Blobrana


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Dark energy is the deus ex machina of cosmology, able to save even the most inflation-prone calculations from destruction or - worse - being provably wrong.  But while we've been busy watching the X-energy apparently accelerating all of creation while hiding in plain sight, some believe it's responsible for much more than that.  It didn't just save the universe - no, no, that's far too small scale - it saved INFINITE universes.
Scientists at Princeton and Cambridge say that most of the universe is regularly destroyed.  It's space-time-twisted into black holes, in fact, which is about as utterly destroyed as you can get without pissing off Zeus.  In each destruction cycle only a small seed of habitable space survives, which grows phoenix-like to provide a new universe due to the apparently all-powerful dark matter.

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Blobrana


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Cosmological Parameter Constraints
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Title: Chandra Cluster Cosmology Project III: Cosmological Parameter Constraints
Authors: A.Vikhlinin, A.V.Kravtsov, R.A.Burenin, H.Ebeling, W.R.Forman, A.Hornstrup, C.Jones, S.S.Murray, D.Nagai, H.Quintana, A.Voevodkin

Chandra observations of large samples of galaxy clusters detected in X-rays by ROSAT provide a new, robust determination of the cluster mass functions at low and high redshifts. Statistical and systematic errors are now sufficiently small, and the redshift leverage sufficiently large for the mass function evolution to be used as a useful growth of structure based dark energy probe. In this paper, we present cosmological parameter constraints obtained from Chandra observations of 36 clusters with <z>=0.55 derived from 400deg˛  ROSAT serendipitous survey and 49 brightest z=~0.05 clusters detected in the All-Sky Survey. Evolution of the mass function between these redshifts requires Omega_Lambda>0 with a ~5sigma significance, and constrains the dark energy equation of state parameter to w0=-1.14±0.21, assuming constant w and flat universe. Cluster information also significantly improves constraints when combined with other methods. Fitting our cluster data jointly with the latest supernovae, WMAP, and baryonic acoustic oscillations measurements, we obtain w0=-0.991±0.045 (stat) ±0.039 (sys), a factor of 1.5 reduction in statistical uncertainties, and nearly a factor of 2 improvement in systematics compared to constraints that can be obtained without clusters. The joint analysis of these four datasets puts a conservative upper limit on the masses of light neutrinos, Sum m_nu<0.33 eV at 95% CL. We also present updated measurements of Omega_M*h and sigma_8 from the low-redshift cluster mass function.

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Blobrana


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Title: Can We Avoid Dark Energy?
Authors: James P. Zibin, Adam Moss, and Douglas Scott

The idea that we live near the center of a large, nonlinear void has attracted attention recently as an alternative to dark energy or modified gravity. We show that an appropriate void profile can fit both the latest cosmic microwave background and supernova data. However, this requires either a fine-tuned primordial spectrum or a Hubble rate so low as to rule these models out. We also show that measurements of the radial baryon acoustic scale can provide very strong constraints. Our results present a serious challenge to void models of acceleration.

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Blobrana


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Dark Energy Stifling Structure Growth



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Blobrana


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Alexey Vikhlinin and his colleagues used Chandra to observe the hot gas in dozens of galaxy clusters, which are the largest collapsed objects in the universe. Some of these clusters are relatively close and others are more than halfway across the universe.
The results show the increase in mass of the galaxy clusters over time aligns with a universe dominated by dark energy. It is more difficult for objects like galaxy clusters to grow when space is stretched, as caused by dark energy. Vikhlinin and his team see this effect clearly in their data. The results are remarkably consistent with those from the distance measurements, revealing general relativity applies, as expected, on large scales.

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Blobrana


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Billions of years ago, the universe was crowded with tight-knit clusters of galaxies. Then, a party crasher got the upper hand. This mysterious force now called dark energy has since been expanding the universe at an increasing pace.
New measurements of this accelerating expansion, which drives galaxies away from one another on large scales but so far shows no effects on small scales (such as within a galaxy), provide details about the nature of the unseen and unknown dark energy that is at work.

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Blobrana


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For the first time, astronomers have clearly seen the effects of "dark energy" on the most massive collapsed objects in the universe using NASA's Chandra X-ray Observatory. By tracking how dark energy has stifled the growth of galaxy clusters and combining this with previous studies, scientists have obtained the best clues yet about what dark energy is and what the destiny of the universe could be.

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