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http://space.newscientist.com/article/dn13975-dark-energy-imaged-in-best-detail-yet.html
Ever since accelerated expansion was observed in 1998 some people (like e.g. David Wiltshire) have been working hard to think up alternative explanations that could account for the observations without invoking a positive cosmo constant or a uniform dark energy density.
This is good, it is good to try out alternative explanations.
But finally we have a study that begins to look like it shoots down these alternatives, and that there really is a positive cosmo constant.
The author is Istvan Szapudi et al.
(Granett/Neyrinck/Szapudi)
Szapudi is at Uni Hawaii--Honolulu
I think the study is an extensive analysis of integrated Sachs-Wolfe effect on the CMB temperature variation. Other people have done this, I believe, but this time it is done more thoroughly. It sounds like it begins to persuade some of the alternative-seekers that dark energy is real.
http://arxiv.org/abs/0805.2974
Dark Energy Detected with Supervoids and Superclusters
Benjamin R. Granett, Mark C. Neyrinck, István Szapudi (IfA, Hawaii)
(Submitted on 20 May 2008)
"The observed apparent acceleration of the universe is usually attributed to negative pressure from a mysterious dark energy. This acceleration causes the gravitational potential to decay, heating or cooling photons traveling through crests or troughs of large-scale matter density fluctuations. This phenomenon, the late-time integrated Sachs-Wolfe (ISW) effect, has been detected, albeit at low significance, by cross-correlating various galaxy surveys with the Cosmic Microwave Background (CMB). Recently, the best evidence has come from the statistical combination of results from multiple correlated galaxy data sets. Here we show that vast structures identified in a galaxy survey project an image onto the CMB; stacking regions aligned with superclusters produces a hot spot, and supervoids, a cold spot. At over 4 sigma, this is the clearest evidence of the ISW effect to date. For the first time, our findings pin the effect to discrete structures. The ISW signal from supervoids and superclusters can be combined with other cosmological probes to constrain dark energy and cosmological parameters. In addition, our findings make it more plausible that the extreme Cold Spot and other anomalies in the CMB are caused by supervoids."
===sample quote from NewSci===
...A photon gains energy when it enters a dense region with enhanced gravity – such as a galaxy cluster – as though it is falling into a well. When it leaves the cluster and climbs back out of the gravitational well, it loses energy.
In a universe without dark energy, the energy gained and lost during the crossing would be equal and would cancel out. But in the presence of dark energy, the universe expands quickly enough to stretch the gravitational well while the photon is still inside. This makes the well shallower and easier for the photon to climb out.
That means that a photon traveling through a cluster gains more energy than it loses, giving it a little energy kick so that it creates a hotter spot than would be expected on images of the CMB. Similarly, a photon that has passed through a void would leave a cold spot.
It's tough to detect this effect because dark energy gives only a slight nudge to the temperature, which is easily swamped by the normal temperature variations seen in the CMB, says Szapudi.
Extreme density
To get around this, his team looked at regions of extremely high and extremely low density, where you would expect to see the biggest effect.
Using data from the Sloan Digital Sky Survey, they chose over 3000 superclusters of galaxies and 500 "supervoids" of relatively empty space, and they found that the regions did indeed tally with enhanced hot and cold spots in the CMB.
Other teams have reported signs of this effect in the past, but those have been open to alternative explanations, says Szapudi. By contrast, his calculations suggest that there is less than a 1 in 200,000 chance that the match up his team saw is down to anything other than dark energy...
==endquote==
Ever since accelerated expansion was observed in 1998 some people (like e.g. David Wiltshire) have been working hard to think up alternative explanations that could account for the observations without invoking a positive cosmo constant or a uniform dark energy density.
This is good, it is good to try out alternative explanations.
But finally we have a study that begins to look like it shoots down these alternatives, and that there really is a positive cosmo constant.
The author is Istvan Szapudi et al.
(Granett/Neyrinck/Szapudi)
Szapudi is at Uni Hawaii--Honolulu
I think the study is an extensive analysis of integrated Sachs-Wolfe effect on the CMB temperature variation. Other people have done this, I believe, but this time it is done more thoroughly. It sounds like it begins to persuade some of the alternative-seekers that dark energy is real.
http://arxiv.org/abs/0805.2974
Dark Energy Detected with Supervoids and Superclusters
Benjamin R. Granett, Mark C. Neyrinck, István Szapudi (IfA, Hawaii)
(Submitted on 20 May 2008)
"The observed apparent acceleration of the universe is usually attributed to negative pressure from a mysterious dark energy. This acceleration causes the gravitational potential to decay, heating or cooling photons traveling through crests or troughs of large-scale matter density fluctuations. This phenomenon, the late-time integrated Sachs-Wolfe (ISW) effect, has been detected, albeit at low significance, by cross-correlating various galaxy surveys with the Cosmic Microwave Background (CMB). Recently, the best evidence has come from the statistical combination of results from multiple correlated galaxy data sets. Here we show that vast structures identified in a galaxy survey project an image onto the CMB; stacking regions aligned with superclusters produces a hot spot, and supervoids, a cold spot. At over 4 sigma, this is the clearest evidence of the ISW effect to date. For the first time, our findings pin the effect to discrete structures. The ISW signal from supervoids and superclusters can be combined with other cosmological probes to constrain dark energy and cosmological parameters. In addition, our findings make it more plausible that the extreme Cold Spot and other anomalies in the CMB are caused by supervoids."
===sample quote from NewSci===
...A photon gains energy when it enters a dense region with enhanced gravity – such as a galaxy cluster – as though it is falling into a well. When it leaves the cluster and climbs back out of the gravitational well, it loses energy.
In a universe without dark energy, the energy gained and lost during the crossing would be equal and would cancel out. But in the presence of dark energy, the universe expands quickly enough to stretch the gravitational well while the photon is still inside. This makes the well shallower and easier for the photon to climb out.
That means that a photon traveling through a cluster gains more energy than it loses, giving it a little energy kick so that it creates a hotter spot than would be expected on images of the CMB. Similarly, a photon that has passed through a void would leave a cold spot.
It's tough to detect this effect because dark energy gives only a slight nudge to the temperature, which is easily swamped by the normal temperature variations seen in the CMB, says Szapudi.
Extreme density
To get around this, his team looked at regions of extremely high and extremely low density, where you would expect to see the biggest effect.
Using data from the Sloan Digital Sky Survey, they chose over 3000 superclusters of galaxies and 500 "supervoids" of relatively empty space, and they found that the regions did indeed tally with enhanced hot and cold spots in the CMB.
Other teams have reported signs of this effect in the past, but those have been open to alternative explanations, says Szapudi. By contrast, his calculations suggest that there is less than a 1 in 200,000 chance that the match up his team saw is down to anything other than dark energy...
==endquote==
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