Rutgers: New Galaxy Clusters Revealed by Cosmic Shadows

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Rutgers: New Galaxy Clusters Revealed by Cosmic Shadows

Post by bystander » Mon Nov 01, 2010 10:50 pm

New Galaxy Clusters Revealed by Cosmic “Shadows”
Rutgers University | 01 Nov 2010
[img3="Four Atacama Cosmology Telescope (ACT) images of cosmic background radiation, top row, with dark blue colors indicating "shadows" cast by galaxy clusters. Below, four optical images of the galaxy clusters, with white contour lines corresponding to the cosmic background radiation intensity levels in the ACT images. (Credit: Top row: Tobias Marriage, Johns Hopkins University, Princeton University. Bottom matrix: Felipe Menanteau, Rutgers University) Hires Image"]http://www.eurekalert.org/multimedia/pu ... 10_web.jpg[/img3]
An international team of scientists led by Rutgers University astrophysicists have discovered 10 new massive galaxy clusters from a large, uniform survey of the southern sky. The survey was conducted using a breakthrough technique that detects "shadows" of galaxy clusters on the cosmic microwave background radiation, a relic of the "big bang" that gave birth to the universe.

In a paper published in the Nov. 10 issue of Astrophysical Journal, the Rutgers scientists and collaborators at the Pontifical Catholic University of Chile (PUC) describe their visual telescope observations of these galaxy clusters, which were essential to verify the cosmic shadow sightings. Both observations will help scientists better understand how the universe was born and continues to evolve.

The research began in 2008 with a new radio telescope in the Atacama Desert of Chile – one of the driest places on Earth. The instrument, known as the Atacama Cosmology Telescope (ACT), collects millimeter-length radio waves that reveal images of the otherwise invisible cosmic background radiation. Millimeter waves are easily blocked by water vapor, hence the telescope's home high in the Andes Mountains of northern Chile, where there is barely any atmospheric moisture.

The research began in 2008 with a new radio telescope in the Atacama Desert of Chile – one of the driest places on Earth. The instrument, known as the Atacama Cosmology Telescope (ACT), collects millimeter-length radio waves that reveal images of the otherwise invisible cosmic background radiation. Millimeter waves are easily blocked by water vapor, hence the telescope’s home high in the Andes Mountains of northern Chile, where there is barely any atmospheric moisture.
...
Theorists Rashid Sunyaev and Yakov Zel’dovich predicted the shadow phenomenon 40 years ago, now known as the Sunyaev-Zel’dovich effect, or S-Z effect. Shortly thereafter astronomers verified it by observing shadows cast by previously known galaxy clusters. The higher sensitivity and resolution of ACT now makes it practical for astronomers to essentially reverse the procedure – to search the cosmic background radiation for shadows that indicate the presence of unseen clusters.

“The ‘shadows’ that ACT revealed are not shadows in the traditional sense, as they are not caused by the galaxy clusters blocking light from another source,” said Jack Hughes, professor of physics and astronomy at Rutgers. “Rather, the hot gases within the galaxy clusters cause a tiny fraction of the cosmic background radiation to shift to higher energies, which then makes them appear as shadows in one of ACT’s observing bands.”
The Atacama Cosmology Telescope: Physical Properties and Purity of
a Galaxy Cluster Sample Selected via the Sunyaev-Zel'dovich Effect
- F Menanteau et al

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SLAC: Galaxy Clusters Shine New Light on Dark Energy

Post by bystander » Thu Dec 09, 2010 4:13 pm

Galaxy Clusters Shine New Light on Dark Energy
SLAC National Accelerator Laboratory | 07 Dec 2010
Scientists' murky understanding of dark energy may have just gotten a little clearer, thanks to recent work by a team of researchers that includes astrophysicist Neelima Sehgal of the Kavli Institute for Particle Astrophysics and Cosmology at SLAC. The team used observations from the Atacama Cosmology Telescope, or ACT, in the Chilean Andes to more narrowly define the properties of dark energy, that enigmatic entity that's thought to make up approximately 70 percent of the mass-energy of the universe and is pushing space apart.

The team's results are summarized online on the arXiv.

Scientists have struggled to come up with a satisfactory explanation for the nature of dark energy ever since astronomical observations in 1998 first suggested its existence. Astronomers were surprised to discover that the universe's expansion was accelerating, a fact that could be explained only by a previously unknown source of energy.

"Basically the problem is that nobody knows what dark energy is," Sehgal said.

Dark energy may be the energy of the vacuum of space, which is one of the simplest theories to explain its existence. Vacuum energy is the background energy in otherwise empty space. Particle physicists theorize that this property of space could be the result of virtual particles constantly forming and disappearing. However, the virtual particle theory predicts a vacuum energy density that is 120 orders of magnitude larger than the observed dark energy density. The vast discrepancy has spurred the development of alternative dark energy theories, including dark energy whose density can vary in time or that results from a breakdown in the behavior of gravity at very large scales. To test competing theories, scientists need to gather more experimental data.

Sehgal, leading an effort by the ACT team, took an important step in this direction by analyzing the formation pattern of large galaxy clusters, enormous structures in the universe comprising dark matter, hot ionized gases and hundreds of thousands of galaxies. The formation of these galaxy clusters is governed by the interactions between dark energy and gravity. By examining the number of clusters and their distances from us, scientists can learn more about dark energy's properties.

The team harnessed the power of ACT to collect high-resolution microwave images of the night sky. Sehgal then identified large galaxy clusters by the telltale way the hot gases within the clusters dimmed or brightened the cosmic microwave background radiation at certain frequencies.

"The CMB acts as a backlight," Sehgal said. "The galaxy clusters scatter the microwave radiation as it passes through them, in effect casting 'shadows' that we have been able to identify. The really special thing about this shadow signal is that it does not fade with distance."

In its first season, the ACT team identified 23 clusters, approximately half of which were previously unknown. The discovery of new clusters highlights the power of CMB observations to spot extremely distant galaxy clusters. Once the galaxy clusters are discovered, optical wavelength observations are used to determine their distance. Sehgal led the effort to analyze the new data in order to distinguish between different competing dark energy theories.

"Each model for dark energy makes a prediction that you should see this many clusters, with this particular mass, this particular distance away from us," Sehgal said.

Sehgal tested these predictions by using data from the most massive galaxy clusters. The results support the standard, vacuum-energy model for dark energy.

These results are an important step toward settling the dark energy debate. Scientists will continue to probe the nature of dark energy by carrying out analyses similar to Sehgal's with additional sets of data, provided by new instruments such as the South Pole Telescope and the Planck satellite.
The Atacama Cosmology Telescope: Cosmology from Galaxy Clusters Detected via the Sunyaev-Zel'dovich Effect - N Sehgal et al
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