Great Void in Eridanus
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In March 2009, astronomers using the the 1.2-metre UK Schmidt Telescope in Australia published a paper which revealed the discovery of an even larger, 3.5- billion-year-wide void while undertaking the Six Degree Field Galaxy Survey in the southern hemisphere (David Shiga, New Scientist, April 4, 2009; and Jones et al, 2009).
In May 2008, two astronomers (Kendrick Smith and Dragan Huterer) announced that the so-called void may be a statistical artifact, in a paper to be submitted to the Monthly Notices of the Royal Astronomical Society. On the other hand, the WMAP Cold Spot is real. Hence, the void could be confirmed by pointing an optical telescope at the cold spot and counting the number of galaxies at different distances to build a 3-dimensional map of the region and see whether there is a large empty patch or not Steve Nadis, New Scientist, May 19, 2008; and Smith and Huterer, forthcoming 2008).
WMAP Cold Spot
Located in Constellation Eridanus, the River, this cosmologically anomalous region was found during 2004 in a map of the Cosmic Microwave Background (CMB) radiation made by NASA's Wilkinson Microwave Anisotopy Probe (WMAP) satellite. The CMB is an imprint of microwave radiation left from about 379,000 years after the Big Bang, the hypothesized beginning of the universe, and so provides a "picture" of conditions at a very early point in time. As measured by the WMAP satellite, irregularities in the CMB show structures already present, representing temperature differences that vary by only millionths of a degree.
Larger microwave image.
Big-Bang-era photons redshifted to
microwaves suggest that the early
universe was very smooth, although
slight energy fluctuations enabled
gravity to create concentrations of
dark and ordinary matter (more at
WMAP and APOD).
Found within a 25-degree depiction of CMB emission, this region was known as the largest WMAP Cold Spot, where colors represented very small variations (parts in 100,000) around the average temperature of 2.7 degrees above absolute zero and where blue is colder. Astronomers wondered if the cold spot was intrinsic to the CMB and so indicated some structure in the very early universe, or whether it could be caused by something more closer through which CMB radiation had to pass on its way to Earth. Given the hypothesized nature of "dark energy" that appears to be accelerating the expansion of the Universe, CMB photons should gain a small amount of energy when they pass through more typical regions of space with ordinary and/or dark matter and so appear warmer (or more cosmologically red-shifted). On the other hand, those CMB photons that pass through a void lose energy and should make the CMB from that part of the sky appear cooler (less cosmologically red-shifted).
To investigate this cold spot, astronomers used radio waves collected by the NRAO Very Large Array Sky Survey (NVSS, see Brian Kent's Powerpoint presentation) to develop a heavily smoothed picture of the region showing blended emission from radio galaxies along each path, where blue colors represent areas with surface brightness that dipped between 20 and 45 percent below the average. The radio study found an unusually low number of cosmic radio sources between Earth and the CMB cold spot, which led them to deduce that the cold spot is the site of a giant void of space. While independent confirmation is still needed, the slightly colder temperature of the CMB in this region appears to be caused by absence of nearly all matter in a tremendous void of around a billion light-years in diameter (280 mega-parsecs) that lies roughly six to 10 billion light-years (1<=z) from Earth (more from APOD; NRAO press release; Rudnick et al, 2007; Cruz et al, 2007; and Martínez-González et al, 2006).
The apparent development of a large void of some billion light-years in diameter appears to be improbable given current cosmological models. However, the astronomers who discovered the great void in Eridanus believe that the void was created as a late "Integrated Sachs-Wolfe" (ISW) effect (Rudnick et al, 2007). More discussion on the late ISW effect can be found in Robert Crittenden's Powerpoint presentation on "Making the Most of the ISW Effect."
Up-to-date technical summaries on the Great Void may become available at: NASA's ADS Abstract Service for the Astrophysics Data System; the SIMBAD Astronomical Database mirrored from CDS, which may require an account to access; and the NSF-funded, arXiv.org Physics e-Print archive's search interface.
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