Great Void in Eridanus
(WMAP Cold Spot)

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, GSFC, NASA

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).

Rudnick et al, NRAO,
AUI, NSF, NASA

Larger microwave and radio images.

Initially, it was unclear whether
the WMAP cold spot was a
primoridal or relatively local
anomaly (more).

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).

Bill Saxton, NRAO,
AUI, NSF, NASA


Larger illustration.



The billion-light
-year wide void
or "hole" is
estimated to be
located between
six to 10 billion
light-years away
(more).