A Big Hole in the Sky

A Big Hole in the Sky

How many of you have been tricked by the question, How much dirt is in a hole that is one foot deep and one foot in diameter?

The typical response is one cubic foot. But the correct answer, of course, is none. There is no dirt in a hole!

Another tricky question is, How much matter is in a hole a billion light-years in diameter? This hole was recently detected by a University of Minnesota astronomer, Lawrence Rudnick, and two graduate students. What they [discovered](https://arxiv.org/abs/ 0704.0908v2) is that the correct—and truly remarkable—answer is none.

In the last several years, astronomers have conducted two surveys of the whole sky. The Wilkinson Microwave Anisotropy Probe (WMAP) mapped the Cosmic Microwave Background (CMB) radiation left over from the big bang; and the NRAO VLA Sky Survey (NVSS) mapped the radio emission at a wavelength of 21 cm coming largely from giant radio galaxies distributed throughout the universe.

While earlier examinations of these survey maps yielded some anomalies, it was Rudnick and his colleagues who made a detailed comparison between the two and realized that the large “cold spot” in the WMAP seen previously had a corresponding dip in the NVSS. They concluded the explanation is likely a large void in space, nearly a billion light-years across and about 6 to 10 billion light-years from Earth. It’s virtually empty of both normal matter (stars, galaxies, etc.) and dark matter.

Astronomers expect to see some “clumpiness” in the distribution of matter throughout the universe. Though this distribution was initially uniform early in cosmic history, gravity caused the matter to aggregate into stars and galaxies and clusters o f galaxies, leaving voids in between. Even in the face of this, though, the large-scale distribution of matter is still expected to be fairly uniform and homogeneous.

What makes this discovery so remarkable is the sheer size of this hole. Given that the approximate diameter of the observable universe is 27 billion light-years, this void is no small part of that total. Finding a void this large is so unusual tha t some astronomers question the result. How could it form? Why has it not been filled in the meantime? All of this goes to remind us that there is a lot left to discover about the Universe in which we live. Even some of the areas of knowledge we consider well established (like large scale uniformity in the Universe) cannot be taken for granted. But we do expect that as we gain further understanding of this creation, it will ultimately bring greater glory to God.