When a research team reported a hole in the cosmos last year, the conclusion went beyond the evidence. But continued study has shown it probably is right.
What it all adds up to ''is a very big volume that is a void, completely empty'' of the galaxies astronomers would expect to find there, says Robert Kirshner of the University of Michigan. There may be some dust and gas or even some galaxies too faint to be seen. But the detectable galaxies expected there seem to be absent. This makes the density of that region of space at least ten times less than the cosmic average.
This void - some 300 million light years in diameter - is the largest distinctive feature yet found in the universe. As such, it has given astronomers much to think about.
Indeed, using current cosmological theory, it is hard to figure out how the universe could evolve to leave such a hole in its fabric. Thus the hole's existence was questioned when it was first reported in October 1981 by Kirshner and his colleagues Augustus Oemler Jr. of Yale University, Dr. Paul L. Schechter of Kitt Peak National Observatory, and Stephen A. Shectman of the Mt. Wilson and Las Campanas Observatories.
Studying the distribution of galaxies, they had found a gap with no visible galaxies between 360 million and 540 million light years from Earth along three lines of sight. This suggested a great void in the northern hemisphere in the direction of the constellation Bootes. Skeptics noted that the researchers may simply have picked three sight lines that missed galaxies in the suspected void.
Observations reported last spring by Vicki Balzano and Daniel Weedman of Pennsylvania State University seemed to reinforce this skepticism. Looking at a sampling of one particular type of galaxy (called the Markarian galaxies), they found these distributed in such a way as to suggest there really isn't any void at all.
Kirshner, having looked into this, now says that would be a superficial interpretation. The Markarian sample by no means covers all the expected galaxies. The Balzano-Weedman survey does not rule out the void, Kirshner says.
Meanwhile, he and his colleagues, at this writing, were getting ready to publish their own detailed survey of the void. They now have many lines of sight. ''The evidence is that, in the northern hemisphere, there is a very large feature of very low density,'' Kirshner says. He adds that similar observations of the southern hemisphere show no break in the expected distribution of galaxies.
This means that there now is strong evidence that the void exists and is a unique feature of the universe. ''In time,'' Kirshner says, ''the controversy (about the void) will go away and we will understand it a bit better. Meanwhile, it not only challenges astronomers to recognize it and explain it, but it reminds them that the cosmos still can produce a major surprise.''