Deep images favor expanding universe.

According to the Big Bang theory, the universe began in a giant explosion.

Ever since, the cosmos has ballooned in size, carrying galaxies farther apart.

But gravity slows that expansion and determines the fate of the universe. If gravity is weak, the universe will remain open, expanding forever. However, if the universe contains sufficient mass, it will eventually close in on itself: Gravity will halt cosmic expansion, crashing galaxies together in a Big Crunch. In a third scenario, the universe is delicately balanced between expansion and collapse, possessing just enough gravity to halt inflation without contracting.

Although cosmologists have by no means reached a consensus, recent evidence suggests that the cosmos will grow indefinitely. An analysis of some of the deepest cosmic images ever taken supports this idea.

In the new findings, presented last week in Liverpool, England, at the Royal Astronomical Society's National Astronomy Meeting, Thomas Shanks, Nigel Metcalfe, Ana Campos, and their colleagues at the University of Durham in England relied on a standard method. Gravity curves space in such a way that the volume of universe within a given radius from Earth is greater for an open universe than a closed one. The number of galaxies in that volume will therefore also be greater, so counting them could indicate the nature of the universe.

So far so good. But the difference in volume between an open and a closed universe becomes significant only when astronomers consider a chunk of universe containing distant objects. Unfortunately, galaxies located far from Earth are often too dim to detect. Enter the Hubble Deep Field, a set of Hubble Space Telescope images that provide detailed portraits of galaxies deep in space and far back in time (SN: 1/20/96, p. 36). Armed with these images, as well a smaller set his team took with the William Herschel Telescope in the Canary Islands, Spain, and the United Kingdom Infrared Telescope atop Hawaii's Mauna Kea, Shanks' team found that the number of galaxies increases sharply as their brightness decreases.

In interpreting this finding, the astronomers tried to avoid some well-charted pitfalls. A dim galaxy doesn't necessarily signify a distant galaxy, and as telescopes peer further back into time, the galaxies they detect may differ in size or brightness from galaxies seen today. In particular, if more distant galaxies shine more brightly, astronomers might overestimate their number.

After taking these confounding effects into account, the team still finds a large number of faint galaxies. The count, Shanks says, in most cases falls in the range predicted for an open universe, approximately four times that expected for a cosmos poised between expansion and contraction. He notes, however, that the data from ground-based infrared images don't distinguish between the models.

Mark Dickinson of the Space Telescope Science Institute in Baltimore notes that using the Hubble images to count galaxies is an important step, 'and these are the first people to make a careful attempt.'