Harvard astronomer Margaret Geller gives a demonstration during her May 7 lecture in Rockefeller Hall. Adriana Rovers/University Photography
By Larry Bernard
Mapping the nearby universe over the past 15 years has yielded only a small fraction of the observable universe, but new telescopes on the horizon will help make the job easier, according to a scientist who has been making those maps.
"The universe is a very big place. We have mapped only a small fraction of it," said Margaret J. Geller, astronomer at Harvard University and the Harvard-Smithsonian Center for Astrophysics. "If the universe were the size of the Earth, we've mapped a portion as big as the state of Rhode Island."
Geller made her remarks in a talk, "So Many Galaxies . . . So Little Time," at the Bethe Lecture May 7 in Schwartz Auditorium, Rockefeller Hall. The Bethe Lecture Series was established by the Physics Department and College of Arts and Sciences almost 20 years ago to honor Hans A. Bethe, Cornell professor emeritus of physics.
Geller, who was born in Ithaca but whose family moved when she was 2, showed a variety of slides of galaxies to explain how galaxy mapping is accomplished. The first step, she said, is to take pictures of the sky, which yields a two-dimensional map. But only in the last 15 years have computers and better optics made it possible to map the universe in three dimensions. Light from distant galaxies is shifted toward the red end of the spectrum because the galaxies are receding from us. The further away they are, the greater the recession velocity. Thus, by measuring the redshifts of galaxies, astronomers determine their distance from us.
"The goal is to determine, what does the universe look like? Are there any big patterns in the universe, any large structures?" Geller said.
Using the 60-inch telescope atop Mount Hopkins in Arizona, she and colleagues can measure redshifts of about 1,000 galaxies per year. A three-dimensional slice of the universe indeed shows a large pattern, known as the "Harvard Stickman," for its resemblance to a stick figure. A larger slice of the universe shows a great density of galaxies, known as the Great Wall -- thousands of galaxies clustered along the plane -- with great black voids in front and beyond them 150 million light years across.
"The patterns are obvious," Geller said. "There is not uniform distribution of galaxies in the universe." But, she cautioned that the map only shows about 4 percent of the observable universe, so it may not be representative. "We've learned the universe does have very large patterns, but we still really don't know how big."
In an effort to understand how the universe came to be the way it is, Harvard is building a 6.5-meter telescope on Mount Hopkins. "This telescope can easily reach 30 to 40 percent of the visible universe, which means we can see the universe when it was 40 percent as old as it is today," Geller said.
The new telescope, which is expected to see first light in 1997, will enable Geller and her associates to observe 1,000 galaxies a day, not the 1,000 a year they currently observe.
Geller concluded her talk by showing a clip from her movie, "So Many Galaxies ... So Little Time." In it, a scientific visualization takes the viewer on a trip "not available from your travel agent," Geller said -- through the Great Wall of galaxies, a 500-million-light-year excursion in about two minutes.