Mapping Galactic Foam : SMITHSONIAN ASTRONOMER MARGARET GELLER PLOTTED THE BUBBLE STRUCTURE OF THE UNIVERSE. NOW SHE'S WORKING TO FIND OUT HOW IT GOT THAT WAY.

On a wall of her office Margaret Geller has hung a picture of the stickman--her stickman. It is not large, perhaps a foot on each side. As stickmen go, in fact, this one is just average--or mind-bogglingly huge, depending on how you look at it. It is made up of astronomical structures extending for hundreds of millions of light-years. As most of us will see it, however, it's just this, a cartoon figure outlined by glowing galaxies set against the dark emptiness of space.

For the past 20 years Geller, an astronomer and professor at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, has mapped the universe by plotting positions of galaxies. In 1986 the first of her maps, often called the stickman map, was evidence of something few had believed possible: on the largest scale, the universe has a distinct structure. With more plots, it became clear that the stickman was part of a pattern in which "walls" of galaxies surround vast areas with very few galaxies. Suddenly, the stickman map heralded a sea change in human perception.

In public lectures on her work, Geller likens the universe's 3-D pattern to soap bubbles or foam. (Imagine, if you will, a universe comprised of your kitchen sponge, its air pockets delineated by walls of galaxies instead of sponge material.) Though astronomers have argued over which--bubbles or foam--is more accurate, Geller hasn't let that bother her. Having convinced the astronomical community with the stickman map, she is set on cracking the universe's next big puzzle: how it got this way.

"One of the great challenges of modern cosmology is to discover what the geometry of the universe really is."

Margaret Geller's clear, ringing voice, a remnant of childhood acting lessons, reaches even the farthest corners of a sloping lecture hall at the Harvard Science Center. Freshmen to seniors, English to economics majors, listen attentively. Their professor has notes, but rarely consults them. For 15 years, she has taught Astronomy 14, "The Universe and Everything," for anyone interested in the space we inhabit. The course is nearly always filled.

Today's lecture, on Einstein's general theory…