Lars Hernquist, professor of astronomy. Photo by Jon Chase.

The key to the structure and history of the universe may lie not in stars and galaxies but in the space between them, according to astronomer Lars Hernquist. Based on the idea that what's in this space is similar wherever you look, the newly tenured professor of astronomy is building a model universe in a computer at the Harvard-Smithsonian Center for Astrophysics (CfA).

For Hernquist, looking from Earth at distant, very intense sources of light is like seeing a flashlight shining through the dust and gas of the universe. "The distribution and movement of this material traces the large-scale structure of the universe," says Hernquist. "That distribution, we think, is determined by how the universe evolved. Using this information, it should be possible to build an actual model of the universe in a computer."

This is the kind of thinking and research that led Harvard to lure Hernquist away from the University of California, Santa Cruz, to its Department of Astronomy.

"Professor Hernquist works in a very exciting and frontier area of cosmology, using large-scale computer simulations to study the evolution of the universe," says Ramesh Narayan, professor of astronomy and chairman of that department. "Few others are doing this and we feel it's very important for us to get into this area. Hernquist is one of the best in the world, and we are delighted that he accepted our offer. Along with being an extremely good scientist, he is a very nice guy. I love the idea of having him as a colleague both for his scientific expertise and human qualities."

"Hernquist's computer simulations are widely appreciated for their important and surprising insights into the behavior of galaxies and cosmological structure," adds George Rybicki, professor of the practice of astronomy and associate director of the CfA. "His appointment definitely puts Harvard in the forefront of a field that promises to become increasingly important in the future."

Hernquist came here in August and, so far, likes the place. "People in the Department [of Astronomy] are approachable and friendly, and the environment is supportive," he notes. "I haven't taught undergraduates yet, but I've talked with a lot of graduate students. They impress me with their motivation, and, in turn, seem interested in what I'm doing."

Building a Supercomputer

In December, Hernquist and some California colleagues made observations with the Keck telescope, the largest in the world. It sits atop 13,000-foot-high Mauna Kea, an extinct volcano on the island of Hawaii. The group measured gases illuminated by a powerful quasar, the light from which may come from the collision of two galaxies, each containing billions of stars.

Material between here and the quasar absorbs light from that colossal celestial flashlight, allowing the astronomers to identify its composition and map its distribution. Hernquist compares this information about what is actually there with what his computer model predicts should be there.

"Such models help us to understand features we observe directly and to put realistic limits on our theories of the structure of the universe," he explains.

Hernquist is building a supercomputer at CfA by hooking together 70-80 off-the-shelf personal computers. These computers will be connected in parallel so the machine can work on different aspects of the same problem simultaneously. Besides projects such as the structure of the universe and the collision of galaxies, Hernquist plans to work on the formation of the first stars in the early universe.

He describes his work on colliding galaxies as "fun and exciting." Collisions of spiral disks and spheroids, each containing hundreds of billions of stars, are the most powerful source of energy in the universe when they evolve into quasars. Most are so far away, however, that they appear only as pinpricks of light. To better understand them physically and to appreciate their awesome beauty, Hernquist brings them down to Earth in computer simulations.

One of his simulations, the violent merger of two spiral galaxies like the Milky Way, is part of a movie called Cosmic Voyage shown at the Air and Space Museum in Washington, D.C.

In these enormous clashes, gravity drives huge volumes of interstellar gases to the center of the surviving galaxy. Heated to millions of degrees, the electrified gases spin around and fall into black holes, which may already be there or may form during the collision.

"One goal of working on the large-scale structure of the universe is to predict where and when galaxies formed," Hernquist notes.

Little Green Men

Now 44, Hernquist's interest in astronomy and science goes back to elementary school. Growing up in Princeton, N.J., where his father worked as an electrical engineer, he enjoyed his own small telescope. At a private high school in the area, he took all the courses available in chemistry, physics, and math.

Hernquist began his studies at Colgate but switched to Cornell after two years. During the break between schools, he worked as a research assistant for a company that was designing a fusion reactor. Such machines attempt to produce power by the fusion of hydrogen atoms, the same processes that fuel the hydrogen bomb and the burning of the sun.

After earning a degree in physics from Cornell in 1977, Hernquist chose the California Institute of Technology for graduate school. There, he began leaning more toward basic physics than astronomy until he met astrophysicist Roger Blandford. With Blandford, Hernquist started working on neutron stars, sunlike stars that have burned up their hydrogen fuel and collapsed.

Some neutron stars rotate rapidly and give off regular, intense bursts of radio waves. When first discovered in the 1960s, astronomers thought these were messages being sent by alien civilizations. They called these waves LGMs for "Little Green Men."

By the 1980s, neutron stars had become unoccupied objects of scientific interest only. Hernquist studied them for his Ph.D. thesis, which he received in 1984.

While at Caltech, Hernquist also began working on computer simulations of galaxies. "I wanted to try something different, and this was very appealing to me," he recalls.

Twin interests in astrophysics and computer simulations carried him through academic positions at the University of California, Berkeley; Princeton University; and the University of California, Santa Cruz. He did research and taught at Santa Cruz from 1990 to last year.

At Caltech, Hernquist met his wife, Dale, then a postdoctoral fellow in applied mathematics. They now have two children, Kirsten, 8, and Thomas, 5, and a house in Lexington. Together, they enjoy exploring the Boston area's historic sights and engaging in winter activities like ice-skating and cross-country skiing.