Dark Age of the Universe
Abraham Loeb lights it up with his research
By William J. Cromie
Abraham Loeb has a reputation as a tireless worker who peppers his colleagues and students with new ideas for scientific research, and who ministers to an unusually large congregation of graduate students.
The newly appointed professor of astronomy sees tenure as "an opportunity to do long-term research with greater peace of mind. I can now focus without distractions on what I regard as substantial problems in our understanding the universe," he says.
Loeb, 34, works with seven graduate students in both astronomy and physics. "I have more ideas than I can address alone. I enjoy sharing them with students because they work hard, have fresh perspectives, and show none of the biases that established scientists often do."
"Avi is enormously productive, energetic, and dedicated," says George Field, Robert Wheeler Willson Professor of Applied Astronomy. "He worked as a physicist in Israel then switched to astronomy; now he lectures in both fields."
"Avi is the only person I know who has more good ideas for research than he has fingers and toes," notes astrophysicist John Bahcall of the Institute for Advanced Study in Princeton, N.J., where Loeb worked before coming to Harvard in 1993. "In addition, he can calculate just about anything that is calculable."
Shining Light on the Dark Age
Loeb is most interested in exploring what he refers to as "the dark age of the universe," the 1-2 billion years between the faded glow of creation and the lighting of stars.
After its creation 10-15 billion years ago in the mother of all explosions, called the Big Bang, the universe was a homogenous sea of cooling gas. Slight fluctuations in gravity rippled this smoothness, however, and gravity's force pulled the ripples into waves of matter that condensed into the first objects with separate identities.
"Each of these first structures had a mass of 1,000 to 100,000 suns, very small on the scale of the universe," Loeb believes. "They then clustered together to form galaxies that we see today. That probably occurred when the universe was 10 percent of its present age, or 1-2 billion years old."
Loeb is a theorist, one of the few ever promoted to tenure from the ranks of Harvard astronomers. He realizes that, to be valid, his theories must make predictions that his colleagues eventually confirm by observations.
"Philosophers argue questions for centuries without resolution, but theories in science must be pinned to reality," Loeb says. "Observations provide that reality."
"New generations of instruments such as the Hubble Space Telescope enable us to see farther out in space and so further back in time," Field points out. "Avi is working on a theory that predicts what we will see."
For example, quasars are the farthest points of detectable light. Their incredible brightness, equal to 100 galaxies like our Milky Way, makes them visible. According to most theorists, the stupendous energy of a quasar comes from a black hole located at the center of a distant galaxy. The hole, the most powerful gravitational attractor known, is sucking gas into its maw the way a sink drain pulls in water. The renting apart, rapid spinning, then "swallowing" of gases releases visible light, X-rays, and other radiation that astronomers see shining from quasars.
According to this idea, the galaxy came first, then the black hole. Loeb thinks it happened the other way. "How does a black hole form in a galaxy?" he asks. "How can you get gas spinning into a hole unless the hole is already there?"
Loeb believes that some of the original ripples in the primordial sea of space became gravity sinkholes rather than stars. These tiny whirlpools measured no more than our solar system, a scant 3-4 billion miles across.
"One out of 1,000 or 10,000 objects in original space became black holes rather than groups of stars," Loeb estimates. "When a galaxy formed in the same neighborhood, a black hole 'seed' sank into its center and began devouring the gas of which stars are made. That's not a popular idea, but it's my view of how things had to happen."
Search for Planets
A national panel of scientists recommended last month that the focus of future space research should be a search for the origins of the universe, planets, and life. As a man of many ideas, Loeb has thought of an efficient way to look for unknown planets.
When light passes near a star, it is deflected by the star's gravity. If a distant background star lies directly behind a closer star, then an observer on Earth could see light from the former as a ring around the latter.
"The closer star acts like a lens, focusing and intensifying light from the background star," Loeb explains. "If planets orbit the closer star, they, too, could magnify light from the distant source. Thus, they could be detected as occasional blips of light on the ring."
If Earth was lined up with a star halfway to the bright center of the Milky Way, it should be possible to see a planet orbiting the star at a distance of about 500 million miles. If aliens are looking carefully at the sun this way, they should see Jupiter.
"Using different methods, 10 massive planets have recently been found orbiting sunlike stars," Loeb points out. "With the lensing technique, it should be possible to see smaller, Earthlike planets."
His colleagues say that Loeb never stops coming up with such ideas, and he is quick to share them with students. He currently works with three graduate students from the astronomy department and four from the physics department, as well as collaborating with two at Columbia University, where his wife goes to school.
"I knocked on his door one day," recalls Daniel Eisenstein, now a postdoc at the Institute for Advanced Study. "I didn't know it was his first day here; his computer hadn't even been hooked up. He welcomed me and we talked about two hours. He wound up giving me an interesting problem dealing with the evolution of galaxies."
Last year, Loeb won a Hoopes Prize for excellence in undergraduate teaching. "I enjoy interacting with students," he comments. "When I share problems with them, both of [us] learn from the effort to solve them."
From Farm to Physics
Loeb grew up on a farm in the small village of Beit-Hanan, near Tel Aviv. "At school I was interested in philosophy but good at physics and sports," he recalls. "At age 18, I was selected for the Talpiyot project, which allows about 25 high school graduates from all over Israel to participate in a program of study and research as part of their compulsory military service."
For the next eight years, Loeb did research in plasma physics while earning graduate degrees in physics at the Hebrew University of Jerusalem. In 1985, he was picked as the best master-of-science student. Two years later, he won the Kennedy Prize for his Ph.D. research.
In addition to academic pursuits, Loeb did applied research that resulted in a patent for a method of accelerating projectiles to high velocity using a plasma, or electrically charged gas. This work received support from the U.S. Department of Defense, a connection that led him to visit the Institute for Advanced Study (IAS) on a trip to this country in 1986. A lunch with John Bachell led to an invitation for a four-week stay at IAS.
The year after receiving his Ph.D. in 1987, the IAS offered Loeb a five-year position. The switch from physics to astronomy was unusual, but Loeb liked the idea of trying to solve the biggest mysteries of the universe. Bachell and his colleagues felt that Loeb would be good at it.
After Princeton, Loeb came to Harvard as an assistant professor in 1993, then advanced to associate professor two years later. While here, he received tenure offers from Cornell, the University of California at San Diego, and the Weizmann Institute in Israel.
Loeb wanted to stay at Harvard, but didn't know if the University would give him tenure. It had been decades since the astronomy department had promoted an associate professor.
While waiting, Loeb went to New York City every few weeks to be with his wife, Israela, who is a research scientist and Ph.D. student in psychology at Columbia University. During a visit last October, they came home from an afternoon at the New York Botanical Gardens to a recorded message from Jeremy R. Knowles, Dean of the Faculty of Arts and Sciences. Loeb remembers that "when I called the Dean back, he said, 'President Rudenstine and I are delighted to offer you a tenured position.' I responded positively, then Dean Knowles said, 'You should celebrate with an expensive bottle of wine.' I don't drink, but my wife went down to the grocery store and bought some grape juice."
When Israela graduates in 1997, there will be four Ph.D.'s in the family. After raising her children, Loeb's mother, Sarah, went back to school and earned a Ph.D. in comparative literature last month. His sister, Shoshana, holds the fourth Ph.D.
"I could have gone in a number of completely different directions during my life," Loeb muses. "I never really worried about having a job; I felt that I could always go back to the family farm. Now, I can spend the rest of my life doing research. Tenure at Harvard is something to be proud of, but 'professor' is just a title; you have to fill it with substance, such as good research."
Copyright 1998 President and Fellows of Harvard College