Energy matters

Tufts cosmologist contemplates the universe

It is Alexander Vilenkin’s job to ponder major questions such as the origin of the universe. He does so from a pale, jumbled office where books are toppled on shelves and several chairs are covered with papers and still more books. Vilenkin himself, however, is crisp and organized. Dressed in gray slacks and a light gray shirt, he speaks clearly about his discoveries in a field that is complex and difficult for most to understand.

Alexander Vilenkin © JODI HILTON

Vilenkin’s field is cosmology—the branch of astrophysics that studies the origins and structure of the universe. He is a professor in the Department of Physics and Astronomy and director of the Tufts Institute of Cosmology. By the end of June, he will have two new published contributions in that area. The first is an article in Science magazine that he was invited to write; the second is a book that will be published on June 27.

The article appeared on May 4 on Sciencexpress, a website for Science magazine at The book is called Many Worlds in One: The Search for Other Universes (Hill and Wang.)

A cosmic puzzle
Science asked Vilenkin to respond to an article written by two other physicists, P.J. Steinhardt of Princeton University and N.G. Turok of Cambridge University, and give background information for discussion. In the article, Vilenkin addresses what he calls a “puzzle,” namely understanding the energy in a vacuum. A vacuum is empty space, Vilenkin explains, yet according to the principles of quantum mechanics, it has energy, with particles popping up in and out of existence.

“You can’t utilize this energy,” he says. “The energy is locked there, and it’s always the same, so you can’t run a hair dryer on it. But it does have an effect through gravity, and it turns out that the gravitational field of the vacuum is very unusual because it is repulsive.”

The problem, he says, is that theoretical calculations of this vacuum energy density give “ridiculously large numbers…energy density that would simply blow the universe apart. So these calculations are certainly wrong, but they come from theories which have been confirmed with very high accuracy. So that is the puzzle: What went wrong? Theory does not agree with observation.”

A telltale sign
Most physicists, he says, thought that based on observation, the vacuum energy must vanish, and most theoretical efforts were directed at finding a new principle that would make vacuum energy zero. But in the late 1990s, astronomers discovered that the expansion of the universe—instead of being slowed down by gravity as one would expect—was actually accelerating. This is a telltale sign of a nonzero, but very small, energy density of the vacuum.

Vilenkin says that one idea is that vacuum energy density is not really a fixed constant, but that it takes different values in different parts of the universe. “You have this huge universe, some say multiverse, and density has high values in some regions and low in others. Where vacuum energy is high, there’s gravitational repulsion, and it will prevent stars from forming. It’s only in the regions where vacuum energy is small that galaxies can form, and there will be stars and planets.”

Because scientists who observe the universe will only exist in regions where vacuum energy is low, that is, where stars and planets can form, the typical observer will detect a low vacuum energy. “Yet most of the universe is inhospitable to life,” Vilenkin says.

Vilenkin’s new book about the search for other universes discusses the energy density puzzle as well as other topics, including an explanation of the Big Bang. In its early moments, the universe, he says, was filled with unstable high energy vacuum that was gravitationally repulsive. “That’s what blew the universe apart, and then the unstable vacuum decayed and dumped all this energy into a fireball of matter.”

Marjorie Howard is a senior writer in Tufts’ Office of Publications. She can be reached at