# Cosmologist to ponder the alpha and omega of the universe

Andrei Linde to explore the mysteries of string theory, dark energy and shape of the cosmos in free public lecture set for Dec. 15

You've heard about the Big Bang—but what about the Big Bagel? Physics Professor Andrei Linde will give cosmology buffs something new to chew on at a free public lecture about the origin and the fate of the universe. Linde's lecture, scheduled for Dec. 15 at 8 p.m. in Kresge Auditorium, is part of the 22nd Texas Symposium on Relativistic Astrophysics, to be held on campus Dec. 13-17 but initially launched from the Lone Star State. About 500 physicists will attend to discuss cosmology, gravitation, particle physics and more.

"There are three standard textbook models of the universe—a homogeneous closed universe, looking like a sphere of a finite size, an infinite open universe and an infinite flat universe," Linde wrote in an e-mail. "I will argue, however, that it is rather unlikely that our universe is described by any of these models. The reason is that typically it requires a lot of energy to create a homogeneous, spherically symmetric universe, but it is relatively easy to create a topologically nontrivial universe looking like a bagel. In a certain sense, one can say that spheres are expensive but bagels are free."

That's food for thought. And it may necessitate rewriting cosmology textbooks, which for the last 50 years have presented models of the universe that are simple, popular and probably unrealistic, according to Linde.

"The usual textbook open or flat spherically symmetric universes are infinite, which makes them difficult to create," he said. "Closed universes are finite, so if they are small enough, they can be easily created. However, small closed universes tend to collapse very soon. A long-living closed universe typically must be huge from the very beginning—though there are some exceptions from this rule. Meanwhile, a bagel-like universe can be born small, and despite this initial smallness it can survive for a long time, until the beginning of cosmic inflation. Then inflation makes it huge and suitable for our life."

But enough about bagels. Linde's talk is also about subatomic spaghetti—string theory. A mathematical description of the universe based on vibrating strings rather than conventional, point-like elementary particles, string theory promises to explain the four fundamental forces—electromagnetism, gravity, the weak nuclear force, the strong nuclear force—in a unified way.

String theorists posited in 2003 that our part of the universe eventually may become 10-dimensional. While we experience four dimensions—three of space and one of time—string theorists suggest the existence of at least six other spatial dimensions that are compacted so much that they're invisible. Consider an ant crawling across a picnic blanket. It might experience the East-West and North-South aspects of its two-dimensional environment without sensing a third, Up-Down, dimension, until it reaches a corner that has curled under. Similarly, what if our three dimensions of space are linked to compacted dimensions?

In 2003, Linde with co-authors Shamit Kachru and Renata Kallosh, both of Stanford, and Sandip Trivedi of the Tata Institute of Fundamental Research in India proposed a theory of dark energy—the residual energy in empty space that causes expansion of the universe to speed up—based on string theory. The theory grew in popularity in 2004 and provided the basis for string theory landscape, a term coined by Stanford's Leonard Susskind, the Felix Bloch Professor in Physics.

"The idea is that string theory has an enormously large number of possible vacuum states; some people estimate this number to be 10^{1000}," explained Linde. "The set of all possible vacua is called the string theory landscape, because each of these vacua corresponds to a local minimum in a specific surface describing potential energy; this surface resembles a mountain landscape. Our universe may jump from one vacuum state to another, and becomes divided into an enormously large number of exponentially large parts with different laws of physics in each of them."

The fate of our part of the universe in the distant future depends on the properties of this landscape, according to Linde: At first, it will expand and become empty, and then it may either collapse or become 10-dimensional. But before you reach for a piece of cake to comfort yourself about the gloomy future, Linde offers some icing: "The whole universe will exist forever, even if each of its parts is doomed to die."

Symposium organizers include the Kavli Institute for Particle Astrophysics and Cosmology, the Stanford Linear Accelerator Center and the Department of Physics. Superstring theorist Brian Greene, author of *The Elegant Universe* and host of the TV series of the same name, will open the weeklong symposium with a technical talk.