Since the earliest days, stargazers have looked to the sky and wondered: Where did the stars come from? Is the universe infinite? If not, where does it end? What would that look like?

Though many of the mysteries surrounding the size and age of the universe persist, very recent discoveries in the field of cosmology have led scientists to develop a new picture of the universe.

Penn’s Department of Physics and Astronomy is at the center of that discussion, part of a worldwide collaborative effort to gain an understanding of what the universe looked like less than a nanosecond after the Big Bang.

What we know now, says Mark Trodden, the Far R. and Eugene L. Langberg Professor of Physics in the School of Arts and Sciences, is that less than 5 percent of the universe is composed of atoms—the visible mass that makes life on Earth possible. Another 22 percent of space is thought to be dark matter, an undetectable form of mass thought to affect the orbits and evolutions of galaxies.

“Through experiments and observation, we know that there’s a lot of dark matter out there in the universe,” Trodden says. “The big question for people like me [theorists] is, ‘What is this stuff?’”

Trodden recently co-authored an article in Scientific American on the composition of dark matter and dark energy, a mysterious, invisible force that is believed to make up the remaining 74 percent of the universe. In the article, Trodden asserts that the universe is expanding at a rate faster than previously thought. This requires either a new form of energy in the universe—dark energy—or a more radical possibility that Einstein’s Theory of General Relativity might “need revising.”

The bulk of Trodden’s work is done in his office, with pen and paper, working out complex mathematical equations. When he arrived at Penn two years ago, there was, he says, an extremely respected and well-established group studying particle physics—the elementary laws of nature underlying physics at its most basic level. Penn also had a long-standing and world-class group studying cosmology—people who are interested in theory and the observational practice of looking at the universe and extracting how and why it looks the way it does.

Traditionally, the two groups worked separately. But Trodden and co-director Bhuvnesh Jain saw great potential for building on existing efforts at Penn and bridging the gap; the Center for Particle Cosmology was born.

“Observational cosmology can tell you a great deal about what is actually happening—namely that the universe is expanding rapidly. The question for a physicist is: Why is that happening? What are the fundamental physical laws that underlie that process?” Trodden says. “The purpose of the Center is to make interactions between the two groups easier and more fluid. We want to open new collaborative opportunities between them and to bring in visitors from the outside to facilitate the kind of work that joins those two fields together.”

Upcoming experiments in both cosmology and particle physics will provide new data on dark matter, dark energy and the physics of the early universe. Trodden is particularly excited about Penn’s involvement with breakthrough ATLAS research taking place at the Large Hadron Collider, the world’s largest particle accelerator. Planned experiments there could answer decades-old questions about the nature of dark matter. “There are a number of reasons to think that the physics responsible for making the standard model of particle physics work the way it does also should be responsible for a new particle in the universe, which should have the right sort of features to be the missing dark matter of the universe.”

There’s good reason to think the two are tied together, he explains.

“If that’s the case, then not only will the Large Hadron Collider help us better understand fundamental physics, it may also provide a window into dark matter. We may be able to create and study dark matter there.”

In Woody Allen’s movie “Annie Hall,” the thought of an expanding universe that could collapse upon itself leads a nine-year-old to stop doing his homework. “What’s the point?” he asks. When Trodden looks at the stars, he’s filled with great optimism about what the next decade of research may yield.

“One thing we’ve discovered in the last six to seven years is that changing Einstein’s theory in a theoretically consistent way that fits in with all we observe is a very tricky thing and very hard to do,” he says.

With pen and paper in hand, ready to crunch data provided by his peers studying the night sky, Trodden, for one, is looking forward to the challenge.