Expanding Horizons

Star-gazers take note: Your favorite nighttime guides are on an increasingly rapid retreat. It's a phenomenon at odds with Einstein's theory of relativity, which would see the universe falling prey to gravitational pull and expanding ever more slowly, a phenomenon Mark Trodden and his colleagues at the Center for Particle Cosmology have dedicated their careers to understanding.

"Not only is the universe expanding, but it's doing so at an increased rate," says Trodden, the Far R. and Eugene L. Langberg Professor of Physics. "Dark energy is the putative name we use to describe the force that is causing this. In terms of our understanding of gravity, expansion should be slowing, not speeding up."

This expansion, deemed cosmic acceleration, was discovered by astronomers using supernovae—think of them as the flashlights of the universe—to precisely track the rate at which the universe was growing. Einstein's theory dictates that the way in which space and time behave depends on the contents of the universe, so there must be a better understanding of the "stuff" it is made of, Trodden explains.

"Regular matter like protons and electrons accounts for less than 5% of the universe, and it has the almost universal capability to interact with light. But you have things in the universe that you don't see because they don't glow, yet you know they affect it in other ways. This is called dark matter and our best understanding of it is that it clumps up, sucking normal matter in, and creating what we know as galaxies."

This dark matter Trodden describes, not to be confused with dark energy, makes up 25% of the universe, five times that of regular matter. Due to its enigmatic nature, researchers are attempting to re-create it with technology like the Large Hadron Collider. As the world's most advanced particle accelerator, the Collider is able to mimic the conditions of the extremely volatile "early" universe, as close as one tenth of one nanosecond to the Big Bang.

"Regular matter like protons and electrons accounts for less than 5% of the universe, and it has the almost universal capability to interact with light. But you have things in the universe that you don't see because they don't glow, yet you know they affect it in other ways." – Mark Trodden

So what about the other 70% of the universe not defined by matter? It's the missing component of the energy budget that is causing cosmic acceleration, Trodden says—dark energy. And though distinctly different from the matter that makes up the other 30% of the universe, it may not be entirely independent in its behavior.

In a recent Scientific American article he co-authored, titled "Hidden Worlds of Dark Matter," Trodden investigated the potential interactions between dark matter and dark energy. He believes laboratory experiments and cosmic observation will eventually lead to a better understanding of their relationship, and how it affects expansion as a whole.

Is dark energy really responsible for cosmic acceleration, or does Einstein's theory need revising?

"The latter is a dramatic possibility," Trodden admits. "But Penn researchers have been at the forefront of exploring it. This type of collaboration between cosmologists and particle physicists is essential to solving the mysteries surrounding the unknown forces and energy in the early universe. That's what the Center for Particle Cosmology is all about—combining these fields. We're quite excited that through it, Penn is playing a leading role in understanding these fascinating phenomena."