Every night our sky is lit up with pinpricks of light constituting the billions of stars and galaxies that fill our universe. And every night, although imperceptible to the human eye, those pinpricks of light are farther and farther away. Our universe is expanding, but contrary to logic, gravity is not slowing down the expansion. In fact, the rate of expansion is accelerating, and physicists, specifically researchers in the UC Davis Cosmology Group, are just beginning to figure out why.
The culprit is dark matter — a form of matter permeating the entire universe and believed to be the underlying cause of the universe’s accelerating expansion.
“There is more dark matter than there is anything else in the universe,” said Andreas Albrecht, chair of the UC Davis physics department. “About three-quarters of the stuff in the universe is dark matter.”
Dark matter, as its name implies, is dark. It is invisible to the eye, and invisible to every astronomical sensor we have. We are only aware of its existence through indirect observation and the effect it has on surrounding celestial bodies such as galaxies and galactic clusters. Dark energy is related to dark matter in the same way energy is related to mass in that famous equation, E = mc².
Imagine throwing a ball into the air. After the ball reaches a certain height, gravity pulls it back down. Now imagine that instead of the ball slowing down as it moves away from you, it accelerates, moving faster and faster until it is gone. That is essentially how dark energy functions in our universe. It is a force that is pushing the universe apart.
“The only force that matters over a long distance is gravity,” said Robert Becker, a professor in the UC Davis physics department and a member of the Cosmology group. “It is the reason that matter clumps. It is why we have galaxies and planets.”
The gravity that should be pulling the ball back down is only important on the relatively small scale of solar systems and galaxies. On a universal scale, gravity is ineffectual.
John Conway is physics professor at UC Davis and an off-site researcher working with data from the Large Hadron Collider (LHC), the largest particle accelerator ever built, at CERN in Geneva. Part of his research focuses on searching for quantifiable signs of dark matter.
“We know there is something there,” Conway said. “If we can produce collisions at the LHC where we produce dark matter particles, we will be able to see signs of its existence.”
One of the ways we have observed dark matter is through a phenomenon called gravitational lensing. Gravity has the ability to bend light, so when light from a bright object such as a supernova or a quasar passes through an area dense with dark matter, the light bends and we observe it as multiple points or as a crescent. Based on how sheared the images are, we can deduce how much dark matter is present.
The universe is about 14 billion years old, and as it continues to expand and the acceleration increases, dark energy will become a more dominant force.
“Eventually, galaxies, solar systems and even individual atoms will be blown apart by dark matter expansion,” Becker said.
The elusive nature of dark matter is what makes it such an active area of interest. The American Association for the Advancement of Science recently named dark matter one of the top questions in science, and the 2011 Nobel Prize for Physics was awarded to Saul Perlmutter, Brian Schmidt and Adam Riess for dark matter’s discovery.
“We are far from a deep understanding. We know that it works, but not why or how,” Albrecht said. “The puzzle is figuring out where that force originates from.”
Topics such as dark energy comprise the grand questions of our universe. Every researcher in the field started out searching for the answers to these grand questions.
“The exciting thing about physics is how solid it is,” Albrecht said. “Newton’s laws, Maxwell’s equations…they are tested to death. Revolutions in physics only happen under careful scrutiny, and nothing short of a revolution in physics is needed to explain the acceleration of the universe.”
HUDSON LOFCHIE can be reached at firstname.lastname@example.org.