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The Mystery of Dark Energy

Mercury Winter 2008 Table of Contents

by Robert Naeye

Ten years ago a startling discovery stunned the astronomical community. Contrary to expectations, the expansion of the universe is accelerating, and remote galaxies are speeding away from us so rapidly that they will someday disappear from view. This finding was so profound that it has revolutionized our understanding of the universe's past, present and future.

expanding universe illustration

This illustration reveals the changing rate of cosmic expansion: the shallower the curve, the faster the rate. The curve changes noticeably five or six billion years ago.
Image courtesy of Ann Feild (STScI)

To picture the effect of a runaway universe, imagine an inflating balloon that can grow for eons without bursting. At first, an operator pumps air into the balloon at a constant rate, causing it to inflate in a smooth and steady fashion. As the operator ages, the pumping slows and the balloon's rate of expansion also starts to slow. But after a few billion years, the operator's teenage son takes over and cranks up the pressure. Suddenly, the balloon begins expanding faster and faster. This scenario is somewhat analogous to what's happening today.

Michael Turner of the University of Chicago coined the term "dark energy" to describe the unknown force that is hastening cosmic expansion. Whatever it is, dark energy makes up the bulk of the universe's mass-energy budget — about 74%. Of the rest, 22% consists of dark matter and a mere 4% of the cosmos contains the type of matter we can actually see. To put it another way, scientists are in the same position with respect to dark energy as someone who has no idea what water is despite the fact it covers more than 70% of Earth's surface. Explaining dark energy's nature is one of the greatest challenges in modern physics.

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