|
In the late 1990s astronomers made a revolutionary discovery: the expansion of our Universe is accelerating. One of today's greatest scientific challenges is to understand what is causing this acceleration. Lacking any deep understanding of this mysterious force, and without the ability to probe it directly with existing technology, physicists coined the generic term "dark energy" to describe it. Whatever dark energy actually is, detailed measurements from independent techniques reveal that it comprises a whopping 74% of our Universe's total mass-energy.
When Einstein formulated his General Theory of Relativity in 1915, he assumed the Universe was neither expanding nor contracting. But he knew that the gravitational pull of all the Universe's mass would tend to make the Universe contract, so he postulated that even empty space would still contain an energy—a "cosmological constant"—that counters gravity to maintain the balance. When Edwin Hubble discovered the expansion of our Universe in 1929, Einstein rejected his own idea and called it his greatest scientific blunder.
When astronomers attempted to measure the Universe's expansion rate in the 1990s, they expected that it would be decelerating due to gravity. To their astonishment, they found the opposite effect. The expansion rate, which indeed appears to have slowed in early cosmic history, is revving up again. We live in a runaway Universe!
Richard Feynman and other physicists who developed quantum theory proposed that space is filled with "virtual" particles that continually form and destroy themselves. This idea has since been verified in numerous laboratory experiments. Could the energy associated with empty space exert an outward pressure that causes space to inflate, sort of like blowing up a balloon from within? Is the current accelerated expansion related to the hyper-accelerating expansion of the early Universe (inflation)?
To answer these questions, scientists have proposed three competing concepts for the Beyond Einstein Joint Dark Energy Mission, which will be a partnership between NASA and the Department of Energy. Whichever JDEM mission is eventually selected, it will measure how cosmic expansion has changed over time.
By seeing how dark energy evolves, cosmologists will probably be able to determine whether this energy is a property of empty space as Einstein proposed, or whether it is a dynamic force field that evolves. This, in turn, may enable cosmologists to predict whether the universe will continue expanding forever, or whether it will someday collapse back upon itself in a Big Crunch. It will also likely have profound implications for our understanding of elementary particle physics and the nature of our Universe.
|
 |
|
