Decoding the Mystery of Dark Energy

Mercury, September/October 2003 Table of Contents

Image Courtesy of NASA/STScI/Ann Feild. (Click here for a larger version of this image)

by Amanda Gefter

About 70% of the universe’s energy consists of a force that accelerates cosmic expansion, and scientists know virtually nothing about it. Astronomical observations and bold theories may help solve the mystery.

Some mysteries grow deeper the closer you look. From the unexpected supernovae announcements of 1998, a new world has unfolded before our eyes. Like Alice emerging from the rabbit hole and finding herself in Wonderland, we must untangle the meaning of a strange new terrain ruled by the covert forces of dark energy. The latest observational and theoretical exploration of this mystery will point the way to a more fundamental understanding of cosmic evolution and the inner workings of the universe itself.

The story begins, of course, at the beginning. In the first fraction of a second following its birth, the universe apparently expanded exponentially in a dramatic event called inflation, increasing its size by a factor of 10⁵⁰ in far less than the blink of an eye. The expansion slowed as time passed, allowing galaxies and clusters of galaxies to coalesce. With gravity applying the brakes, the expansion of the cosmos should still be slowing down. So when two teams of observers, the Supernova Cosmology Project and the High-Z Supernovae Search team, began their work in the mid 1990s, they were trying to measure the current rate of deceleration.

Using Type Ia supernovae, which result from the thermonuclear explosions of white dwarfs locked in binary systems (see "Searching for Supernovae To Be," Jan/Feb 2003), the teams were able to determine the recent expansion history of the universe. Type Ia supernovae shine with near uniformity, so by comparing their apparent brightness with their intrinsic brightness, the teams can infer a supernova’s distance. A supernova’s spectrum reveals how its light has been stretched, or redshifted, by the expansion of space itself. When the two teams completed their analyses, they were confronted with the astonishing result that the expansion of the universe was not slowing down as expected — it was speeding up.

And so cosmology’s adventures in Wonderland began. What could be causing space to expand faster and faster when it should be slowing down? Most cosmologists think there is some mysterious energy lurking in the shadows, some devious and furtive "antigravity" force called dark energy. Although cosmologists were at first loath to admit the existence of dark energy, it seems to be the most natural solution to the puzzle.

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