Edwin Hubble discovered that the universe was expanding in 1926. Scientists built new models that assumed that the expansion was slowing down as gravity tugged on stars and galaxies, pulling them back toward each other. This model seemed logical. However, a few, highly technical problems existed with the mathematics associated with this model.
Einstein tried to explain these problems by creating something he called the “cosmological constant", a force that opposed gravity. But he then rejected the idea as his greatest scientific blunder.
After receiving a Ph.D. in physics in 1986, Saul Perlmutter worked at Lawrence Berkeley National Laboratory and headed the Supernova Cosmology Project (SCP). This group used the Hubble Space Telescope to find and study distant supernovae (exploding stars). They chose supernovae because they are the brightest objects in the universe. Type Ia supernovae produce a constant amount of light, and it is believed that all Ia supernovae shine at about the same brightness. This made them ideal for Perlmutter's study.
Over the 10 year period from 1987 to 1997, Perlmutter developed a technique to identify supernovae in distant galaxies and to analyze the light they produce. His team searched tens of thousands of galaxies to find a half dozen type Ia supernovae. When Perlmutter found an Ia supernova, he measured its brightness to determine its distance from Earth. (The brighter it is, the closer it is.) Perlmutter also measured the red shift of the supernova's light. This is a technique based on Doppler shifts. If a star is moving toward Earth, its light is compressed and its color shifts a little toward blue. If the star is moving away, its light is stretched and the color shifts toward red. The faster the star is moving, the greater its color shift. By measuring the supernova's red shift, Perlmutter could calculate the star's velocity away from Earth. Now came the hard part. Other factors could ac count for a red shift, and Perlmutter had to prove that the red shifts he measured were the result of only the star's motion away from Earth. Space dust can absorb some light and shift its color. Some galaxies have an overall color hue that could distort the color of the light com ing from a supernova. Each of a dozen possible sources of error had to be explored, tested, and eliminated.
Finally, in early 1998, Perlmutter had collected reliable distance and velocity data for a dozen Ia supernovae spread across the heavens. All were moving at tremendous speeds away from Earth. Perlmutter used mathematical models to show that these galaxies couldn't have been
traveling at their current speeds ever since the Big Bang. If they had, they would be much farther away than they really are. The only way Perlmutter's data could be correct was for these galaxies to now be traveling outward faster than they had in the past.
The galaxies were speeding up, not slowing down. The universe had to be expanding at an accelerating rate!Perlmutter's discovery showed that some new and unknown force (named “dark energy” by Michael Turner in 2000) must be pushing matter (stars, galaxies, etc.) outward.
More recent research using new specially designed satellites has shown that the universe is filled with this “dark energy.” (Some estimates say that two thirds of all energy in the universe is dark energy.) Over the next few years this new discovery will rewrite human theories of the origin and structure of the universe.
