link to explorelink to learning resourceslink to missionslink to whats up
link to our place in spacelink to big banglink to black holeslink to dark energy

You have arrived at an archival site.

The Universe Forum's role as part of NASA's Education Support Network concluded in September, 2009. Please visit NASA at for current information about NASA's science, education, and public outreach activities.

exploring: dark energy > where did the idea come from

Something is driving the universe apart.

For nearly a century, we've known that the universe is expanding: Distant galaxies are moving further away from us and from each other. Until recently, scientists thought this expansion must be gradually slowing down, as gravity put on the brakes. But the big surprise of the past decade was the discovery that the expansion of the universe is speeding up! Something is driving the universe apart. Scientists do not yet understand the source of this acceleration, but they have given it a name: dark energy. Whatever it is, it appears to make up most of the mass of the universe.

How do we know that the expansion of the universe is speeding up?
After all, even through the most powerful telescope, no astronomer can see galaxies move directly, even watching for an entire lifetime. It's a bit like looking at a car that is so far down the highway that we can't see whether it is moving at all. Fortunately, we can tell how fast galaxies are moving away from us by analyzing the light coming to us from the galaxy. The method is related to one used by traffic police to tell how fast a car is moving.

There's another, more difficult, problem as well. The expansion of the universe does not change fast enough for us to detect over our lifetime. We can only tell if the universe is speeding up by comparing its expansion today with how fast it was expanding in the distant past.  

You might think it would be impossible to tell what the universe was doing in the past; no one was there to observe it. But amazingly, wonderfully, nature gives us a way to see the past directly. The universe is so large, that it takes a very long time for light to reach Earth from distant galaxies. Therefore, the image that we capture today shows us what the object looked like in the past, when light left it - not what it looks like now, when we are receiving that light. By observing the motions of galaxies at different distances, astronomers can tell how fast the universe was expanding at different times in the past.

A key breakthrough was finding a technique to determine the distance to a galaxy, and therefore to tell how far back in the past we are looking. The technique rests on the happy accident that when a certain type of star dies, it explodes with a spectacular flash whose inherent brightness is known. These exploding stars, called supernovae, enable astronomers to determine the distance to the star and the galaxy in which it lives and dies: The dimmer the flash appears, the further away the star must be. In its death, a star is a beacon to all observers throughout our universe, marking out distance in the dark reaches of space.

From all this information, astronomers have been able to piece together how fast the universe was expanding at different times in its history. The result is clear: Sometime around 5 billion years ago, the universe began accelerating - its expansion getting faster and faster, rather than gradually slowing down. The conclusion: Something, which scientists now call dark energy, must be driving the acceleration of the universe. The dark energy was presumably there since the beginning of the universe, but its effect becomes more important as the universe expands.

Astronomers are able to determine how much dark energy there is as well, based on the measured rate of expansion of the universe. (They are able to do this because Einstein's theory of gravity relates the expansion rate of the universe to the total amount of all forms of mass or energy in the universe.) The result is that about 70% of the content of the observable universe must be dark energy. In other words, most of the universe is made of some mysterious form of energy whose nature is completely unknown.

Important conclusions should be based on more than a single line of evidence. Astronomers now have other independent lines of evidence that confirm the reality of dark energy. Now a new and extraordinary chapter begins: the search for the nature of dark energy. Join the search in the next section >




















Where did the idea come from?
What might Dark Energy be?
Why do we care?-

link to nasa link to smithsonian link to harvard HOME | ABOUT THE FORUM | SITE MAP | CREDITS