Dark Energy Is Ancient History

Nov. 17, 2006 — The mysterious anti-gravity force known as dark energy has been pushing apart the universe a lot longer than expected — for at least nine billion years — say astronomers who drew that conclusion by studying light from distant stellar explosions.

The idea that this repellant force is so persistent comes as a surprise to some cosmologists. Previous evidence supported the idea that dark energy was pushing galaxies apart as long as five billion years ago, but the new observations suggest that it has been around — in much the same form — much longer.

The discovery also supports the cosmological constant, the mathematical expression of dark energy required to make Einstein's theory of General Relativity work.

Ironically, Einstein didn't much like the cosmological constant and later expressed regret at ever coming up with something as weird as a repulsive force that perpetually carves cosmic real estate out of utterly nothing.

Dark energy is "a rather mysterious energy that appears to make up 70 percent of the energy in the universe," said astrophysicist Adam Riess of the Space Telescope Science Institute and Johns Hopkins University.

To find out how long dark energy has been at work, Riess and his colleagues used the Hubble Space Telescope to look at 24 very distant supernovae, large stars that explode at the ends of their lives.

The extreme faintness of the 24 was compared to the brightness of similar supernovae known to be closer to home. The stellar explosions recently seen by Hubble were nine billion light-years away, which means they happened nine billion years ago.

Those ancient supernovae allow astrophysicists to calculate how fast the universe has been expanding over those same nine billion years — which in turn tells them how much dark energy has played a role.

"These supernovae provide cosmic mile-markers that allow us to measure the growth rate of the universe for nine billion years," said Riess. That growth rate suggests that the role and power of dark energy hasn't changed over time.

"It's an extremely important clue" toward understanding just what dark energy is, said astrophysicist Mario Livio, also of the Space Telescope Science Institute. "We are only starting to understand its properties."

Livio drew a comparison to the early scientific understanding of water. First scientists studied water's properties and only much later did they discover it was made of hydrogen and oxygen.

Dark energy is a "complete puzzle to us," confirmed astronomer Louis-Gregory Strolger of Western Kentucky University in Bowling Green. "These observations rule out or speak against the idea of recent changes (in dark energy's nature). It remains for us to understand this 70 percent of our universe."