Bringing Dark Energy to Light

There’s a mysterious force that makes up about two-thirds of the universe. And it has nothing to do with Star Wars.

Scientists call it dark energy, and it is believed to be causing galaxies to move away from each other faster and faster. Now, researchers who have been trying to figure it out for more than 20 years by studying supernovae – stars that exploded billions of years ago – are hoping to send a telescope into space, where they’ll be able to get a better look.

“You can see hundreds of times more sky at a time,” said Saul Perlmutter, a professor of physics at the University of California, Berkeley. “And it’s also designed for just the wave length range, just the colors, where we need to study the supernovae and the other galaxies in order to study dark energy.”

The new NASA telescope is known as WFIRST, which stands for wide-field infrared survey telescope. If Congress approves initial development funds of $50 million to $100 million by the end of the year, WFIRST could launch sometime between 2022 and 2025.

A telescope like it has been in the works for more than 15 years, at one point developed by a Joint Dark Energy Mission made up of NASA and the Department of Energy. Now the project is back in NASA’s hands.

Despite the project’s bumpy history, Perlmutter is optimistic.

“At this point things are sounding good,” he said.

In 1929, astronomer Edwin Hubble discovered that the universe is expanding, with galaxies moving away from each other. Before Hubble’s discovery, even Albert Einstein believed that the universe was static.

Eighty years later, in 2011, Perlmutter, who also is an astrophysicist at Lawrence Berkeley National Laboratory, shared the Nobel Prize in physics for his contributions to the discovery that the expansion of the universe started to accelerate seven billion years ago.

“Just imagine that you are living here on a galaxy, and there’s galaxies forever going in all directions, nothing but galaxies, no end,” said Perlmutter gesturing with his arms. “And the only thing I mean when I’m saying that the universe is expanding is that we’re sort of pumping extra space between the galaxies. And when we say it’s accelerating, we just mean that that extra pumping is happening faster and faster and the distances are growing bigger and bigger more and more quickly.”

To figure out that the expansion was accelerating, Perlmutter and his team used the light from supernovae – stars that exploded billions of years ago – to plot out the history of the universe.

A particular kind of supernova stars, called Type 1a, explode in a very similar way every time, brightening like fireworks and then fading away. And they reach the same peak brightness every time.

Their predictability makes these exploding stars what researchers call “standard candles.” Their initial brightness is constant and grows fainter with distance. Since researchers know light always travels at 186,000 miles per second, they’re able to calculate how long ago these supernovae exploded.

When a supernova explodes, the light starts spreading out in all directions, much like the ripples on the water spread out when you drop a pebble into a lake.

The supernovae Perlmutter studies exploded billions of years ago. As the light from their explosions was traveling toward our galaxy, our solar system had time to develop, dinosaurs had a chance to come and go, and humans made their grand entrance and had time to build telescopes.

“While the light is traveling to us through the universe, the universe is expanding. And everything in the universe that’s not nailed down expands with the universe,” said Perlmutter. “That includes the very wavelengths of the photons of light that are traveling to us from the supernova.”

If the light is moving away from the observer, it appears red, in a phenomenon known as “redshift.”

“Now with these two ingredients – the brightness of the supernova and how much the light has been shifted towards the red in its appearance – you now can just read off the history of the expansion of the universe,” said Perlmutter, “because the brightness tells you how far back in time any given supernova event occurred, and the red shift tells us how much the universe has expanded since that time. And now we just do this for five, ten, 20, 40 supernovae at different times back in history and they, one after another, tell us for each time in history how much the universe has stretched since that time.”

With WFIRST, astronomers plan to study supernovae that are farther away.

The telescope, which would be launched to space on a satellite, would also include technology to study dark energy in other ways.

One new technique called Baryon Acoustic Oscillations, or BAO for short, allows scientists to refine their history of the universe by comparing the average distance between galaxies at different points in time with the distances between the hot and cold spots just after the Big Bang. The hotter spots were denser and gave rise to more galaxies.

If Congress were to move forward with WFIRST, it would be an exciting step for scientists trying to figure out what dark energy might be, said Perlmutter.

“You really would be able to probe into the history of the expansion of the universe in a way that we’ve never done before,” he said. “This would be the big chance of finding out what dark energy is in our lifetime.”