card
346

dark energy

OLogy Series
astronomy
card
346

dark energy

OLogy Series
astronomy

Since the 1920s, observations of distant galaxies reveal that our universe was expanding. Astrono- mers thought that gravity, which draws matter together, would eventually slow, stop, or even reverse this expansion. But in 1998, astronomers determined that the expansion of the universe is actually speeding up! There is a mysterious pressure working in opposition to gravity, pushing matter apart. Scientists call it "dark energy."

How do scientists determine the rate at which the universe is expanding? They charted the distances and speeds of extremely remote galaxies by studying their exploding stars, or supernovae.

Stars explode fairly often. In a galaxy like ours, about three stars explode every century. So if you observe enough galaxies, you can spot a new supernova every few nights.

These supernovae are very bright. They also have a uniform brightness, so scientists could use an observed supernova's brightness to determine its distance. It's similar to car headlights on a highway: the farther away the car, the dimmer its headlights.

They also measured the supernova's light over time to determine how fast it was moving away from us. By analyzing the data from hundreds of supernovae, they could measure how the universe's rate of expansion has changed over time.

What percent of the universe is dark energy?

20%

50%

70%

Are you right?

### Correct!

Dark energy makes up 70 percent of the total "stuff," or mass-energy, of the universe. The normal matter in planets, stars, and galaxies is actually less than five percent. Dark matter makes up the rest.

How do scientists calculate the amount of dark energy in the universe?

by calculating the amount of darkness in the universe

by calculating the energy needed to overcome gravity

by calculating the amount of dark matter in the universe

Are you right?

### Correct!

Scientists can measure dark energy's effect by calculating the energy needed to overcome the gravity of all the dark matter and normal matter in the universe.

Dark energy gets its name from dark areas of the universe.

or
?

### Fiction

The name "dark energy" is just a label. "Dark" describes how little we know about it.

Definition: a mysterious pressure that is causing the expansion of the universe to accelerate
Discovered: in 1998 by two independent teams of astronomers
Measured: In 2002, the Wilkinson Microwave Anisotropy Probe determined that more than 70% of the universe is made of dark energy.
Cool fact: The force of dark energy seems to be growing stronger as the universe expands!

Image credits: © AMNH.