Dark Energy

Our universe is speeding up, but no one knows why. Hubble's discovery demonstrated how much more we have yet to learn about the cosmos.

A sparkling galaxy with a bright, glowing core and patchy spiral arms dotted with pale blue star formation.

To understand the mystery that is dark energy, you have to start at the beginning. Of the universe, that is.

The theory of how the universe came to exist starts with an event called the big bang, when the universe suddenly expanded from a hot, dense point at incredible speeds, inflating like a balloon as it grew and cooled. Eventually atoms and molecules formed, then collections and clumps of matter that formed stars, galaxies, and everything within them.

The universe continues to expand today. For many years, the fate of that expansion was unknown. Did the universe have enough matter, and thus enough gravitational pull, to stop or even reverse that expansion? Or would it keep expanding forever, slowing but never quite stopping?

By the late 1990s, scientists had found that the universe had enough matter to slow the expansion but not bring it to a halt. Furthermore, they had determined the current rate of the expansion. Now they just needed to find out how much it had been slowing over time.

Or so they thought.

What astronomers discovered was that the universe isn’t slowing at all. The expansion of the universe is speeding up, faster and faster, propelled by some force we still don’t understand. We call this phenomenon “dark energy.”

Dark Energy Expansion Graph : Animation illustrating the changing rate of expansion due to dark energy.
NASA Goddard's Scientific Visualization Studio

Hubble made the key observations that led to the discovery of dark energy. Astronomers looked at a special kind of supernovae, called Type 1a, that give off a set amount of light when a certain type of star explodes. Because the brightness is consistent and known Type 1a supernovae can be used to measure distances to extremely distant objects like faraway galaxies. Astronomers compare the actual, known brightness of the supernova to how bright it appears ― similar to the way you can estimate the distance to a distant car in the dark because you know approximately how bright the headlights should be.

This image contains six boxes with Hubble pictures. The top three boxes show distant galaxies, which are small and somewhat blurry, and lack much detail. The bottom three boxes are the same galaxies, but with the bright point of a supernova explosion. An arrow points to the supernova in each galaxy.
These Hubble images show a series of supernovae ― explosions that occur at the death of massive stars ― detected in distant galaxies. The arrows indicate the supernovae. Type 1a supernovae like these have a known brightness that can be used to measure distances to faraway objects.
NASA and A. Riess (STScI)

Only Hubble had the vision to detect these supernovae in distant galaxies. It found that the supernovae were dimmer than they should have been, which meant that their host galaxies were farther away than expected. And that only made sense if the universe’s expansion was accelerating.

Scientists now theorize that the universe slowed down under the power of gravity for a time after the big bang and then, at a point when it had grown large enough that those gravitational connections weakened, began to speed up as dark energy took over.

Dark energy and its mysteries are a reminder of how much we have yet to learn about the universe. Hubble continues to investigate the enigma as it gazes into the vast reaches of the cosmos.

Big Bang Infographic showing the timeline of the history of the big bang and the formation of the building blocks of the universe. he history of the universe is outlined in this infographic. It starts with Inflation, then the first particles in 1 microsecond, followed by first nuclei (10 seconds); first light (300,000 years); first stars (200 million years); galaxies and dark matter (400 million years); dark energy (10 billion years); present (13.8 billion years). NASA
The history of the universe, as we currently understand it, looks something like this. The universe began expanding with the big bang. Around 10 billion years later, it strangely began to accelerate, a phenomenon we call dark energy.
NASA
Spiral galaxy UGC 9391 is shown sprinkled liberally with red circles where Cepheid variable stars have been found and a blue cross where a Type 1a supernovae was seen.
This galaxy, UGC 9391, contains two types of objects that astronomers use to calculate accurate distances to galaxies. The red circles mark the locations of Cepheid variable stars. These stars pulsate at rates that correspond to their true brightness, which can be compared with their apparent brightness as seen from Earth to accurately determine their distance.

The blue "X" at bottom right denotes the location of supernova 2003du, a special class of exploding star called a Type Ia supernova. These supernovae are another commonly used cosmic yardstick. They flare with the same brightness and are brilliant enough to be seen from longer distances.
NASA, ESA, and A. Riess (STScI/JHU)
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