What is cosmic acceleration and dark energy?

The universe is expanding and it is growing a little faster all the time. Scientists call the acceleration of this cosmic acceleration of expansion. This growth increases the distance between points in the universe, just as stretching a rubber sheet would cause the points on that sheet to move farther and farther apart.

The universe has experienced two distinct periods of cosmic acceleration. The first, called inflation, occurred a fraction of a second after the big Bang. The second is the long period of cosmic acceleration that began about 9 billion years after the Big Bang and continues today. Scientists discovered the growing expansion of the universe in 1998 through observations of distant supernovae (exploding stars). Scientists who discovered cosmic acceleration received the 2011 Nobel Prize in Physics.

This discovery posed a new question that scientists continue to explore today: What is “dark energy” that overcomes the effect of gravity and separates our universe? Dark energy can be an inherent characteristic of the universe, or it can be related to new and unknown particles or forces. It could also indicate that Einstein’s general theory of relativity is not a complete description of gravity.

Cosmic acceleration and dark energy quick facts

• Scientists are conducting studies to determine if dark energy is consistent with the cosmological constant, a term Albert Einstein originally included in his equations to counterbalance gravity. Alternatively, dark energy may not be constant, but something that changes over the history of the universe.
• Dark energy makes up about 70% of the total mass-energy of the universe. In contrast, dark matter represents about 25% of the mass energy of the universe and ordinary matter only 5%.

DOE Science Office: Contributions to Cosmic Acceleration and Dark Energy

The Department of Energy supports researchers seeking to understand cosmic expansion and dark energy. Scientists supported by the DOE are partnering with the National Science Foundation and other organizations to build specialized and sensitive detectors. Teams of scientists are conducting experiments to measure the characteristics of the cosmic microwave background, the faint light left by the warm, primitive universe. Their work may provide clues to the early inflation of the universe. Scientists are also using large-scale ground-based telescope surveys to collect data on the past and present universe that will improve our understanding of the long-term history of the universe. These investigations will help shed light on the nature of dark energy.