The universe is expanding and expanding slightly faster all the time. Scientists call the acceleration of this expansion cosmic acceleration. This growth increases the distance between points in the universe, just as stretching a rubber sheet will cause points on the skin to move further and further apart.
The universe experienced two different periods of cosmic acceleration. The first, called inflation, has a fraction of a second after the Big explosion. The second is the extended period of cosmic acceleration that begins about 9 billion years after the big bang and continues today. Scientists discovered the increasing expansion of the universe in 1998 through observations of distant supernovae (exploding stars). The scientists who discovered cosmic acceleration received the 2011 Nobel Prize in Physics.
This discovery posed a new question that scientists are still investigating today: what is the ‘dark energy’ that overcomes the effect of gravity and pulls our universe apart? Dark energy can be an inherent feature of the universe, or it can be something related to new and unknown particles or forces. It may also be a hint that Einstein’s theory of general relativity is not a complete description of gravity.
The Dark Energy Spectroscopic Instrument at the Mayall Telescope at Kitt Peak National Observatory will use 5,000 robotic “eyes” to map the history of cosmic expansion. Credit: Photo courtesy of Marilyn Chung, Berkeley Lab
Cosmic acceleration and dark energy fast facts
- Scientists are conducting studies to determine whether dark energy corresponds to the cosmological constant, a term that Albert Einstein originally included in his equations to weigh gravity. Alternatively, dark energy may not be constant, but something that changes in the history of the universe.
- Dark energy accounts for about 70% of the total mass energy of the universe. In contrast, dark matter makes up about 25% of the mass energy of the universe, and ordinary matter only 5%.
DOE Office of Science: Contributions to cosmic acceleration and dark energy
The Department of Energy supports researchers who want to understand cosmic expansion and dark energy. Scientists supported by DOE work with the National Science Foundation and other organizations to build specialized, sensitive detectors. Teams of scientists are conducting experiments to measure the properties of the cosmic microwave background, faint light left over from the warm early universe. Their work can provide clues about the early inflation of the universe. Scientists also use large-scale telescope recordings on the ground to gather data on the past and the present universe that will improve our understanding of the long-term history of the universe. These recordings will help shed the nature of dark energy.