What is cosmic acceleration and dark energy?

The universe is expanding and expanding a little faster all the time. Scientists call the acceleration of this expansion cosmic acceleration. This growth increases the distance between the points of the universe, just as stretching a rubber sheet would cause the points on that sheet to move further and further 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 extended period of cosmic acceleration that began 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). Scientists who discovered cosmic acceleration received the 2011 Nobel Prize in Physics.

This discovery raised a new question that scientists continue to explore today: what is the “dark energy” that is overcoming the effect of gravity and pulling our universe apart? Dark energy can be an inherent feature of the universe, or it could be related to new and unknown particles or forces. It could also be an indication that Einstein’s theory of general relativity is not a complete description of gravity.

Dark energy spectroscopic instrument

The Dark Energy Spectroscopic Instrument on the Mayall Telescope at Kitt Peak National Observatory will use 5,000 robotic “eyes” to map the history of the cosmic expansion. Credit: Photo courtesy of Marilyn Chung, Berkeley Lab

Quick Facts on Cosmic Acceleration and Dark Energy

  • Scientists are conducting studies to determine whether dark energy is consistent with the cosmological constant, a term that Albert Einstein originally included in his equations to counteract gravity. Alternatively, dark energy may not be constant, but rather something that changes throughout the history of the universe.
  • Dark energy represents approximately 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 Office of Science: Contributions to Cosmic Acceleration and Dark Energy

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

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