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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.
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.
Photo courtesy of Marilyn Chung, Berkeley Lab

The universe is expanding, and it expands a little faster all the time. Scientists call the speeding up of this expansion cosmic acceleration. This growth increases the distance between points in the universe, just like stretching a rubber sheet would make points on that sheet 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). The 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 the “dark energy” that is overcoming the effect of gravity and pulling our universe apart? Dark energy may be an inherent feature of the universe, or it could be something related to new and unknown particles or forces. It could also be a hint that Einstein’s theory of general relativity is not a complete description of gravity.

DOE Office of Science: Contributions to Cosmic Acceleration & Dark Energy

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

Cosmic Acceleration and Dark Energy Quick Facts

  • Scientists are conducting studies to determine whether 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 accounts for about 70% of the total mass-energy of the universe. In contrast, dark matter accounts for about 25% of the universe’s mass-energy, and ordinary matter only 5%.

Resources and Related Terms


Definition contributors: Michael Cooke (DOE)


Scientific terms can be confusing. DOE Explains offers straightforward explanations of key words and concepts in fundamental science. It also describes how these concepts apply to the work that the Department of Energy’s Office of Science conducts as it helps the United States excel in research across the scientific spectrum.