Science Made Simple: What is Cosmic Acceleration and Dark Energy?
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.