Definition: Dark energy is a hypothetical form of energy that permeates space and exerts a negative pressure, which would have gravitational effects to account for the differences between the theoretical and observational results of gravitational effects on visible matter. Dark energy is not directly observed, but rather inferred from observations of gravitational interactions between astronomical objects, along with dark matter.
The term "dark energy" was coined by the theoretical cosmologist Michael S. Turner.
Dark Energy's Predecessor
Before dark energy, there as the notion of a cosmological constant, which was a feature of Einstein's original general relativity equations that caused the universe to be static. When it was realized the universe was expanding, the assumption was that the cosmological constant had a value of zero ... an assumption that remained dominant among physicists and cosmologists for many years.Discovery of Dark Energy
In 1998, two different teams - the Supernova Cosmology Project and the High-z Supernova Search Team - both failed at their goal of measuring the deceleration of the universe's expansion. In fact, they measured not only a deceleration, but a totally unexpected acceleration. (Well, almost totally unexpected: Stephen Weinberg had made such a prediction once)Further evidence since 1998 has continued to support this finding, that distant regions of the universe are actually speeding up with respect to each other. Instead of a steady expansion, or a slowing expansion, the expansion rate is getting faster, which means that Einstein's original cosmological constant prediction manifests in today's theories in the form of dark energy.
Also Known As: vacuum energy, vacuum pressure, negative pressure, dark energy
