Dark Energy (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. The term dark energy was coined by the theoretical cosmologist Michael S. Turner. Dark Energys Predecessor Before physicists knew about dark energy, a cosmological constant  was a feature of Einsteins 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 universes expansion. In fact, they measured not only a deceleration but a totally unexpected acceleration  (Well, almost totally unexpected: Stephen Weinberg had once made such a prediction). 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 Einsteins original cosmological constant prediction manifests in todays theories in the form of dark energy. The latest findings indicate that over 70% of the universe is composed of dark energy. In fact, only about 4% is believed to be made up of ordinary, visible matter. Figuring out more details about the physical nature of dark energy is one of the major theoretical and observational goals of modern cosmologists. Also Known As: vacuum energy, vacuum pressure, negative pressure, cosmological constant