Equations of General Theory of Relativity led inevitably to a dynamic Universe, either contracting or expanding, as bending of space-time makes matter move. Einstein was thinking that his equations brought a contracting Universe. Main opinion at the time being that of a static Universe however, Einstein inserted what was called the "cosmological constant" in his equations. This was counterbalancing the inherent logics to Relativity that Universe was contracting. A little later Hubble (to whom Hubble telescope name is paying hommage) showed that Universe was expanding. Einstein did not need the "cosmological constant" anymore
In 1998, as Alan Guth had already "invented" the inflation theory and worked about quantum "void energy" showing that such a void had tremendously expanded ("inflated") Universe shortly after the Big Bang, two teams simultaneously and fortuitously discovered that Universe was accelerating since 7 billion years. They found this acceleration as they were studying faraway supernovae. The latter were found farther than where they should have been found. This really was meaning that Universe had expanded more swiftly than thought since that time. Such an acceleration was ascribed to what the discoverers dubbed "dark energy". Further studies proved that dark energy is accounting for about 70 percent of the Universe energy budget
No real explanation was given about the nature of dark energy however. Although numerous theories have been built, two most adopted are that dark energy is either Einstein's "cosmological constant", or is associated with a "quintessence" i.e. a changing energy field similar but milder, than the one which caused inflation. If dark energy is the "cosmological constant" it should be unchanging and having a prescribed strength. If a quintessence its features would be different
Late Hubble study is showing that dark energy might display a character of permanence, hence being close to be the cosmological constant. If this would not be the case, the pace of change of dark energy is low and would bring Universe to its end in 30 billion years from now only. Recentest study by NASA WMAP satellite which is studying the CMB (Cosmic Microwave Background), leftover of the Big Bang, is showing that Universe will expand forever. This new Hubble study may question the study about this latter point in case dark energy would not be the cosmological constant. In this case, dark energy not having a character of permanence, it would lead Universe either to a "Big Rip" or to a "Big Crunch", either tearing apart everything in the Universe -down to the smallest atom, or as dark energy might flip, bringing back the whole Universe to a single point in a final implosion. First case would threaten us in about 30 billion years as second in only 10 to 20 billion years