Related to the concept of Dark Matter is that of Dark Energy. It is a conjectural type of energy which is said by some to be the mechanism behind the acceleration of the universes’ expansion and that it infuses all of space. Based upon the Standard Model, this Dark Energy constitutes approximately 68% of the universe. Although the density of Dark Energy is very low, its very uniformity across the universe is what causes this dominance. It doesn’t appear to react with any of the fundamental forces of nature other than perhaps gravity.
There are a number of competing explanations for Dark Energy but two of the most mainstream ones are the Cosmological Constant and, Quintessence.
The Cosmological Constant version explains that a given volume of space has some essential and fundamental energy. In other words Dark Energy comes from the very fabric of space itself. This is the Cosmological Constant, often referred to as Λ, Lambda. It was of course invoked by none other than the great man himself in order to cater for a static universe by solving gravitational field equations. The General Relativity theory predicts that this energy will have a gravitational effect. As you might imagine it’s a pretty small number; somewhere around 10-29 g/cm3. Interestingly, Einstein however soon recanted and thought that his ‘invention’ of the Cosmological Constant was a big mistake calling it his “greatest blunder”. Today though, it is a pivotal component in the most accepted explanation of Dark Energy. If indeed this is how the universe works then what we would expect is that the universe will continue to exhibit an accelerating expansion forever.
Quintessence models on the other hand refer to the potential energy of a dynamical field and differ from Cosmological Constant models in that they can vary in space and time. To date there is no direct evidence of quintessence, but it is predicted that observed violations of Einstein’s equivalence principle might indicate its existence. The future of the universe however, is radically different in this model, as the strength of the Dark Energy Field could vary over time. The implication of this is that the universe could end in a big crunch where it contracts in on itself or a big rip where all of the structures in space rip apart.
Why does any of this matter? (Pardon the pun)
Because Dark Energy may constitute the majority of the universe we live in, that’s why.
There are other possibilities however. In fact Dark Energy may not even exist. It is fairly well known that Einstein’s work on gravity is not well adapted to describe events on the quantum scale. Equally well known is that it is mightily unassailable at galactic scales. There are whisperings however that when we zoom all the way out to the most cosmic of scales that it may again not be adequate; or that at least it needs some form of modification to be complete. If in fact this is the case then we may need to find other mechanisms than dark energy to explain reality. As far as any predictions as to the fate of universe based upon this thinking…unknown.