Dark energy, what could it be?
The best current explanations for the Universe’s accelerating rate of expansion
The most widely held answer is something called the cosmological constant (known by the Greek letter lambda, Λ). It originally appeared in Einstein’s general theory of relativity as a way to counteract gravity and keep his notion of a static Universe that isn’t expanding. Once Edwin Hubble showed the Universe was indeed expanding in 1929, Einstein labelled his cosmological constant “the greatest blunder of my life”. Yet the idea gained renewed traction after the discoveries of 1998 hinted at the presence of dark energy. At its heart is the idea that even empty space contains energy and the more that space expands the more this energy dominates.
It has had the same value everywhere in space throughout the Universe’s history. Quintessence is the other leading dark energy contender. It says that dark energy is in fact a substance that pervades space, rather than a property of space itself. Its name means ‘fifth essence’ as it would be the fifth different component in the Universe after ordinary matter, light, particles like neutrinos and dark matter. Unlike the cosmological constant, the outward pressure exerted by quintessence can evolve over time - which explains why the Universe’s expansion has only started to accelerate relatively recently.