BBC Sky at Night Magazine

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.

 ??  ?? Big question: is dark energy a property of space itself?
Big question: is dark energy a property of space itself?

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