What is dark energy?

The existence and origin of dark energy is one of sciences most intriguing mysteries.

23rd April 2017
What is dark energy? (Getty)

Asked by: Fred Thomas, London

During the 1990s, astronomers measuring the rate at which the Universe is expanding made a shock discovery: it’s actually accelerating, as if the whole cosmos is being propelled by some invisible source of energy. This is so-called dark energy and its origin is one of the deepest mysteries in science. Various explanations have been put forward, with arguably the simplest being that it’s a manifestation of so-called quantum vacuum processes. According to the laws of the subatomic world, there is always some uncertainty about the amount of energy filling even empty space. This vacuum fluctuation energy has been detected in the lab, and theorists have shown it can have the ‘anti-gravitational’ effects of dark matter. So far, however, they have struggled to produce a detailed theory of its cosmic effects.

This has led to suggestions that dark energy may simply be a force-field left over from the Big Bang. Sometimes called quintessence, it’s capable of getting stronger over time, but again details remain elusive. There have even been claims that dark energy is leaking out from hidden extra dimensions of space that failed to expand following the Big Bang. Until there’s a breakthrough in the underlying theory, however, all this is little more than speculation. 


Get more fascinating Q&As from BBC Focus magazine by following @sciencefocusQA