5 WHAT IS DARK ENERGY?

It’s invisible, fills all of space and its repulsive gravity is speeding up the expansion of the Universe. ‘Dark energy’ was discovered by astrophysicists in 1998. They were studying type 1A supernovae – stellar explosions believed to unleash a fixed amount of energy and burn with a standard luminosity like a cosmic 100W lightbulb. The problem was that the most distant supernovae were fainter than expected. Cosmic expansion had speeded up, pushing them further away.

At the time, the only force thought to be operating in the large-scale Universe was gravity, which acts like an invisible web between the galaxies, braking cosmic expansion. The discovery that the expansion of space was speeding up gobsmacked cosmologists who were forced to postulate the existence of a substance that accounts for an astonishing two-thirds of the mass-energy of the Universe. This ‘dark energy’ overwhelmed gravity and gained control of the Universe about five billion years ago.

One possibility is that dark energy is a cosmological constant, an intrinsic repulsion of space. Such repulsion might arise from quantum energy fluctuations in the vacuum. However, when quantum theory, our best theory of the submicroscopic world, is applied to the vacuum, theorists predict an energy density that is 10 followed by 120 zeros bigger than that of the dark energy: the biggest discrepancy between a prediction and an observation in the history of science. Conceivably, the discrepancy will disappear when we finally manage to combine quantum theory with Einstein’s theory of gravity. Meanwhile, space experiments may help. In 2022, the European Space Agency will launch Euclid, which will measure how dark energy varies with cosmic time, hopefully providing a vital clue to solving what is the biggest puzzle in science.