What is Dark Energy?

Dark energy is the name given to a strange entity that appears to make up most of the mass-energy density of our Universe. This mysterious 'energy' is pushing apart our Universe, and its identity appears to be at least as elusive as that of dark matter.

Evidence for dark energy comes from multiple sources. Cosmologists have long known that Universe as a whole has a shape – it can be 'closed', analogous to the surface of a sphere, 'open', like the surface of a saddle, or flat. The exact shape the Universe takes depends on its total mass-energy content. Scientists have measured the shape of the Universe by observing the cosmic microwave background radiation that fills all of space almost uniformly and is a relic of the Big Bang. The shape of the Universe affects the size of the slight variations that we expect to see in the cosmic microwave background, and measurements of these variations tells us that the Universe is flat. Intriguingly, we know from observing galaxies in clusters that all the matter in the Universe makes up only 27% of the total mass-energy density needed for a flat Universe; in order for our observations of clusters to match with those of the cosmic microwave background, we need to account for the missing 73% of the Universe's energy content. Dark energy is thought to make up this 73%.

In addition to this missing energy, more direct evidence for dark energy comes from supernovae observations. A supernova is an immense explosion that happens when a massive star reaches the end of its life, and for a brief period (usually a few hours or days) they shine as brightly as an entire galaxy of 100 billion stars. This means that we can see supernovae that happen in distant galaxies. Astronomers can use these explosions as 'standard candles' because the light we receive from a supernova always follows the same pattern with time. We can thus deduce how bright the supernova really is and by comparing it to how bright it appears on the sky, work out how far away it is.

It has been known for a long time that the Universe is expanding, meaning objects in the Universe are getting further apart over time. A receding object will have its light redshifted to lower frequencies due to the Doppler effect. Since the Universe is expanding, we expect the light from supernovae in distant galaxies to be shifted towards the red end of the spectrum, and indeed this is what we observe. However, when we calculate the expected amount of redshift for supernovae based on how far away we know they are, they appear to be too strongly redshifted to account for the calculated expansion rate of the Universe. This means that distant galaxies (and the supernovae that occur within them) are actually moving away from us much more quickly than expected, and as a result the Universe must be expanding more quickly than previously thought. Not only is the Universe expanding too quickly, but it is accelerating in its expansion, when everything we know about the history of the Universe tells us the expansion should be slowing down. What could be causing this accelerated expansion? No one knows for sure, but the term dark energy was invented to describe this phenomenon, and to account for the huge proportion of missing mass-energy.

The identity of dark energy remains a mystery, but the simplest explanation is known as the cosmological constant, so called because its density is constant over space and time. It is also known as vacuum energy as it is thought to be the intrinsic energy of empty space. In many ways, the cosmolgical constant is an elegant solution to the problem of dark energy because it explains many cosmological observations with one simple number – the density of the vacuum energy. Vacuum energy is known to exist thanks to an experiment by Hendrik Casimir, however there is a huge discrepancy between the amount of dark energy that cosmologists observe and that predicted by particle physics. If dark energy really is vacuum energy, particle theory predicts that there is 120 orders of magnitude (1 followed by 120 zeros) times more vacuum energy than what we actually observe. Clearly this is a serious problem for the cosmolgical constant, which has led scientists to put forward other suggestions for the identity of dark energy. Many of these proposed solutions require the density of dark energy to vary with time or modifications to the laws of gravity, and all are more complicated than the cosmological constant model.

Dark energy has a uniform – but extremely low – density everywhere in space. For this reason, detecting dark energy directly the way we search for dark matter may be impossible. In order to unveil the identity of dark energy, more precise cosmological measurements from weak lensing are needed. Accurately determining the distance to galaxies using their redshifts in surveys such as CFHTLenS will allow us to work out if and how the dark energy density has varied over the history of the Universe. The time-variation of dark energy should allow us to discern between different dark energy theories and may well be the key to unlocking the true nature of this mysterious entity.

Author: Emma Grocutt