A mysterious repulsive force
seems to dominate the Cosmos

The distribution of galaxies as revealed in the UK-supported 2dF Galaxy Redshift Survey. The clustering of matter supports the dark energy scenario

In 1998, astronomers made a shocking discovery that has forced them to re-think their ideas about the nature of the Cosmos. Surveys of exploding stars – supernovae – in distant galaxies indicated that they were much further away than expected. We know that the Universe has been expanding ever since its birth in the Big Bang, but the new observations suggested that this expansion was accelerating, starting about 6 billion years ago.

This was not what cosmologists wanted to hear; they had developed a robust description of how the Universe has evolved into the galaxies and clusters of galaxies we see today. A favoured idea was that the Universe contained a critical density of matter and energy to keep it ‘flat’: according to Einstein’s General Theory of Relativity, the gravitational force exerted by matter makes space curve round, which is then counterbalanced by the energy of expansion.

This critical density included the visible stars and galaxies, and a larger amount of invisible dark matter. The acceleration of expansion suggests, however, that to retain a flat geometry there must be another component – a repulsive energy field that is pushing everything apart. This ‘dark energy’ actually accounts for a huge 70 per cent of the critical density, with about 25 per cent dark matter and 5 per cent ordinary matter.

	Three of the most distant supernovae known, discovered with the Hubble Space Telescope and used to trace the expansion rate of the Universe

The new results
The evidence for dark energy was based on a certain type of supernova, which emits a characteristic amount of light, and therefore acts as a ‘standard candle’ for calculating distance. Combining the supernova data with measurements of the redshifts of their host galaxies allowed the survey teams to calculate the expansion rate at different epochs. (The light from galaxies is shifted to the red end of the spectrum depending on how fast they are moving away from us; the faster they recede, the further away they are, and the further back in time we are seeing them.)

Although doubts remain about the reliability of supernovas for measuring cosmological distances, other observations provide supporting evidence for dark energy. First, measurements of fluctuations in the cool radiation from the first structures that formed after the Big Bang and developed into galaxies – the cosmic microwave background (CMB) – have given information about the Universe’s geometry. The scaling of the fluctuations indicates that the Universe is flat. In addition, large-scale surveys of galaxy redshifts looking at the distributions of galaxies and how they cluster together over time under the effects of gravity have provided a measure of their total average mass. Extrapolating the results to the whole Universe gives a mass density that is only 30 per cent of the critical density.

What is dark energy?
Faced with these uncomfortable results, theorists have re-visited the basics of modern physics – relativity and quantum theory – to find an explanation for dark energy. The simplest idea lies in Einstein’s gravitational equations as applied to cosmology. Einstein had introduced a constant to keep the Universe static, but later discarded it when Edwin Hubble showed that galaxies were, in fact, flying apart. This Cosmological Principle has been attributed to the energy of virtual particles popping in and out of empty space – an intrinsic concept in quantum theory. Unfortunately, this simple interpretation predicts an energy density that is 120 orders of magnitude larger than is observed and would have been too large in the early Universe to allow galaxies to have formed under gravity.

Another candidate is a dynamic form of energy called quintessence which evolves over time, working like a field of springs to exert a negative pressure on space. It is a gentler version of a phenomenon called ‘inflation’ – when the Universe blew up very rapidly just after the Big Bang to become spatially flat, with the structure we see now. Other, more exotic ideas have been proposed, such as phantom energy which gets stronger with expansion leading to a ‘big rip’ when all matter is just torn apart.

Finally, it may be that Einstein’s theory of gravity needs to be modified over large scales. One suggestion is that our four-dimensional Universe is embedded in higher dimensions; gravity leaks into them, so that its grip on matter weakens and causes the cosmic expansion rate to increase. Another speculative idea is that gravity could vary in different parts of the Universe and that we are just lucky to be in a region where the conditions are suitable for our existence.

Further observations
The next steps must be to measure the expansion in more detail and establish how it evolves with time. Recent supernova surveys and also
X-ray studies of hot gas in galaxy clusters suggest that dark energy does behave like the Cosmological Constant. But what is needed is a variety of independent tests. For example, sound waves generated in the primordial gas that comprised the early Universe leave their fossil imprint on the clustering of galaxies. The scale of these acoustic oscillations at different epochs offers a complementary yardstick for expansion. Another approach is to measure the distortion in images of distant galaxies, which is created when their light is bent by the gravity of galaxies lying in front of them. The statistics of this weak gravitational lensing depend on the rate at which galaxies clustered – and thus the expansion rate. Finally, subtle variations in the CMB, arising from the effects of a repulsive energy on the interplay between light and matter in the early Universe, are also a useful probe.
The UK is committed to playing a leading role in such studies. Dedicated international programmes such as the Dark Energy Survey will observe supernovas, galaxy clustering and weak lensing over the evolution of the Universe. Galaxy clustering will be measured with a new spectrograph, WFMOS, built for installation on one of the world’s 8-metre telescopes, and also by a giant radiotelescope, the Square Kilometre Array, now being planned as a global project.

Dark energy is also being investigated at the subatomic scale. For example, some theories predict a value for dark energy similar to the masses of neutrinos – wispy particles thought to fill the Universe –
so their role is being explored. Furthermore, the new Large Hadron Collider shortly to start up at Europe’s main particle physics laboratory, CERN, will be able to probe modified gravity ideas by searching for evidence of hidden dimensions.

In fact, dark energy causes just as many headaches for particle physicists as it does for cosmologists, since it is not easily explained by theories of particles and forces such as superstrings. The discovery of this mysterious new energy opens a new window on our understanding of existence and there are clearly exciting, if unpredictable, times ahead.

From left to right: LSST, Dark Energy Survey, WFMOS, Pan-STARRS, SNAP

For more information on dark energy, go to:
www-astro.physics.ox.ac.uk/darksector
www.scienceatstake.com
http://en.wikipedia.org/wiki/Dark_energy
http://physicsweb.org/articles/world/17/5/7
www.darkenergysurvey.org
http://snap.lbl.gov
www.lsst.org/lsst_home.shtml

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Thanks go to Robert Crittenden of the University of Portsmouth and Roberto Trotta of the University of Oxford for their help with this paper

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