IT IS 15 head-scratching years since we noticed that some mysterious agent is pushing the universe apart. We still don’t know what it is. It is everywhere and we can’t see it. It makes up more than two-thirds of the universe, but we have no idea where it comes from or what it is made of. “Nature has not been ready to give us any clues yet,” says , a theoretical physicist at the California Institute of Technology in Pasadena.
We do at least have a name for this most enigmatic of beasts: dark energy. Now the hunt for it is really on. Later this year, astronomers will begin a new sky survey to look for signs of the stuff among exploding stars and ancient galaxy clusters. A pack of space missions and gigantic Earth-based telescopes will soon join the chase. Meanwhile, some physicists are pursuing an unorthodox idea: that we might snare dark energy in the lab.
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As yet, our knowledge of the quarry is desperately scarce. It is limited to perhaps three things. First, dark energy pushes. We first noted that in 1998, in the unexpected dimness of certain supernova explosions which told us they were further away than we expected. Space seems at some point to have begun expanding faster, as if driven outwards by a repulsive force acting against the attractive gravity of matter.
Second, there is a lot of the stuff. The motion and clustering of galaxies tells us how much matter is abroad in the universe, while the cosmic microwave background radiation emitted 380,000 years after the big bang allows us to work out the total density of matter plus energy. This second number is much bigger. According to the latest data, including microwave observations from the European Space Agency’s Planck satellite, about 68 per cent of the universe is in some non-material, energetic, pushy form. That works out at about 1 joule per cubic kilometre of space.
Third, dark energy makes excellent fuel for the creative minds of physicists. They see it in hundreds of different and fantastical forms.
The tamest of these is the cosmological constant, and even that is a wild thing. It is an energy density inherent to space, which within Einstein’s general theory of relativity creates a repulsive gravity. As space expands there is more and more of the stuff, making its repulsion stronger relative to the fading gravity of the universe’s increasingly scattered matter. Particle physics even seems to provide an origin for it, in virtual particles that appear and disappear in the bubbling, uncertain quantum vacuum. The trouble is these particles have far too much energy – in the simplest calculation, about 10120 joules per cubic kilometre.
This catastrophic discrepancy leaves room for a menagerie of alternative theories. Dark energy could be quintessence, a hypothetical energy field that permeates space, changing over time and perhaps even clumping in different places. Or it might be a modified form of gravity that repels at long range, or an illusion born of Earth’s position in the cosmos. Maybe dark energy could take the form of radio waves trillions of times larger than the observable universe – or something altogether more exotic than that (see “Arcane energies“).
“Many clever people have tried to devise something better than the cosmological constant, or understand why the cosmological constant has the value it does,” says Carroll. “Roughly speaking, they have failed.”
The dark is rising
One way to cut to the chase might be to find out whether dark energy is changing over time. If it is, that would exclude the cosmological constant: as an inherent property of space, its density should remain unchanged. In most models of quintessence, by contrast, the energy becomes slowly diluted as space stretches – although in some it actually intensifies, pumped up by the universe’s expansion. In most modified theories of gravity, dark energy’s density is also variable. It can even go up for a while and then down, or vice versa.
The fate of the universe hangs in this balance. If dark energy remains steady, most of the cosmos will accelerate off into the distance, leaving us in a small island universe forever cut off from the rest of the cosmos. If it intensifies, it might eventually shred all matter in a “big rip”, or even make the fabric of space unstable here and now. Our best estimate today, based mainly on supernova observations, is that dark energy’s density is fairly stable. There is a suggestion that it is increasing slightly, but the uncertainties are too large for us to worry about this increase just yet.
The , an international project due to start collecting data this September, aims to tighten things up. It uses the 4-metre-wide Víctor M. Blanco telescope at the in Chile, attached to a specially designed infrared-sensitive camera, to look for several telltale signs of dark energy over a wide swathe of the sky. “This is not the world’s biggest telescope, but it has a very large field of view,” says of the University of Chicago, who is director of the project.
For a start, the telescope will catch many more supernovae. The apparent brightness of each stellar explosion tells us how long ago it happened. During the time the light has taken to reach us, its wavelength has been stretched, or redshifted, by the expansion of space. Put these two things together and we can plot expansion over time.
The survey will also draw an intricate sky map that marks the positions of a few hundred million galaxies and their distances from us. Sound waves reverberating around the infant cosmos gave vast superclusters of galaxies a characteristic scale. By measuring the apparent size of superclusters, we can get a new perspective on the expansion history of the universe (see diagram).
Eyes on the sky
The map will also reveal dark influences on smaller scales. Dark energy hinders galaxies from coming together to form clusters. The survey team will count clusters directly, and also follow their growth using an effect known as gravitational lensing, which happens when clusters bend light passing through them from even more distant cosmic objects.
These various measurements should give us a handle on how dark energy has changed over time – if at all. The survey should reduce the uncertainty on existing results by a factor of four, says Frieman. With its first analysis due in 2016, it will begin to distinguish between some of the different theoretical models.
A full posse of dark energy hunters will set out just a few years later. The , a US-led project, is due to open its great eye in 2021. Other mega-scopes such as the in Hawaii and the and the , both in Chile, should also swing into action around the same time. So should a huge cosmic radio receiver based in Australia and South Africa, the Square Kilometre Array, which will trace cosmic structure through the radio glow of hydrogen clouds. In 2020, the European Space Agency and NASA plan to launch a dark-energy hunting space mission called that will trace gravitational lensing and galaxy clumping to even earlier cosmic times. The US may follow soon after.
This chase through space will be thrilling, but the quarry may still elude us. Say we find that dark energy maintains a near constant density over time. That would seem to support the cosmological constant, but it would not rule out some quintessence fields that just happen to have a nearly constant density. Even if we find the dark energy density to be increasing or decreasing, we might not be able to tell whether that is due to quintessence, or to some kind of varying gravity.
That leads some physicists to suggest laying traps for the beast here on Earth. “If you introduce a new field or particle to be your dark energy, then it will also act as the carrier of a new force,” says at the University of Nottingham, UK. Something like quintessence would produce a fifth fundamental force, separate from gravity, electromagnetism and the nuclear forces. The same holds for most forms of modified gravity. “But we don’t see a fifth force within the solar system,” says Burrage.
“If dark energy is a new field or particle it will create a fifth force we don’t feel in the solar system”
Theorists generally extricate themselves from this sticking point by adding a screening mechanism that weakens the fifth force in comparatively dense environments such as the solar neighbourhood. A project called the , at Fermilab in Illinois, is already looking for one particular screened dark energy field called the chameleon.
So far GammeV has seen nothing, but now Burrage aims to search for a much wider range of dark energies, and with higher sensitivity. Along with Nottingham colleague Edmund Copeland and of Imperial College London, she wants to expose it using a cloud of cold atoms called a Bose-Einstein condensate, which oscillate together in a collective quantum wave. Dark energy should just slightly slow down the frequency of this oscillation. The team plans to split a condensate in two and place a dense object near one of the halves. If the object screens out dark energy, waves in the two halves will get out of sync, and when brought back together the two condensates will interfere.
Electric effects
At the University of Washington in Seattle, the is probing other forms of cosmic repulsion. In one theory, extra dimensions of space less than a millimetre across can play host to dark energy. That would also increase the strength of gravity on these scales. A type of screened quintessence called the symmetron would generate a similarly small-scale extra force – a tiny effect that the subtle twistings of the Eöt-Wash pendulums should expose.
Meanwhile, Michael Romalis at Princeton University and Robert Caldwell at Dartmouth College in Hanover, New Hampshire, proposed earlier this year that if ordinary photons or electrons can feel quintessence even very faintly, then a magnetic field on Earth should generate a tiny electrostatic charge. This effect is potentially simple to spot, although any apparatus designed to do it would need to be very precise ()
Carroll points out that we might see another electromagnetic effect in space. If photons interact with dark energy, it could rotate their polarisation as they travel across the universe. When the Planck team announce their measurements of the polarisation of photons from the cosmic microwave background within the next few years, “it is conceivable they will announce they have detected quintessence”, he says. We may then have an uneasy wait of a decade or two while the telescopes look out to see which way dark energy is likely to slide, before we can breathe easy that the space around us is not liable to collapse into a new and unhealthy state.
Few imagine that the hunt will be over soon. “Dark energy is one of the greatest mysteries, and I don’t expect to still be around when we solve it,” says at the University of Oregon in Eugene. After 15 years of puzzlement we have no clue as to dark energy’s identity. But on the bright side, we do have some clues to where the clues may lie.
Arcane energies
Energy is ignorance, say and Orlando Luongo of the University of Naples – and that could explain where dark energy comes from (see main story). If different eras of the universe are in entangled quantum states, when we try to measure some property of the early universe such as its expansion speed, quantum theory puts a limit on the information we can extract. Information loss is intimately linked to a rise in entropy, which implies energy sloshing around the cosmos. “It is possible to interpret this as dark energy,” says Capozziello. By his calculations our cosmic energy of ignorance would have the right properties to cause the real acceleration of the universe ().
Or perhaps dark energy is a hologram. The holographic principle, devised to help link gravity with quantum mechanics, says that nothing can contain more energy or information than a black hole. A black hole’s total energy increases with its circumference, not its volume, so perhaps the universe’s energy is related to the length of some boundary akin to a black hole’s event horizon. “It is an appealing idea because you get the right energy scale,” says one of the idea’s originators, Stephen Hsu at the University of Oregon in Eugene. This could reduce the excessive energy of quantum particles popping out of the vacuum to the level of dark energy. Even so, there are all sorts of technical issues, he says – not least that it is difficult to define such a boundary of the universe.
Meanwhile, and colleagues at the University of Nottingham, UK, think an unimaginable tumult of dark energy could sit beneath our serene existence. In an obscure paper from 1974 they found the most general mathematical form of scalar-tensor theories, in which an added energy field allows the strength of gravity to vary from place to place. When they adapted these equations to produce an accelerating universe like ours, they found it includes a huge amount of vacuum energy – perhaps more than 1060 joules of energy per cubic metre of space (). We don’t feel the powerful anti-gravity effect of this dark energy because it is almost entirely blocked by the added field. If that is right, you can hold enough dark energy between your hands to disintegrate a million galaxies.
Read more: “The search for dark energy”
This article appeared in print under the headline “Chasing shadows”
