A new field, a new force, the power of our own ignorance? It’s two-thirds of the cosmos but it just keeps us guessing
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 Sean Carroll, 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.
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.
