Dark matter, dark energy, dark… magnetism?

WE WILL be lonely in the late days of the cosmos. Its glittering vastness will slowly fade as countless galaxies retreat beyond the horizon of our vision. Tens of billions of years from now, only a dense huddle of nearby galaxies will be left, gazing out into otherwise blank space.

That gloomy future comes about because space is expanding ever faster, allowing far-off regions to slip across the boundary from which light has time to reach us. We call the author of these woes dark energy, but we are no nearer to discovering its identity. Might the culprit be a repulsive force that emerges from the energy of empty space, or perhaps a modification of gravity at the largest scales? Each option has its charms, but also profound problems.

But what if that mysterious force making off with the light of the cosmos is an alien echo of light itself? Light is just an expression of the force of electromagnetism, and vast electromagnetic waves of a kind forbidden by conventional physics, with wavelengths trillions of times larger than the observable universe, might explain dark energy's baleful presence. That is the bold notion of two cosmologists who think that such waves could also account for the mysterious magnetic fields that we see threading through even the emptiest parts of our universe. Smaller versions could be emanating from black holes within our galaxy.

It is almost two decades since we realised that the universe is running away with itself. The discovery came from observations of supernovae that were dimmer, and so further away, than was expected, and earned its discoverers the Nobel prize in physics in 2011.

Prime suspect in the dark-energy mystery is the cosmological constant, an unchanging energy which might emerge from the froth of short-lived, virtual particles that according to quantum theory are fizzing about constantly in otherwise empty space.

Mutant gravity

To cause the cosmic acceleration we see, dark energy would need to have an energy density of about half a joule per cubic kilometre of space. When physicists try to tot up the energy of all those virtual particles, however, the answer comes to either exactly zero (which is bad), or something so enormous that empty space would rip all matter to shreds (which is very bad). In this latter case the answer is a staggering 120 orders of magnitude out, making it a shoo-in for the least accurate prediction in all of physics.

This stumbling block has sent some researchers down another path. They argue that in dark energy we are seeing an entirely new side to gravity. At distances of many billions of light years, it might turn from an attractive to a repulsive force.

But it is dangerous to be so cavalier with gravity. Einstein's general theory of relativity describes gravity as the bending of space and time, and predicts the motions of planets and spacecraft in our own solar system with cast-iron accuracy. Try bending the theory to make it fit acceleration on a cosmic scale, and it usually comes unstuck closer to home.

That hasn't stopped many physicists persevering along this route. Until recently, Jose Beltrán and Antonio Maroto were among them. In 2008 at the Complutense University of Madrid, Spain, they were playing with a particular version of a mutant gravity model called a vector-tensor theory, which they had found could mimic dark energy. Then came a sudden realisation. The new theory was supposed to be describing a strange version of gravity, but its equations bore an uncanny resemblance to some of the mathematics underlying another force. "They looked like electromagnetism," says Beltrán, now based at the University of Geneva in Switzerland. "We started to think there could be a connection."

So they decided to see what would happen if their mathematics described not masses and space-time, but magnets and voltages. That meant taking a fresh look at electromagnetism. Like most of nature's fundamental forces, electromagnetism is best understood as a phenomenon in which things come chopped into little pieces, or quanta. In this case the quanta are photons: massless, chargeless particles carrying fluctuating electric and magnetic fields that point at right angles to their direction of motion.

Alien photons

This description, called quantum electrodynamics or QED, can explain a vast range of phenomena, from the behaviour of light to the forces that bind molecules together. QED has arguably been tested more precisely than any other physical theory, but it has a dark secret. It wants to spit out not only photons, but also two other, alien entities.

The first kind is a wave in which the electric field points along the direction of motion, rather than at right angles as it does with ordinary photons. This longitudinal mode moves rather like a sound wave in air. The second kind, called a temporal mode, has no magnetic field. Instead, it is a wave of pure electric potential, or voltage. Like all quantum entities, these waves come in particle packets, forming two new kinds of photon.