Dark energy just opened a can of wormholes. An extension of Einstein's gravity may explain this strange force and give us stable wormholes to navigate quickly through the cosmos – but there is a catch.
Building one of these cosmic shortcuts would require talking with aliens made of exotic matter, who can only send us messages via space-time ripples called gravitational waves.
The universe is expanding at ever-increasing speeds, and the prevailing theory is that a mysterious force called dark energy is driving it. But Manuel Hohmann at the University of Tartu in Estonia has been working on an alternative called multimetric gravity, which involves copies of the standard model of particle physics that each have slightly different properties.
Particles within a copy interact with each other as usual to build matter. But those from different copies would only interact via gravity, rather like dark matter, the invisible substance thought to make up about 80 per cent of all matter in the universe. The difference is that two particles made of different kinds of multimetric matter would repel rather than attract each other.
Galaxies made from dark multimetric matter could exist in the huge cosmic voids between regular galaxies, says Hohmann. Such structures would repel our galaxies and so could be contributing to what we perceive as dark energy.
The theory could also generate wormholes, says Hohmann. Most schemes to make stable wormholes require matter with negative energy – something we have never seen. But if a wormhole is built with equal amounts of regular and repulsive matter, that would hold it open.
To build such a wormhole, civilisations made from each type of matter would need to cooperate, and they could only talk via gravitational waves.
"Each civilisation would have to be able to manipulate a whole solar system to generate these waves, then they would need a working wave detector to hear this communication," says Hohmann. In other words, don't expect construction to start any time soon.
"From the purely mathematical point of view, it is clever and quite elegant," says Aurélien Barrau at the University of Grenoble-Alpes in France. "But it is far from being convincing from the physical viewpoint." The theory of multimetric gravity is a highly speculative extension of general relativity, says Barrau. And even if the theory is correct, actually constructing a wormhole would require what he describes as a miraculous situation.
Hohmann accepts this scenario has its difficulties. But if dark galaxies are out there, he thinks we can find them. Normal, massive galaxies warp space-time in a way that can focus light and enhance objects behind them, rather like a magnifying lens. A dark galaxy would defocus light due to its repulsive gravity, creating a faint smudge in the void we see where these galaxies might exist. Future telescopes may be able to spot such a smudge.