THERE’VE been some pretty strange theories to explain the identity of the missing mass that must exist to make the Universe behave the way it does. Now there’s another one: the lost mass is made of echoes from the hidden dimensions of space.
One reason physicists seriously contemplate the possibility that higher dimensions exist is because of superstring theory. This is the best candidate for a “theory of everything”, and it says that there are six extra space dimensions in addition to the three of space and one of time that we know.
We can’t see or move in these extra dimensions. And they must be minuscule. If they were any more than a fraction of the size of an atom, we would have detected signs of them already. But if they exist, physicists say there must be a whole load of extra particles out there too.
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These are known as Kaluza-Klein particles, and were first suggested in the 1920s. They would arise when force fields such as the electromagnetic field and the gravitational field bounce around in small, hidden dimensions, rather like the sound of a voice bouncing around in a small room. This would create “echoes” or distortions in the fields, which would manifest themselves as particles. If there are higher dimensions, then quarks, electrons and all the other familiar particles would have a whole family of heavier cousins.
No Kaluza-Klein particles have been observed in collisions at any particle accelerator, so the lightest must require an enormous amount of energy to create. They would have been created in abundance in the energetic conditions that existed in the first split second of the Universe. But physicists have always assumed they would have been much too short-lived to survive until now.
Two physicists now say that assumption may be wrong. Tim Tait of the Argonne National Laboratory in Illinois and Géraldine Servant of the University of Chicago have calculated that the lightest Kaluza-Klein particle – the first echo of the photon or neutrino – could be stable after all. “It could have survived to the present day,” says Tait.
The particle would probably interact with normal matter only very rarely. So if is still around today, it could explain why there seems to be a lot more mass in the Universe than astronomers can actually see. Tait and Servant worked out that if the Kaluza-Klein photon or neutrino is 800 to 1000 times as massive as a proton, then the mass of the particles they’d expect to have survived since the big bang would be just about right to explain the amount of dark matter in the Universe. They have submitted their findings to Nuclear Physics B.
“Such things are possible,” admits Joseph Silk of the University of Oxford. If Tait and Servant are right, the super-heavy photon or neutrino could show up in the next generation of particle accelerators as they smash particles together at higher and higher energy. More intriguingly, because the Kaluza-Klein particles are so heavy, they’d be drawn by gravity towards the centre of the Sun and of our Milky Way. Matter and antimatter versions of the particles would then be likely to meet and annihilate each other, possibly creating a characteristic signature of gamma rays and high-energy electrons that physicists could look for to prove the existence of the Kaluza-Klein particles.