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Immediately after the big bang, there was just one force. As the universe cooled, this force split into two, then three, and then the four forces of today鈥檚 universe. Physicists have long been searching for a grand unified theory that combines everything except gravity into one force. Now, a new idea does just that, but without making predictions that had jinxed earlier attempts.

Of the four fundamental forces 鈥� gravity, electromagnetism, and the weak and strong nuclear forces 鈥� experiments at particle colliders have shown that electromagnetism and the weak nuclear force become one 鈥渆lectroweak鈥� force at energies of about 100 gigaelectronvolts (GeV). This takes us back to about a trillionth of a second after the big bang.

鈥淏ut the question is what happened after one trillionth of a trillionth of a trillionth of a second after the big bang?鈥� says at the University of California San Diego. In principle, the electroweak and strong nuclear force should have become one force.

Fornal and , also at UCSD, have now built a grand unified theory on an earlier idea called SU(5), but crucially without requiring that protons should decay.

Breaking symmetry

SU(5) was proposed in 1974 by Howard Georgi and Sheldon Glashow. In SU(5), at the high energies of grand unification, all the particles are represented by mathematical structures that can be thought of as pentagons and decagons.

Take the pentagon. Each side of the pentagon represents either a quark or a lepton, which are fundamental particles. But mathematically, the sides of the pentagon are identical 鈥� so there鈥檚 no way in this model to tell apart a quark from a lepton. It鈥檚 only when the universe cools that this symmetry breaks, and this metaphorical pentagon shatters into pieces, each of which now represents either a quark or a lepton.

One side effect of SU(5) is that it predicts an extra interaction between quarks and leptons. This makes protons liable to decay, for instance, when聽two聽of the three quarks that make up a proton spontaneously turn into聽a lepton and an antiquark

But experiments like the Super-Kamiokande neutrino observatory in Japan, which have been looking for the radiation released by decaying protons, have seen nothing. They have put the lifetime of a proton at more than 10³⁴ years, orders of magnitude more than what鈥檚 predicted by SU(5).

There are theories that predict longer proton lifetimes, but they are far more complicated. For example, grand unified theories can incorporate supersymmetry, which doubles the number of known particles. This can extend the lifetime of protons. But experiments at the Large Hadron Collider at CERN have seen no evidence of supersymmetry either.

Adding dark matter

In their new grand-unified model, Fornal and Grinstein have added two structures to SU(5), one with 40 sides and another with 50 sides. These structures represent heavy fields, which both help unify the electroweak and strong nuclear forces, and also prevent the proton from decaying.

鈥淚t鈥檚 a proof of concept that such a theory can be constructed,鈥� says Fornal. And the duo has done it without resorting to supersymmetry. 鈥淲e pride ourselves on constructing a non-supersymmetric theory,鈥� says Fornal.

, who studies grand unified theories at the University of Split, Croatia, is impressed. He says this 鈥渄emonstrates once again the richness and beauty of this mature subject鈥�. But, the proof of concept can be refuted if proton decay is observed, he says.

The next step is to see if the theory can explain why the mass of the recently discovered Higgs boson is less than expected, Fornal says. 鈥淚nstead of doing the usual nip and tuck to make the Higgs fit their theory,鈥� Fornal says, 鈥渢here聽might be a way to solve this by adding extra fields聽into the theory.鈥�

Sometimes, such tweaks are needed to make the observed universe add up mathematically. The Higgs field was added to solve just such a problem 鈥� the origin of mass for all particles 鈥� and that turned out to be correct.

The team also wants to see if the theory can be modified to incorporate a candidate particle for dark matter, which would require a more general unified group of dark matter plus all the forces and particles we know of. When the symmetry breaks in that case, dark matter would fall out as a candidate particle.

ArXiv,

Read more: Reality guide: The essential laws of cosmology