Mystery: Why does anything exist at all?
THERE is plenty to recommend the standard model, our best description of particles and their interactions. But it has the odd awkward lapse. “It is a somewhat embarrassing fact that it fails to explain our existence,” says Werner Rodejohann at the Max Planck Institute for Nuclear Physics in Germany.
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Actually, it’s worse than that: the standard model positively insists we don’t exist. It says that in the big bang, matter and antimatter should have been created in equal measure. These two famously don’t get along, annihilating one another in a flash of light whenever they come within touching distance. They should have snuffed each other out in a hot orgy of mutual destruction during the first second of the universe’s existence, leaving a cosmos filled with nothing but light. “It would look very different, not least because planets and stars and life could not evolve in such an environment,” says at the University of Liverpool, UK.
And yet here we are. Somehow, matter won.
One possibility is that the antimatter is just hiding: some of it, somehow, escaped the death match, taking refuge in little safe spots that eventually became distant regions as the cosmos cooled and expanded. In that case, there should be stars and galaxies made exclusively of antimatter. But we are yet to spot any hint that they exist.
“Embarrassingly, our theories suggest nothing should exist”
So for now, we have to think something must have tipped the balance in the early universe. We know that there are subtle differences in the outcomes of interactions involving certain matter and antimatter particles. This so-called CP violation is where Shears and her colleagues are seeking an answer. But so far, any differences we have found are roughly a billion times too small to explain the cosmic imbalance.
Shape-Shifters
Some researchers hope to find the answers among neutrinos. What little we know of these elusive, shape-shifting particles that come in three flavours – electron, muon and tau – is already gesturing towards new physics, says Rodejohann. We have hints of significant CP violation in measurements of how often they and their antimatter counterparts switch between flavours while travelling through our planet. We also know they have tiny masses, which goes against the standard model prediction that they should be massless.
The least implausible way to explain this is to invoke the existence of a heavier cousin called the sterile neutrino. In a mathematical trick called the “seesaw mechanism”, these heftier relatives would weigh down one end of the seesaw, lifting the lighter ones to ensure they have a small mass.
The catch is that for this trick to work, neutrinos must be their own antiparticle. If so, then the asymmetry seen today might be explained by these heavy neutrinos having decayed to lighter particles in the maelstrom of the early universe, with more of them choosing to become matter than antimatter.
It could take a decade to firm up hints that neutrinos violate CP and can be their own antiparticles. Even then, we would need to track down their heavy cousins. “But the neutrino story is appealing because the same guys that suppress neutrino mass also explain why matter dominates,” says Rodejohann. “It happens naturally, and it kills two birds with one stone.”
This article appeared in print under the headline “Object: The Universe”
