Scientists keep trying to disprove the Standard Model that governs modern physics. And they keep failing.
The Standard Model of particle physics, sometimes referred to as the “theory of almost everything,” unites three of the four fundamental forces of the universe. It proposes a zoo of 17 particles to mediate these forces. The last to be found is the celebrated Higgs boson.
But the Standard Model has its share of problems. It’s built on the “fundamental constants” of nature, but it doesn’t explain why the constants have the values they do. Nor does it explain dark matter or unite gravity with the other three fundamental forces, a union both physicists and astronomers are convinced must exist.
Physicists’ current favorite of non-Standard theories is supersymmetry, which proposes that each of the 17 particles in the Standard Model zoo has a corresponding “superpartner” with a different spin and probably a heavier mass, too. Supersymmetry solves a lot of problems — most importantly for astronomers, it provides “weakly interactive massive particles” (so-called WIMPs) that might be the dark matter they've been searching for.
But theorists have their work cut out for them. Several teams presenting at the American Astronomical Society (AAS) this week in Long Beach, California, have tried to find non-Standard physics in the distant universe, and failed.
Counting Neutrino Families
The universe created hydrogen and deuterium (a heavy version of hydrogen, with a neutron in its nucleus) in the first 20 minutes of its existence. So measuring how much deuterium and hydrogen there is in distant, pristine gas clouds tells us about the newborn universe, including the number of speedy particles called neutrinos that formed in the chaos. The Standard Model predicts three families of neutrinos, and that’s what particle accelerators have found, too, but some non-Standard theories say that number might have changed over cosmic time.
To measure deuterium and hydrogen abundances some 10-12 billion years ago, astronomers such as Ryan Cooke (University of California, Santa Cruz) examine quasars, beacons radiating from the most distant stretches of the observable universe. That emission carries a message of what lies between the quasars and us, so astronomers have an indirect glimpse of ancient gas clouds as they absorb the quasars’ radiation. Cooke’s team measured the primordial abundance of hydrogen and deuterium in several of these gas clouds, and then they converted that value to the number of neutrino families.
“If we found a value different from standard model, then we could also claim that this could be due to a change in gravitational constant or a whole host of other things,” Cooke said at a press conference at the AAS. But they didn’t. They found the number of neutrino families is 3, with a spread in values of 0.5.
The story’s not over yet, though. The team was surprised by the large spread in values, which couldn’t be explained by error alone. So the researchers are re-analyzing the data in a more consistent way. If the spread is still there when they’re done, they’ll have to come up with an explanation.
Of Protons and Electrons
One of the Standard Model’s fundamental constants is the mass ratio between protons and electrons. This ratio relates to the strong force, the force that holds the subatomic world together by binding quarks together to make protons and neutrons and binding protons and neutrons together to make atomic nuclei.
But if the strong force evolves over the history of the universe, then so will the mass ratio. That’s exactly what some theories attempting to go beyond the Standard Model call for.
Julija Bagdonaite (University of Amsterdam) and her colleagues studied PKS 1830-211, a spiral galaxy seen as it was 7 billion years ago. By comparing molecular transitions with those measured in an Earth-bound laboratory, they found that the mass ratio doesn’t change at all.
“This important new result was like a Christmas present for me,” said Rodger Thompson (University of Arizona), who presented the paper at the AAS. “It presents very serious constraints on current alternative cosmologies.”
Fine Structure Constant
The fine structure constant, another of the fundamental constants of the Standard Model, determines the strength of the electromagnetic force. Some supersymmetry models suggest that this constant should vary too. To measure this constant over time, Jonathan Whitmore (Swinburne University of Technology, Australia) and his colleagues observed absorption lines in distant quasars using the Very Large Telescope in Chile.
At this point, you can probably repeat the refrain with me: their results are consistent with no change in the fine structure constant over cosmic time.
What’s a Theorist to Do?
“All three results simply say that what we’ve found now is consistent with the Standard Model,” Thompson said. “Simplified supersymmetry calculations have not given values that are consistent with what we’re observing.”
But supersymmetry is a wily beast. “There are many different ways supersymmetry can be altered to accommodate these results,” Thompson added.
In other words, the measurements above have put some restraints on the multitude of supersymmetry models that theorists can come up with, but theorists still have plenty of room to play. At least now they are playing with a little reality mixed in.
Observers still have room to play, too. Despite the null results, Cooke, Thompson, Whitmore, and their colleagues remain determined to keep going. Many supersymmetry models predict the fundamental constants varied more in the very young universe, so astronomers will keep pushing to ever-greater distances to see if that prediction’s true.