By Stuart Wolpert and Kavli IPMU
A team of theoretical researchers, including UCLA Professor of Physics and Astronomy Alexander Kusenko, has linked gravitational waves, supersymmetry, and the universe’s matter/antimatter asymmetry, and discovered it may be possible to detect Q-balls in gravitational waves — and their detection would solve the mystery of why more matter than anti-matter was left over after the Big Bang.
Humans exist because at some time in the universe’s first second, more matter was somehow produced than anti-matter. The asymmetry is so small that only one extra particle of matter was produced every time ten billion particles of anti matter were produced. Current theories of physics cannot explain this asymmetry. Standard theories say matter and anti matter should have been produced in exactly equal quantities.
A popular idea among researchers is that this asymmetry was produced just after inflation, a period in the early universe when there was a very rapid expansion, from a condensate of fields that develops during inflation. However, testing this paradigm directly has been difficult, even using the largest particle accelerators in the world, because the energy involved is billions to trillions of times higher than anything humans can produce on Earth.
The condensate owes its existence to what is known as “supersymmetry,” Kusenko explained, saying: “Supersymmetry is a very appealing property of nature that has not yet been confirmed experimentally, but which many scientists believe to be likely. One appealing feature is that supersymmetry can easily explain the matter-antimatter asymmetry (via what is known as Affleck-Dine baryogenesis). Supersymmetry predicts that many ‘scalar’ fields (similar to the Higgs field) can exist and that many of them can be endowed with the properties that distinguish matter from antimatter. In the early universe, these fields develop large values, and then decay into quarks and leptons, the building blocks of matter. Since the supersymmetric fields distinguish between matter and antimatter, the plasma of particles resulting from their decays can have the right amount of matter-antimatter asymmetry. But how are we going to confirm that this process took place? Well, the Affleck-Dine fields are unstable with respect to forming lumps, or Q-balls. Such Q-balls can exist for some time, then evaporate into quarks and leptons. But because the scalar field exists for some time as a lumpy substance, the presence of such lumps can be detected via gravitational waves. As the universe expands, the lumps of field, Q-balls, can come to be the dominant energy before they decay.
“If this is the case,” Kusenko concluded, “then gravitational waves can tell us about their temporary existence in the early universe.”
A team of researchers in Japan and the U.S. including University of Tokyo Kavli Institute for the Physics and Mathematics of the Universe Project Researcher Graham White, Kusenko, a 2021 Simons Fellow and Kavli Institute Visiting Senior Scientist, and Kusenko’s former UCLA Ph.D. student Lauren Pearce report in the journal Physical Review Letters a new way to test this theory by using blobs of field known as Q-balls. Pearce is now an assistant professor of physics at Penn State-New Kensington
The researchers use the fact that the condensate of fields forming after inflation usually breaks up into lumps, called Q-balls, before decaying into ordinary particles. The presence of these lumps in the early universe can be detected in gravitational waves, Kusenko said.
“Q-balls dilute slower than the background soup of radiation as the universe expands until, eventually, most of the energy in the universe is in these blobs,” White said. “In the meantime, slight fluctuations in the density of the soup of radiation start to grow when these blobs dominate. When the Q-balls decay, their decay is so sudden and rapid that the fluctuations in the plasma become violent soundwaves which leads to spectacular ripples in space and time, known as gravitational waves, that could be detected over the next few decades. The beauty of looking for gravitational waves is that the universe is completely transparent to gravitational waves all the way back to the beginning.”
The researchers discovered the conditions to create these ripples are very common, and the resulting gravitational waves should be large enough, and low enough frequency to be detected by conventional gravitational wave detectors.
Caption for attached images:
Asymmetry in the universe may have been the result of the following process: (1) The potential for the inflation has a shape and starts away from its minimum. (2) At the end of inflation, a field starts rolling around to its minimum. (3) In different patches, blobs of field appear. (4) These blobs melt so fast they practically vanish. (5) This sudden vanishing results in enhanced ripples in space and time. A team of theoretical researchers suggests these ripples could be detected by gravitational wave detectors. (Credit: Kavli IPMU)