An international scientific group led by the University of Iowa is trying to solve one of the biggest mysteries in cosmology: why the universe’s expansion is accelerating.
Just two decades ago, scientists realized the universe was growing faster over time rather than at a constant rate. The force assumed to be behind that expansion is called dark energy.
The realization that dark energy is responsible for the universe’s growth came from astronomical observations. But more information is needed to understand what dark energy is.
Spyridon Argyropoulos, a postdoctoral researcher in the UI Department of Physics and Astronomy, showed that scientists can search for dark energy using particle detectors at the Large Hadron Collider (LHC), the world’s foremost particle collider, located near Geneva, Switzerland. Working with the ATLAS detector collaboration in collision experiments at the LHC, Argyropoulos has established some boundaries, or constraints, for how physicists might find dark energy.
“The novelty here is nobody else has looked for such a thing in the colliders,” says Argyropoulos, a particle physicist who works with Department of Physics and Astronomy Professor Usha Mallik. “Now we have. From now on, people who are interested in dark energy should keep in mind that colliders might tell us what this force could be. We have proof of principle that a collider search for dark energy can work.”
The idea of a constant energy permeating the universe has held sway since observations by the Hubble telescope in 1929 upended Albert Einstein’s theory that the cosmos was static and unchanging. But those observations supposed the universe is expanding at a constant rate.
That idea worked until 1998, when astronomers observed distant supernovae—exploding stars in their death throes—and determined the supernovae were farther away than expected. That meant a force must be counteracting the gravitational pull exerted by planets, stars, and other bodies and forcing the universe outward. The simplest explanation for this force was to call it a new type of matter that has a constant energy density, which is how dark energy came to be.
More evidence for dark energy came from clever experiments looking at deep space and echoes of the Big Bang, as well as observing minute oscillations in gravitational waves on Earth. But none of these showed what dark energy looked like or what its physical state might be.
One way to do that would be to recreate the conditions in which dark energy is thought to be created.
Two years ago, European researchers proposed a method to detect dark energy. They suggested that dark energy particles might be created in ultra-high-speed collisions that produce heavy, subatomic particles called top quarks.
Argyropoulos read the paper, then designed experiments to find out whether it would be possible to detect dark energy in particle collisions at the LHC. Working with Mallik and other collaborators in the ATLAS group—one of two main detectors at the LHC searching for known and theorized particles—Argyropoulos conducted a series of high-energy particle collisions to establish the conditions in which dark-energy particles could be found.
“The basic thing you expect to see from dark energy is missing energy,” says Argyropoulos, who joined Mallik’s lab in June 2015 after earning a doctorate from DESY, a national research center in Germany that operates particle accelerators used to investigate the structure of matter. “The model was telling us the most probable scenario is that you would see this missing energy being produced in association with top quarks, or in association with very energetic particles. What we looked for were events with either very energetic particles and missing energy or with heavy quarks and missing energy.”
The group found no evidence of dark energy, but it did establish limits for future collision tests.
“What we can say is that if dark energy is produced, it has to be produced at energies above 1 TeV (units of energy used in particle physics),” Argyropoulos says.
Mallik says those parameters are critical.
“It’s like if I describe someone and I say 'taller than six feet,' you know we’re not looking for a person who’s below that height,” Mallik says.
Argyropoulos credits theorists with the idea that dark energy might be found using particle detectors.
“My hope is that this work will serve as a stepping stone for further developments that will bring the community of cosmologists and particle physicists closer,” says Argyropoulos, whose contributions were included in a recent story by the European nuclear research agency where the LHC is located.
The research adds to the UI’s contributions in the particle physics field. Mallik’s team and a separate group led by UI faculty members Yasar Onel and Jane Nachtman, also in the Department of Physics and Astronomy, are designing, building, and testing a new generation of subdetectors at the LHC that could yield new information about the state of the universe just a trillionth of a second after the Big Bang.