In 2023, physicists were astounded by the detection of nearly imperceptible ripples in spacetime, known as gravitational waves. These ripples, associated with rapidly spinning neutron stars called ‘pulsar timing arrays,’ have a low-frequency hum, initially thought to stem from a phase transition shortly after the Big Bang. However, new research throws a wrench into that assumption.
Theorists have long proposed that these nanohertz gravitational waves originated from a phase transition that occurred in the early universe, shaping the masses of fundamental particles. But Andrew Fowlie, an assistant professor at Xi’an Jiaotong-Liverpool University, points out flaws in this explanation. “Our work uncovers serious problems with that otherwise appealing explanation of their origin,” he stated.
Phase transitions are sudden changes in a substance’s properties, often triggered by reaching a critical temperature. The most familiar example is water freezing into ice. ‘Supercool’ transitions occur when a substance remains in its liquid state, delaying the transformation into its solid phase. Many scientists believe a “first-order phase transition” took place at the dawn of time, initiating gravitational waves. These waves, they believe, could reveal conditions present during the universe’s rapid inflation or even before the Big Bang.
The concept of gravitational waves is rooted in Albert Einstein’s 1915 theory of general relativity. This groundbreaking theory predicts that objects with mass warp the fabric of spacetime, giving rise to the force we experience as gravity. Furthermore, general relativity suggests that accelerating objects generate ripples in spacetime – gravitational waves. While these ripples are insignificant for everyday objects on Earth, their impact becomes substantial when massive cosmic objects like black holes and neutron stars accelerate. These objects, when in binary systems (two objects orbiting each other), continuously emit gravitational waves until their eventual collision, producing a high-pitched ‘screech’ of ripples.
Gravitational waves, like electromagnetic radiation, come in various frequencies. High-frequency waves, like high-frequency light, have shorter wavelengths and more energy, while low-frequency waves have longer wavelengths and less energy. The low-frequency gravitational waves detected by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) pulsar timing array in June 2023 have lower frequencies than those observed from supermassive black hole and neutron star mergers detected by LIGO, VIRGO, and KAGRA. This difference implies a distinct source for these low-frequency waves. The leading candidate? A supercool phase transition immediately after the Big Bang.
“We found that to have created waves with such tiny frequencies, the transition would have to be supercool,” Fowlie explained. However, this poses a challenge. Such supercool transitions during the period of rapid cosmic inflation triggered by the Big Bang would be unexpected. “These slow transitions would struggle to finish, as the transition rate is slower than the cosmic expansion rate of the universe,” Fowlie said. “Even if the transition sped up at the end, it would shift the frequency of the waves away from nanohertz.”
Fowlie and his colleagues conclude that these low-frequency gravitational waves may not be “supercool” in origin. “If these gravitational waves do come from first-order phase transitions, we now know that there must be some new, much richer physics going on—physics we don’t know about yet,” Fowlie said. Their research highlights the need for further exploration and understanding of supercool phase transitions, particularly those that could have occurred at the universe’s beginning.
A better grasp of supercool phase transitions could also have implications beyond the cosmos. “It also has links to applications that are closer to home, such as understanding how water flows through a rock, the best ways to percolate coffee, and how wildfires spread,” Fowlie concluded. The team’s findings are detailed in a paper published in the journal *Physical Review Letters*.
