In 2023, physicists were surprised to discover almost imperceptible ripples in the fabric of space and time that form the unified body known as spacetime. They were waves discovered in association with a cluster of fast-spinning neutron stars called the “pulsar timing array.” This low-frequency background noise of gravitational waves in our universe was originally thought to result from a change or “phase transition” that occurred shortly after the Big Bang. But new research casts doubt on this assumption.
“Theorists and experimental physicists have speculated that nanohertz gravitational waves arise from a known transition that occurred shortly after the Big Bang, a change that created the mass of all known elementary particles,” said Andrew Fowley, assistant professor at Xi’an Jiaotong University. The University of Liverpool said in a statement. “But our work has revealed a significant problem with this fascinating explanation of its origin.” A phase transition is a sudden change in the properties of matter, which typically occurs when a certain material reaches a critical temperature. Perhaps the phase transition we are most familiar with is the transition from water to ice when the temperature drops below freezing. There is also the so-called “supercooling” transition.
In water, the supercooling transition occurs when the material is “stuck” in the liquid phase, slowing down the transformation to ice. Many scientists believe that a “first-order phase transition” occurred at the beginning of time, which triggered the creation of gravitational waves, or ripples, in space-time. Experts therefore believe that these waves could potentially be used to determine the conditions that existed in the first epoch of rapid inflation of our universe, or perhaps even before the Big Bang. The concept of gravitational waves dates back to Albert Einstein’s 1915 theory of gravity, or “General Relativity.” This great physicist’s theory, his masterpiece, predicts that objects with mass will distort the fabric of space-time.
According to the theory, our physical experience of gravity arises from this distortion. General Relativity goes further and suggests that when an object accelerates, it creates ripples in space-time, also known as gravitational waves. This phenomenon is negligible when accelerating objects of the scale we see on Earth, but its effects become noticeable when acceleration affects massive cosmic objects such as supermassive black holes or neutron stars. For example, if these objects exist in a binary star system, that is, if two of them are constantly accelerating around each other, they will continue to radiate gravitational waves until they finally collide, emitting a high-pitched “squawk” of these waves.
Moreover, gravitational waves exist in a range of frequencies, just like electromagnetic radiation. High-frequency gravitational waves have shorter wavelengths and more energy, just like high-frequency light. Low-frequency gravitational waves have longer wavelengths and less energy. Low-frequency gravitational waves also have a long “period.” This refers to the time between when the peak of the wave passes a given point and when the next peak passes that point.
The gravitational waves detected by the North American Nanohertz Gravitational-Wave Observatory (NANOGrav) Pulsar Timing Array in June 2023 are lower in frequency than the gravitational waves emitted by the mergers of supermassive black holes and neutron stars, which are regularly detected by the Laser Interferometer Gravitational-Wave Observatories (LIGO), VIRGO, and KAGRA. This means that there must be another source of these low-frequency nanohertz gravitational waves.
The prime suspect? A phase transition shortly after the Big Bang – a very cool phase transition, to be exact. “It turns out that the transition has to be very cool to generate waves of such small frequency,” Fowley explained. But there’s a problem: such a supercooling transition phase of the universe would be somewhat unexpected during the period of rapid cosmic inflation (in other words, the expansion of the universe) caused by the Big Bang. “Such a slow transition would be difficult to complete because it is slower than the rate of expansion of the universe,” Fowley said.
“What if the transition were to accelerate? Even if this helped complete the transition, we calculated that the frequency of the waves would deviate from nanohertz.” The researchers also added that while nanohertz gravitational waves are cold, they probably do not have an “ultracold” origin. “If these gravitational waves indeed result from a first-order phase transition, we now know that there must be new, more comprehensive physics – physics that we don’t yet know about,” Fowley said.
source: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.132.221001