Low-frequency gravitational waves may open up new physics

In 2023, the NANOGrav observatory detected strange gravitational waves of very low frequency. They were originally attributed to a phase transition that occurred shortly after the Big Bang, but a new study casts doubt on this hypothesis. In particular, the type of transition that occurs is so slow that it is inconsistent with the rate of expansion of the universe.

This suggests that the basis of these waves may lie in unknown physics.

Predicted since 1915 by Einstein's general theory of relativity, gravitational waves are distortions of the fabric of space and time caused by powerful cosmic objects. They can be compared to ripples on the water surface created by a falling object. They can be observed, for example, after the collision of two black holes or neutron stars. These cosmic objects constantly emit gravitational waves when they form at least one binary system (two objects revolving around each other).

Last year, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) detected gravitational waves at a surprisingly low frequency (in the nanohertz range). Since then, the source of these waves has been a matter of controversy, as their frequency is much lower than that of waves generated by supermassive black holes or neutron stars. The researchers hypothesized that these waves may come from more exotic sources.

Among them are cosmic strings (hypothetical thread-like structures that could have appeared during the inflationary period of the universe), primordial black holes and the first-order phase transition – the last option being the most common. A first-order phase transition is a large transition that occurred shortly after the Big Bang, when the universe began to cool and expand.

However, a new study recently published in the journal Physical Review Letters casts doubt on this latter hypothesis. "Theorists and experimentalists have hypothesized that nanohertz gravitational waves arise from a well-known transition that occurred shortly after the Big Bang, the change that gave rise to the masses of all known fundamental particles," explains Andrew Fowley, co-author of the study, on the Xi'an Jiaotong-Liverpool University blog China). "However, our work reveals serious problems with this otherwise attractive explanation for their origin," he notes.

A frequency that does not correspond to the rate of expansion of the universe

Phase transitions are sudden changes in the properties of a substance that usually occur when a critical temperature is reached. For example, when the freezing temperature is reached, water turns into ice. There is also a type of transition known as "supercooled", which occurs when a substance remains in a certain state for a long time, even when it reaches a critical temperature, thereby slowing down its transformation.

In this case, for example, water remains in the liquid phase. After analyzing the gravitational waves detected by NANOGrav, the researchers found that the transition must be superfluid to generate waves with such a low frequency. However, such a transition would be unexpected, since its speed does not correspond to the rate of expansion of the universe. "Such slow transitions would be difficult to achieve because the transition rate is slower than the expansion rate of the universe," Fowley explains.

The researchers also tested whether the data would be able to agree if the transition were accelerated to the end. They found that while this could help complete it, the frequency of the gravitational waves generated would be well above nanohertz. Thus, the experts concluded that the waves detected by NANOGrav cannot be of superfroid origin. And if they really arise as a result of a first-order transition, then unknown physics may be behind them.

These results also emphasize the need for deeper studies of supercooled phase transitions, especially if they are assumed to have occurred in the early universe. "There are many subtleties in the connections between the energy scale of transitions and the frequency of waves. Therefore, we need more careful and sophisticated methods to study gravitational waves and superfroid transitions,” says Fowley.

Understanding this phenomenon can help solve fundamental questions about the origin of the universe, as well as understand physical processes that at first glance seem simple, but are still not fully understood, such as the infiltration of water through rocks and the precise mechanism by which wildfires spread.