Supercooled phase transitions: Could they explain gravitational wave signals?

A new study published in Physical Review Letters explores the possibility that a strongly supercooled, first-order phase transition in the early universe could explain gravitational wave signals observed by pulsar timing arrays (PTAs).

Gravitational waves, first proposed by Albert Einstein in his general theory of relativity, are ripples in the fabric of spacetime caused by violent processes like the merging of black holes.

They were first detected by LIGO in 2016, confirming Einstein's predictions nearly a century later. The most common sources of gravitational black holes are merging black holes, spinning neutron stars, and supernovae.

Recently, the NANOGrav, or the North American Nanohertz Observatory for Gravitational Waves, detected the presence of stochastic gravitational wave background (SGWB) from pulsar timing arrays (PTAs).

SGWB are different because they are isotropic, meaning they spread equally in all directions, indicating that the source of these are distributed uniformly throughout the universe.

This finding prompted the scientists in the PRL study to explore the origin of these waves, which could be from first-order phase transitions (FOPT) in the early universe.

Phys.org spoke to co-authors of the study, Prof. Yongcheng Wu, Prof. Chih-Ting Lu, Prof. Peter Athron, and Prof. Lei W from Nanjing Normal University, to learn more about their work.

"Our probe into the early universe is limited to the period after the formation of CMB [cosmic microwave background]. Although we have some indirect hints about what happened before CMB, gravitational waves are currently the only method to probe the very early universe," said Yongcheng.

Prof. Lei added, "In the past few years, the supercooled FOPT has been widely considered a possible source of the SGWB."

"A new signal seen by PTAs may be evidence of this happening—a very exciting possibility," said Prof. Athron.

Prof. Chih-Ting said that he wanted to understand the connection between the Higgs field and the Higgs boson and its connection to the mechanism of electroweak symmetry breaking. "Linking gravitational wave signals of different frequencies with cosmic phase transitions has opened another window for me to study this," he said.

First-order phase transitions

FOPT are phase transitions in which a system transitions between different phases abruptly or discontinuously. One such example we see in our daily life is the freezing of water.

"The water can stay in a liquid state even if the temperature is below the frozen point. Then, with a small perturbation [change], it suddenly turns into ice. The key signature is that the system stays in the phase for a long time below the transition temperature," explained Prof. Yongcheng.

The electroweak force is a unified description of two of the four fundamental forces of nature: the electromagnetic force and the weak nuclear force.

"We know that in our universe, one drastic change—the breaking of the electroweak symmetry that predicts all weak nuclear interactions—generates the masses of all fundamental particles we have observed today," said Prof. Athron.

This led to the electroweak force splitting into the electromagnetic and weak forces via the Higgs field (which gives all particles their mass). The process by which this happens is the strong first-order electroweak phase transition.

A supercooled FOPT is one where the temperature drop during the phase transition is sudden. The researchers wanted to understand if such a FOPT could be the source of the SGWB observed by the NANOGrav collaboration.