Gravitational waves hint that the universe still has secrets

New research sheds light on the origin of low-frequency ripples in space-time

Just as a stone thrown into water creates ripples, the collision of massive celestial bodies, such as black holes, creates gravitational waves - disturbances that propagate through the fabric of space-time.

A specific type, gravitational waves at frequencies of nanoseconds, was discovered in 2023. The frequency of these waves is so low that it took more than ten years for scientists to see a complete cycle. But it is still not clear how these waves are created.

Some scientists thought they came from a first-order phase transition - a change in the structure of the universe as it expands and cools. But a new study challenges this theory.

Dr. Andrew Pauli, one of the authors of the article says: "Theorists And experimenters hypothesized that gravitational waves at nano-radius frequencies originate from a known transition that occurred very shortly after the Big Bang - a change that created the masses of all known elementary particles.

"But our research reveals serious problems with this explanation of their origin, which is attractive in other respects.

"We found that in order to create waves with such small frequencies, the transition had to be supercooled," says Dr. Pauli.

Transitions in supercooling can be understood when you think of ice and water. We all know that water turns to ice when the temperature cools below freezing. But water can get stuck in the liquid state even below the freezing point, slowing down the transition to ice.

But Dr. Pauli explains why his research team believes that gravitational waves at nanosecond frequencies are not generated by a first-order phase transition in supercooling. "These slow transitions will have difficulty ending, because the transition rate is slower than the cosmic expansion rate of the universe.

"And if the transition accelerated at the end? We calculated that even if the acceleration helped the transition to complete, it would have changed the frequency of the waves from the nanometer range.

"Therefore, even though gravitational waves at nano-radius frequencies are cold, they are probably not supercooled 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 yet know about.

Dr Pauli and co-authors say their results show that more rigor is needed when investigating supercooled transitions.

"Since these are necessarily slow transitions, the usual simplifications of whether transitions end or not are not applicable.

"There are many subtleties in the connections between the energy scale of the transitions and the frequency of the waves, so we need more rigorous and sophisticated techniques when we refer to gravitational waves and supercooled transitions."

"Understanding this field will help us understand the most fundamental questions about the origin of the universe.

"It also has connections to applications that are closer to Earth, such as understanding how water flows through rock, the best way to filter coffee and how forest fires spread."