UC researcher presents interpretation of the recently detected low-frequency gravitational waves

English version: Diana Taborda

Ricardo Zambujal Ferreira, cosmologist and assistant researcher at the Centre for Physics of the University of Coimbra (CFisUC) was one of the scientists who proposed an alternative interpretation to explain the origin of the recently detected ultra-low frequency gravitational waves, a remarkable finding announced by Pulsar Timing Arrays (PTA), which include the North American Nanohertz Observatory for Gravitational Waves (NanoGRAV) and the European Pulsar Timing Array (EPTA).

Albert Einstein's theory of relativity predicted that when massive objects accelerate or undergo strong gravitational interactions, they would emit gravitational waves that propagate through space-time, like a cosmic background "hum". Scientists have being trying to prove Einstein's prediction and try to "hear" this cosmic "hum" from gravitational waves for a long time, and the breakthrough arrived in late June 2023: low-frequency gravitational waves had been detected. But what is the origin of gravitational waves? Although there is a standard explanation among the scientific community, evidence shows that the interpretation now proposed by the researcher from the Faculty of Sciences and Technology of the University of Coimbra is one of the most plausible explanations for the signals detected.

According to Zambujal Ferreira, who co-authored the article, “the most standard interpretation of the origin of the gravitational waves detected is that they were generated by a cluster of millions of supermassive black hole binaries (which orbit each other), formed when galaxies merge. However, this is not the only plausible explanation, there are others also studied in one of the scientific papers about this finding". One of those alternative explanations was proposed by Ricardo and his fellow researchers in 2022, when they suggested that the gravitational waves detected might have originated from specific cosmological structures, known as domain wall networks. These structures form in the early universe when a cosmological phase transition results in the spontaneous breaking of a discrete symmetry.

But what exactly are gravitational waves? If we throw a pebble into a pond, we see waves spreading out from where the pebble hit the water. Now if we think of space, when big objects, like stars or black holes, move around or collide with each other – just like the pebble and the pond – they create ripples. These ripples are called gravitational waves. And what about low-frequency gravitational waves? They are like slow ripples in space: just as we can have big waves and small waves in the ocean, gravitational waves can also have different frequencies, which describe how fast or slow the ripples happen.

Ricardo Zambujal Ferreira explains that “this finding will have a huge impact on several areas of modern physics, from astrophysics to cosmology, providing a unique and direct glimpse into the very early universe and herald a new era for using gravitational waves to study fundamental physics”, further adding that “the arrays detected these waves by monitoring pulsars – neutron stars that emit beams of electromagnetic radiation out of their magnetic poles every millisecond. The passage of a gravitational wave distorts the space-time between us and the pulsars, thus changing the time intervals between their pulses. It was by continuously measuring the signals (pulses) emitted by a network of pulsars for over more than ten years that the PTAs were able to detect the presence of gravitational waves”.

Zambujal states that "the NANOGrav article analyzes our proposal in great detail and confirms that domain wall networks indeed provide an explanation of the data that is as plausible as supermassive black holes, and in some cases even more suitable. We will have to wait for the next few years and for more data to safely determine which interpretation is correct".

The scientific articles related to the recent finding and the interpretation proposed by the CFisUC researcherare available at: https://iopscience.iop.org/article/10.3847/2041-8213/acdc91/pdf; https://iopscience.iop.org/article/10.1088/1475-7516/2023/02/001.