It was Wednesday, September 20, 2023, 10:11 in the morning, Greek time. A historic moment for a Greek university research group and a moment of indescribable joy for the professor of the Physics Department of the Aristotle University of Thessaloniki, Nikos Stergioulas.

It was the first time that the detection of a new candidate source of gravitational waves by the LIGO detectors in the US was announced by researchers from a Greek university, via NASA’s General Coordinates Circular (GCN)! The announcement (with a first error estimate of 1 in 100 years) is about the possible detection of gravitational waves from the merger of a binary black hole system and was sent via GCN, by Mr Stergioulas.

“The lucky shift… that the Universe conspired in Greek”

“I had the great pleasure of sending the announcement. It is the first time it has been sent by a Greek university”Mr. Stergioulas – visibly moved – told APE-MBE, recalling that “I started as a theoretical astrophysicist, my PhD was in theoretical and computational studies in the General Theory of Relativity and I worked on this subject for most of the years, but since 2015 when gravitational waves were detected for the first time, I had the desire to participate in a real discovery, to feel that joy that one feels when discovering something new and not just studying it theoretically.”

“We got a lot of joy, because we are three years into this experiment (ie the LIGO-Virgo-KAGRA International Gravitational Wave Detection Experiment) as a team, but the experimental setups were being upgraded. The observations through shifts of the research teams started around the end of May, this year. There are now gravitational wave detections about twice a week on average and based on the upgrade we have done we estimate that they will be more frequent and by the end of 2024 we hope to have a detection about every day and a half,” explained the professor.

“It is the first time that a research team from a Greek university participates in gravitational wave detections”, he added, clarifying that “the procedure followed is that there is a so-called shift, i.e. every 8 hours the person in charge changes who monitors how the experiment is progressing, if there is any some detection and then is responsible for coordinating the processing of the data, convening a council with other researchers, and finally issuing an announcement through NASA’s GCN service.”

The AUTH Team for the last two years has been involved in all the publications of the experiment and yesterday’s detection will be further analyzed and is to be included in a future publication of the experiment, in which he will participate. NASA’s GCN announcements are sent and announced online to all astronomers worldwide so that they have the opportunity to make additional observations of a new source in the hope of finding another kind of radiation from such a merger, as happened in 2017 with the merger two neutron stars.

“The global gravitational wave detection experiment involves researchers from America through Ligo detectors, from Europe through Virgo to which we belong, from Asia where we have LIGO India in India and Kagra in Japan, while there are also in Australia. During a 24-hour period there are three eight-hour shifts that coincide with the corresponding sunshine on each continent. We start in the morning until the afternoon, then the observation continues from Asia and America”, clarified Mr. Stergioulas. Members of his team had several shifts over the summer but yesterday was the first time the universe … conspired in their favor and the gravitational waves were detected.

“The national team of …space successes”

The gravitational wave research group of Mr. Stergioulas at AUTH is numerous, while he also has two researchers from Athens. He has been focusing in recent years on the application of Machine Learning in Gravitational Wave Astronomy, in collaboration with the research group Deep Learning by Professor Anastasiou Tefa at the Department of Informatics of AUTH. The team participates in the LIGO-Virgo-KAGRA international gravitational wave experiment as well as ESA’s LISA space probe.

The binary black hole system that has more than 99% probability of merging should be about 10 billion light-years away. That is, gravitational waves must have started their journey when the Universe was much younger, to reach Earth today and be detected at 10:11 AM. Greek time from the LIGO detectors.

“We created the gravitational wave group at AUTH, in collaboration with our colleague Anastasios Tefa, professor of the Department of Informatics who specializes in deep learning. We formed the team in 2020 and now focus on sophisticated algorithms for analyzing data produced by gravitational wave detection experiments. In practice we have created a machine learning code that automates the process of detecting gravitational waves, which we are currently testing against past detections and we hope to soon go into real production,” pointed out Mr. Stergioulas.

There are several research groups abroad trying to create such codes, but the code developed by the AUTH team was the one that stood out as the best among four accepted in a competitive process organized by the Max Planck Institute in Germany. “Our code stood out by achieving the best performance compared to others and there was the corresponding publication. I want to highlight the crucial contribution of Dr. Paraskevi Noussi, a member of the Tefa team who wrote the specific code. After the announcement of the results of the competition we were assigned and organized in March 2023 a school for researchers from all over Europe who came to Thessaloniki, AUTH to learn these new methods of applying machine learning to gravitational waves and we had a great participation and success in this event”, concluded the professor.