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14 maltatoday | SUNDAY • 16 AUGUST 2026 NEWS Malta-Istanbul team hunts for cracks in dark energy. They may have found one A machine- learning pipeline built in Malta, in partnership with researchers in Istanbul, found something in the data neither team had told it to look for. Juliana Zammit on what it means, and how far it is from proof JACKSON Levi Said has spent years working on one of physics' biggest open questions: What dark energy actually is. The professor from the Univer- sity of Malta's Institute for Space Sciences and Astronomy, com- pares dark energy to a balloon with dots painted on its surface. The dots appear to drift apart as the balloon is blown up, even though the dots themselves aren't moving. It's the material between them that stretches. Galaxies, he said, behave much the same way, clustered together like those dots, with dark energy acting like the air filling the bal- loon and pushing the space be- tween them wider. The standard explanation for that stretching has long been a fixed cosmological constant, a val- ue that doesn't change no matter how far back or forward you look, and it has served cosmologists reasonably well for decades. More recently, though, cracks have started to show, most visibly in what's known as the Hubble tension—a stubborn disagree- ment between how fast the uni- verse appears to be expanding today and what the early universe seems to predict it should be do- ing. While addressing the disagree- ment, Said's team developed a machine-learning pipeline that learns directly from data, unlike older methods that rely on as- sumptions about the universe, filling gaps where observations are sparse without pre-set notions. That approach is what brought Istanbul into the project. Özgür Akarsu, a professor at Istanbul Technical University, had, since 2020, been building the theoret- ical case that dark energy might not be fixed at all but could have switched from negative to positive at some point in cosmic history. The two groups had already spent years working alongside each other through CosmoVerse, a COST Action research network chaired from Malta that con- nects more than a thousand cos- mologists worldwide, and it was through that collaboration they realised they were circling the same mystery from two different directions. What the machine found When the team combined gal- axy survey data from DESI, Pan- theon+ supernovae, and cosmic chronometers, a pattern emerged. Dark energy's density was nega- tive in the past and positive now. "Our findings suggest a transition of the cosmological constant at about 20% of the universe's cur- rent age, which could reshape our understanding," Said noted. That figure closely matched what Akarsu's group had worked out years earlier from theory alone. "Watching the data alone draw a curve that dives below zero in the past, in every combination we tried, crossing at essentially the epoch we had written down years earlier, that was a special day," Akarsu said. He knows a scientist has to remain his own harshest sceptic but he admitted that "pri- vately it felt as if the universe was winking at us". Beyond that headline result, the team also came across something they hadn't gone look- ing for—hints in some data combinations of a brief extra burst of accelerated expansion clustered around that same period. "At first, nobody ordered that," Akarsu said, though fol- low-up theoretical work has since suggested such a burst can arise natu- rally once dark energy is allowed to switch sign in this way. Not a discovery yet Both researchers were careful to describe the result as a lead worth following up rather than a settled finding. "The signals are at the two-to-three sigma level: Intrigu- ing hints, not a discovery," Akarsu said. The next real test is whether the same pattern holds up once it's checked against early-universe data from missions like Planck, ACT and SPT, alongside what up- coming observations from DESI and Euclid are expected to reveal. The project also gave a number of early-career researchers a cen- tral role in the work, among them Konstantinos Dialektopoulos in Malta together with Jackson Levi Said and, in Istanbul, Emre Onur Kahya, together with postdoctor- al researchers Maria Caruana and Luis Adrián Escamilla Torres. "The Bridging Cosmology [pro- ject] has given me an opportunity to apply my theoretical knowledge in cosmology to data analysis and machine learning," Caruana said. What's at stake If the transition turns out to hold up under further scrutiny, Akar- su said, it would mean the vacu- um of the universe itself changed state at some point in its history. "It's somewhat like water abrupt- ly changing phase." The finding would unsettle the long-standing assumption that dark energy will simply carry on accelerating the universe's expansion forever. "In 1998 we learned that the ex- pansion of the universe is acceler- ating. This decade may teach us that the engine of that accelera- tion has a history of its own," he said. The Bridging Cosmology pro- ject, financed through the Xjenza Malta-TÜBİTAK Joint Call for R&I Proposals 2024, is led by Jack- son Levi Said from the Universi- ty of Malta's Institute for Space Sciences and Astronomy, and Özgür Akarsu of Istanbul Techni- cal University. This is a standard figure that shows the big bang expansion to today. It's a good image to communicate the general idea of the expansion of the universe L to R: Prof. Özgür Akarsu, Prof. Jackson Said UOM, Prof. Dr. Emre O. Kayha (ITU, researcher in the project), Dr Maria Caruana (Turkiye) and Dr Konstantinos Dialektopoulos (Malta)