UBC researchers helped uncover 15,000 new potential strong gravitational lenses - the largest collection of these cosmic tools that help astronomers investigate dark matter.
The first major release of data from the European Space Agency's Euclid space telescope comprises 1.7 million gigabytes of data. Analysis by a global consortium of more than 2,000 scientists, including more than 30 Canadians, revealed more than 900 million galaxies, 60 million quasars, the brilliant cores of distant galaxies and almost five per cent of the entire night sky from 13 months of observation.
"Strong lenses have been difficult to find - the odds were about one in 10,000," said co-author Hrishabh Srivastava, doctoral student in the UBC department of physics and astronomy (PHAS). "Before Euclid, we had less than a thousand confirmed strong lenses. Now we have 15,000 new candidates."
Dark universe hunter
The Euclid satellite launched in 2023 at a cost of more than $1 billion with the aim of mapping more than one-third of the sky to build a 3D map of the universe, which would help scientists determine the properties of dark energy and dark matter on a universal scale.
Dark energy and dark matter remain two of the greatest mysteries of modern physics despite making up about 95 per cent of the universe. Dark energy is an unknown force pushing the universe apart and observed only by the accelerating expansion of galactic objects away from each other in all directions.
Dark matter is some form of invisible, unknown matter that we can only detect through its gravitational effects - including the bending or 'lensing' of light.
"It turns out, the stuff humans are made of is relatively rare in the universe," said co-author Dr. Douglas Scott, PHAS professor and a member of the Euclid collaboration. "And what dominates - dark matter and dark energy - is invisible so we're not used to dealing with it. But it makes the universe what it is, it drives how the universe expands and what the past and future of the universe will be. Where did these things come from? What properties do they have? Euclid's data gives us the potential to answer those questions."
Canadian organizations involved in the Euclid mission include UBC, University of Waterloo, NRC's Herzberg Astronomy and Astrophysics Research Centre, the Canada-France-Hawaii Telescope and many others.
Nature's magnifying glasses
Gravitational lensing occurs when the light from one galaxy is bent by a foreground galaxy acting as a lens. The light from the more distant galaxy appears, in a perfect example, as a ring of light around the foreground galaxy, known as an Einstein ring thanks to his prediction of the phenomena back in 1936. The composition and distribution of matter in the foreground, including dark matter, is revealed by how the light is bent.
"I like to call these gravitational lenses 'nature's own magnifying glasses' to probe the dark matter distribution of our universe," said Srivastava.
"It took many decades to verify Einstein's prediction of gravitational lenses, but if he had the Euclid data it would have been easy to confirm, since they're all over the sky," added Dr. Scott.
Gravitational lenses can also be used to investigate dark energy by plugging their parameters into the current cosmological model and seeing if the numbers match. They also magnify the background galaxy for a clearer picture.
Srivastava was part of the team finding gravitational lens candidates. This four-part process involves machine learning to initially parse the huge amount of data for likely lenses. Then, thousands of citizen scientists pore over the AI-sorted data to further refine the candidate pool. Third, experts including Srivastava check the citizen scientists' work, looking at about 6,000 to 10,000 galaxies each. The final step involves verifying that the candidates are indeed lenses using follow-up data with other telescopes.
The new catalogue is larger than all previous collections of strong gravitational lenses combined. Because the observations were gathered by the same telescope and analyzed using the same methods, researchers can compare them more reliably than ever before.
Srivastava will attempt to estimate the dark matter distribution of the universe. He also aims to build a machine-learning model strong enough to identify strong lenses without verification.
A universal map
Unlike ground-based telescopes, Euclid operates above Earth's atmosphere, which blurs astronomical images. That allows Euclid to spot faint and distant features that would otherwise be difficult or impossible to detect.
The data release, described in 26 papers submitted to journal Astronomy & Astrophysics today, also includes spectra from more than 160 million sources. A spectrum is the data obtained by splitting the light from galaxies into 'rainbows', including distance and sometimes the elemental composition of the emission source.
Scientists can use the dataset to determine distances to galaxies and place them into the 3D structure of the universe, building a giant map. The current dataset covers about 14 per cent of the total sky area Euclid will survey in its six years of service.
Euclid is supported in part by the Canadian Euclid Consortium
Interview language(s): English (Scott, Srivastava), Hindi (Srivastava)
