Hubble shows gravitational lenses ahead of Roman
· BehindLines newsroom
NASA released on 12 November a mosaic of gravitational lenses photographed by the Hubble Space Telescope. According to the agency, this varied sample prepares the observations of the Nancy Grace Roman Space Telescope, launched in August 2026.
An effect predicted by Einstein
A gravitational lens appears when the mass of a galaxy, or of a cluster, bends and magnifies the light of a more distant object lying behind it. In the pictures shown, the alignment of a background galaxy with a foreground galaxy produces distorted arcs. NASA describes several configurations: doubled arcs, a nearly complete Einstein ring and galaxies imaged four times.
What astronomers expect from them
The agency says that studying how the arcs are distorted makes it possible to estimate the mass of the foreground galaxy, invisible dark matter included. Colours also reveal stellar populations: according to NASA, bluish arcs point to young stars and reddish arcs to older ones. Most of the galaxies acting as lenses are ellipticals, but not all, which will allow different evolutionary paths to be explored.
With Hubble's crisp pictures and exact models, researchers ought to be capable of spotting small irregularities in the arcs, a possible trace of small clumps of dark matter. NASA specifies that cold dark matter theory predicts many of them, from one million to one billion solar masses, whereas the halo of our own galaxy weighs around 1,300 billion.
Hubble and Roman in tandem
Also according to NASA, Hubble observes mainly in visible and ultraviolet light with a very sharp view of individual objects, while Roman, mostly infrared, covers a much wider field and will capture more lenses. If Roman spots an interesting lens, researchers could look at it again with Hubble. The researchers plan to use these images to build a search tool, which NASA calls a "super-intelligence", meant to find similar arcs in Roman's images.
Work already under way
NASA states that these lenses were first spotted with the DESI instrument at Kitt Peak, then observed by Hubble, and that Roman will make further observations. The image is credited to X. Huang of the University of San Francisco. On arXiv, a preprint whose first author is X. Huang presents the DESI Strong Lens Foundry project: about 3,500 new lens candidates found by a neural network, 51 of them observed with Hubble and all confirmed. The sources do not establish that NASA's image and this study concern the same lenses: the link is not specified.
Why it matters
NASA presents this sample as a foundation for Roman's follow-up observations. It also raises the possibility of confirming or refuting a theory about dark matter, without announcing any result at this stage.