lensed arcs

Imagine you're looking at a distant flashlight through a glass marble — the marble bends the light and creates multiple distorted images of the flashlight. Now imagine tiny invisible lumps scattered around the marble. Those lumps would subtly warp the images in ways we can measure. That's gravitational lensing! Dark matter forms these invisible lumps (called subhalos), and different theories of what dark matter IS predict different sizes and numbers of these lumps. This paper combines two ways of studying those warped images — the brightness of the multiple images AND the smeared arc of light from the galaxy around the flashlight — to get a much sharper picture of those tiny lumps. They also built a mathematical shortcut that makes the calculations 100 to 1000 times faster. The upshot: they can now test whether dark matter clumps exist down to sizes smaller than has ever been probed before, helping us rule out certain types of dark matter particles.