Strong gravitational lensing and microlensing of supernovae (2024)

Einstein predicted that mass warps spacetime, bending the path of light like a river bending around a boulder. But when a supernova—a star exploding with the energy of a billion suns—aligns perfectly behind a massive galaxy or cluster, the universe performs a trick that would make even the most jaded illusionist blush. The light from that single explosion takes multiple paths to reach us, arriving at different times. We see the same star die, twice, or even four times, like a cosmic replay button. This isn't just a neat party trick; it's a precise laboratory for measuring the expansion rate of the universe, because the time delay between those images is directly tied to the Hubble constant—and it's a measurement that's currently at the heart of a major cosmological crisis. But the real weirdness kicks in when you zoom in. The foreground galaxy isn't a smooth sheet of mass; it's made of billions of individual stars. As a supernova image passes behind one of these stars, the star's own gravitational field acts as a "microlens," causing the supernova's brightness to flicker and flare in ways that are exquisitely sensitive to the fine-grained structure of the lensing galaxy. This microlensing signal is like a stethoscope pressed against the belly of a galaxy, letting us hear the subtle pops and crackles of its stellar population. More tantalizingly, it can also reveal whether dark matter is made of compact objects—like primordial black holes—or is truly diffuse and wispy. Each flicker is a clue. This paper, hosted at EPFL's Infoscience repository, sits at the intersection of observational astronomy and theoretical prediction. It reminds us that the universe is not just a collection of objects, but a dynamic, curving stage where gravity writes the script. The phenomena described here were once considered impossibly exotic, the stuff of science fiction. Now they're routine tools for cosmologists. And yet, every time a lensed supernova is caught in the act, it still feels like a small miracle—a confirmation that the universe is far stranger, and far more connected, than our everyday senses would ever suggest. The light from a star's death, bent and delayed by the gravity of another galaxy, is a message from across the cosmos, and we're finally learning to read it. Source: [EPFL Infoscience – Strong gravitational lensing and microlensing of supernovae](https://infoscience.epfl.ch/entities/publication/644cad8a-6c9b-4b02-bcf3-b8b6e8c614c5)