Black Hole Collisions Hint at How Fast the Universe Expands ✛

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Predicted by Albert Einstein, gravitational waves are ripples in spacetime set off by colliding black holes. Earth-based detectors pick up these signals and gauge the distance to the cosmic crash. But to pin down the expansion of the universe rate, you need one more clue — how fast the source is receding. Nearly a century ago, Edwin Hubble and Georges Lemaître found that galaxies are fleeing from each other, and the farther they are, the faster they go. This motion is measured by the redshift of light, as space expands, light waves stretch and lose energy. The trouble is, most black hole mergers are invisible to telescopes. Such sources are dubbed dark sirens: you hear the sound, but see no flare.

To still catch that fleeing speed, scientists look for galaxies in the same patch of sky where the wave came from. All these stellar cities are suspects. Using spectroscopy, splitting light into colors, they measure each one’s redshift. Then, Bayesian inference (which takes all available data and recalculates probabilities) finds the most plausible expansion rate. Uncertainty sharpens into precision.

A new numerical simulation shows that ten dark sirens with well-known sky positions are enough. Then, just one night on a powerful survey telescope (like DESI). It would measure the spectra of hundreds of candidate galaxies in that square of sky. After number crunching, you get about one percent precision. Four hours of telescope time, and a decade-long dispute is resolved.

Such precision not only bridges the gap between measurements from the cosmic microwave background and supernovae. It also lets us test whether modified gravity changes across vast distances and how the enigmatic dark energy behaves. Future gravitational-wave detectors will make thousands of these measurements, and the cosmic ocean will become transparent to science.