Eight bright dots in radio telescope images. They all come from the same quasar (a distant active galactic nucleus) called HS 0810+2554. Its light passed through a massive galaxy lens, and the gravity bent the rays, producing multiple images. Usually, such gravitational lensing is explained with a smooth cloud of dark matter — an invisible substance that emits no light but exerts a gravitational pull on everything around it. However, calculations with a smooth cloud gave a small but noticeable error: the predicted positions of the dots deviated from reality by a couple of milliarcseconds.
The cause of the error might lie in the quantum nature of dark matter. According to the ‘fuzzy’ dark matter hypothesis, it consists of ultralight particles that behave not like little pellets but as waves (wave-particle duality). These particles merge into a single Bose-Einstein condensate (a quantum fog), and their waves overlap, giving rise to superposition (a mixture of all possible positions). On galactic scales, this creates a constant density ripple, like swells on the surface of an invisible ocean. To test this idea, scientists ran hundreds of numerical simulations (computer calculations), solving quantum equations for the evolution of such a halo over billions of years.
The result was impressive. For particles of a specific mass (about one ten-thousandth of a billionth of the electron mass), the discrepancy with observations plummeted to 6–12 milliarcseconds. That’s far more accurate than what old models give. Moreover, some simulations landed directly within the instruments’ margin of error, and all this without any parameter tuning. If the particles were slightly lighter, the error grew again. So nature itself hinted at the permissible mass range. Apparently, the quantum trembling of dark matter really does warp spacetime — an effect known as spacetime curvature — and shifts the rays of light. Now we have a way to ‘feel’ this trembling with future radio telescopes like SKA. They will be able to measure image positions with unprecedented precision and directly determine the properties of the quanta of the invisible world. Eight dots in the sky are just tiny ripples, but they tell us what the Universe is made of.
The quasar HS 0810+2554 is unique: telescopes captured not one bright spot, but two, and each split into four images. That gave scientists eight independent points to test the theory.