In absolute silence, you need to hear a feather drop. But someone keeps rustling a plastic bag. For the space-based LISA observatory, that 'rustling' will come from charged particles in space.

LISA consists of three satellites that use laser beams to measure the distance between freely floating gold-platinum cubes inside them. When a gravitational wave passes, space stretches slightly, and the distance changes. But the solar wind and galactic cosmic rays bombard the cubes. Each incoming particle leaves its charge on the cube, like dust on glass. Gradually, charge accumulates, and the cube begins to feel the electric fields of the walls of its container. A force arises that pushes the cube, and this push could be mistaken for a gravitational wave.

The main suspect was low-energy secondary electrons. When a proton hits the cube, it knocks out several electrons from its surface. If these electrons fly away to the walls, the cube's charge grows. But if some of them return, the balance changes. To catch this effect, physicists set up an experiment at an accelerator. They bombarded a mock-up of the LISA measurement system with protons and, for the first time, directly measured how charging depends on the electric potential of the cube itself. It turned out that at a small positive voltage, the cube charges less — some of the knocked-out electrons return home.

Now engineers will be able to calculate more precisely what electric 'climate' needs to be maintained inside the satellites to clean the signal from noise. And then, in the cosmic silence, we will be able to hear the Universe itself breathe.