Take two heavy balls and place them a meter apart: they attract each other with a force that weakens by a factor of four when you double the distance. Newton discovered this law, and it works in a room, in the solar system, in a galaxy. But does it still hold when we're talking about hundreds of millions of light-years?

Astronomers found a way to test this directly. They took maps of the cosmic microwave background, the faint glow left over from the Big Bang, and overlaid them on a galaxy catalog. When light from this background passes through a cloud of hot gas around a massive galaxy cluster, its energy shifts slightly. This shift, called the kinematic Sunyaev-Zel'dovich effect, reveals how fast the cluster is moving.

By comparing the velocities of thousands of pairs of clusters at different distances, the scientists calculated how the gravitational force depends on distance. It turns out that it falls off as 1/r² within the measurement uncertainty. This agrees with general relativity and the standard model of cosmology.

So Newton's law has passed the test on the largest rulers available. Now cosmologists have a new tool to look for deviations from the usual gravity — if they exist at all.