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Astronomers measured how giant galaxy clusters attract each other and confirmed Newton's law on scales of hundreds of millions of light-years.
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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.
Light from the cosmic microwave background has been traveling to us for 13.8 billion years. Along the way, it 'bumped' into gas in galaxy clusters, and those bumps can be measured.
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.
The Sunyaev-Zel'dovich effect is named after two Soviet physicists who predicted it in the 1970s. Today it is used to find galaxy clusters across the sky.
g \propto \frac{1}{r^n}, \quad n = 2.1 \pm 0.3
The gravitational acceleration between galaxy halos is inversely proportional to distance raised to the power n; the measured value of n is close to 2, as in Newton's law of universal gravitation.