Jeff McMahon

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.

Imagine looking at a distant mountain range through morning fog. The peaks' outlines are blurred, and if the fog is patchy, some slopes fade more than others. That's roughly how astronomers see the ancient light of the universe, which has been traveling to us for 13.8 billion years. This light was born when the universe was just 380,000 years old, long before the first stars.