Judging only by the blistering summer sun, you’d be forgiven for assuming that space is a hothouse. But despite the tremendous energy pouring out of trillions and trillions of stars, our universe is surprisingly arctic.
To understand why, we first need to wrap our heads around temperature because its true nature isn’t obvious when you burn your hand on the stove or dive into an icy lake. When scientists talk about hot and cold, they’re referring to the average kinetic energy of a system (whether it be a snowball or an entire galaxy), which is based on the motion of its particles: the more they jiggle, the hotter they are.
But far from stars, planets, and other cohesive objects, in empty tracts of the cosmos, the concentration of particles drops precipitously — and so does the thermometer.
“Most of space is cold,” says Emily Hardegree-Ullman, an astronomy professor at Colorado State University, “because there is just nothing to possess kinetic energy.”
