Ask Ethan: How does the Universe's temperature change over time?

Ask Ethan: How does the Universe's temperature change over time?

Our Universe, as best as we understand it, hasn’t been the way it appears today forever. While today’s Universe is filled with rich structures like a cosmic web littered with galaxies, each one containing stars, planets, and enormous possibilities for inhabited worlds, if we look back billions of light-years away — and hence, billions of years into the cosmic past — we find that there was once an early time where none of these things yet existed…