The universe is expanding. But depending on where we look, it's doing so at bafflingly different speeds.
The problem is known as the Hubble tension, and it centers around figuring out a number for the universe's expansion rate, called the Hubble constant. To find it, scientists have pored over tiny fluctuations in the cosmic microwave background (CMB) — an ancient relic of the universe's first light — and built cosmic distance ladders to remote, pulsating stars called Cepheid variables.
