The universe is expanding, but how fast exactly? The answer appears to depend on whether you estimate the cosmic expansion rate—referred to as the Hubble's constant, or H0—based on the echo of the Big Bang (the cosmic microwave background, or CMB) or you measure H0 directly based on today's stars and galaxies. This problem, known as the Hubble tension, has puzzled astrophysicists and cosmologists around the world.