Is the Universe’s expansion rate 67 km/s/Mpc, 73 km/s/Mpc, or somewhere in between? The Hubble tension is real, and not so easy to resolve.
There’s a mystery over how quickly the Universe is expanding, and it doesn’t appear to be going away anytime soon. Back in the early 2000s, after decades of astronomers arguing over whether the expansion rate of the Universe was closer to 50 km/s/Mpc or 100 km/s/Mpc, the Hubble Key Project was completed. Their results — the most precise in history — measured an expansion rate of 72 km/s/Mpc, with an uncertainty of just 10%. For the next decade or so, all the results remained consistent with this value, but then in the 2010s things got more precise. Measurements of the cosmic microwave background, the large-scale structure of the Universe, and of the cosmic distance ladder all improved, and led to a puzzling situation: where more-precise measurements using different methods led to incompatible results.
The groups using an early relic method, where you start at the Big Bang, calculate a phenomenon, and evolve the Universe forward in time until you see what it gives you, measure an expansion rate of 67 km/s/Mpc, with an uncertainty of just ~1%. But if you start nearby and measure progressively more distant…
