The Hubble constant is a number we use to quantify the rate of expansion of the universe. The Hubble tension refers to the fact that when we use 2 different methods to measure this rate of expansion, we keep getting 2 different results that don’t match up. They both can’t be correct, but we don’t know why we’re getting 2 different answers. Over the years, these 2 numbers have diverged from each other even more as our measurements have gotten more precise. So we have 2 really good measurement methods with precise numbers that don’t match.

One possibility is that there’s some systematic error in one of our measurement methods that’s throwing off the results for that method. It’s also possible that there’s some aspect of cosmology at play that we don’t understand, which would mean we’re missing something in our laws of physics somewhere.

There are two main ways of determining the expansion rate of the universe, and those two methods produce different results. One method starts with what we see in the cosmic microwave background radiation, which is the earliest light in the universe, and works forward from there, using what we understand about the universe’s evolution/development, to calculate how fast the universe should be expanding. Another method is to measure the distance of certain stars in distant galaxies which gives us insight into the current and past expansion rates. Both of these methods are legitimate ways to calculate the expansion rate of the universe, but they give incompatible results. We used to think we just needed better measurements, but even as the Hubble was superceded by the JWST and measurements improved, the Hubble tension remains. In fact, it’s a bit worse.

We measure that the universe is expanding. There are different ways to measure that, they mostly fall in two categories:

* Measure how quickly the distance to stars and galaxies is increasing today

* Measure light from the very early universe, and extrapolate the expansion rate from that time to today.

Both measure the same thing, so we expect their values to agree, but the first method measures values that are ~5-10% larger. Both measurements are complicated and rely on a lot of individual measurements and calculations that come together. We might overestimate some distances, underestimate the brightness of some objects, have some wrong assumptions about what changed between the very early universe and today, or tons of other potential issues. At the moment we don’t know where the difference comes from.

There are some measurements that are completely independent of the established methods, but currently their uncertainties are too large, so they can’t tell us which set of measurements is right yet.

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