the hubble tension
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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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