“If you’re puzzled by what dark energy is, you’re in good company.” -Saul Perlmutter
We normally assume that the fundamental constants of the Universe are actually constant, but they don’t have to be that way. They could vary in space, in time, or with the energy density of the Universe, in principle. Before believing in such an extraordinary claim, however, you’d need some remarkable evidence. It’s arguable that exactly that sort of evidence is emerging: from the tensions in the expansion rate of the Universe.
If you measure the expansion rate from the cosmic microwave background, you get a value for the expansion rate of 67 km/s/Mpc. But if you measure it from the traditional cosmic distance ladder, you get a value closer to 74. This tension could be a systematic error in the measurement, but it could also point towards the value of dark energy changing with time. Interestingly, a large survey independent of the Universe’s expansion but dependent on weak lensing shows an increasing dark energy might be the answer.