Ask Ethan: Is the Hubble tension the same thing as dark energy?
Briefly

Ask Ethan: Is the Hubble tension the same thing as dark energy?
"After all, we had a law of gravity - Einstein's General Relativity - that allowed us to calculate how the Universe would evolve based on the amount, density, and distribution of matter and energy within it. We observed the Universe to be roughly uniform in all locations and all directions, and we learned back in the 1920s and 1930s that the Universe was expanding."
"Oh, if only we realized how little we knew back then. We had erroneously assumed that the Universe was 100% made up of matter, that the expansion rate would be decelerating, and that there wouldn't be any surprises. Of course, there wound up being many, including the existence of dark energy and the uncovering of a more recent puzzle: the Hubble tension."
"Are they one and the same, though? That's what Matt Williams wants to know, asking: "Do the Hubble Tension and Dark Energy describe the same thing? Both appear to be based on observations that the expansion of the Universe has been speeding up in more recent cosmic history." We're going to start with the surprises that we learned about the Universe in the late 20th century, and then, one at a time, we'll come up to the puzzles we have here in 2025."
For much of the 20th century cosmology focused on measuring the Hubble constant and the deceleration parameter to determine the Universe's age, history, composition, and fate. General Relativity allowed predictions given the matter and energy content under assumptions of homogeneity and isotropy. Early assumptions that the Universe consisted entirely of matter predicted decelerating expansion. Observations later revealed unexpected phenomena, notably dark energy causing recent accelerated expansion and a newer discrepancy known as the Hubble tension. The Hubble tension questions the consistency of measured expansion rates, prompting investigation of whether it and dark energy represent the same underlying physics.
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