"It's also led to a profound existential question: how will the Universe end? If it's expanding, which it is, and the expansion rate has decreased over time, which the evidence also supports, then what will happen in the far future? Will the expansion continue forever? Will it continue to decrease, or will it increase again? And if it does decrease, will it ever drop to zero and then, afterward, reverse itself?"
"In so many of the popular consumption articles that discuss the possibility that the cosmological constant (or dark energy) might not be a constant - and in particular may be undergoing a recent weakening - always the next sentence states that this might mean that eventually the universe could/would/might be heading for a 'Big Crunch.' But why the heck is this the automatic 'go to' future scenario?"
"That last scenario, once favored by many, would lead to a set of circumstances that would behave like a "Big Bang in reverse," also known as a Big Crunch. It would have seemed to many, with the discovery of dark energy (and the consistency of dark energy with being a cosmological constant), that the Big Crunch would have been inconsistent with how we observed the Universe to actually be."
Observations since the 1920s established that the Universe is expanding, leading to the hot Big Bang model, the cosmic microwave background, and the growth of large-scale structure. Measurements show the expansion rate has decreased over time, raising questions about the ultimate fate of the cosmos: perpetual expansion, slowing then reacceleration, or eventual reversal to a collapse. The discovery of dark energy and the plausibility of a cosmological constant made an eventual Big Crunch appear unlikely. New DESI results indicate dark energy may be evolving and currently weakening, which revives consideration of a future collapse. The assumption that weakening dark energy must imply a Big Crunch is now under scrutiny.
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