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My dark energy/expansion of the universe thought experiment is still this: when you throw a ball up in an expanding universe, how fast does it come back down? This is (clearly) not my field but this basic question just seems to lead fun places. No matter how I think about this question I always come back to the idea that matter seems special in an expanding universe since it seems to extract energy from nothing if the universe is expanding uniformly. As we learn more I hope to get a good explanation for this question one day.