For the first several billion years of our Universe’s history, the Universe’s expansion rate is decreasing and distant galaxies slow in their recession from ours, as the matter and radiation densities drop. However, for the past ~6 billion years, distant galaxies have been speeding up in their recession, and the expansion rate, though still dropping, is not headed towards zero. (Credit: NASA/STSci/Ann Feild)

One of the biggest surprises in all of science history came at the very end of the 20th century. For the prior ~70 years…

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