How Fast Is the Universe Really Expanding?

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One of the biggest mysteries in cosmology seems to keep getting bigger. Astronomers have known since the 1930s that the universe is expanding, but in the 1990s, the discovery that this expansion is accelerating rather than slowing down came as a huge shock to the field. Something had to be driving that acceleration, and dark energy was proposed as the cause. What began as a seismic shock in cosmology eventually led to a Nobel prize . But this story has a sequel, and it comes with another major plot twist. The two main methods for estimating the universe's present-day expansion rate are producing significantly different answers. As a result, how fast the universe is really expanding has…