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Not a constant: the Hubble “constant” changes over time
The Universe is expanding, and the Hubble constant tells us how fast. But how can it be a constant if the expansion is accelerating?
In all of physics, some of the most important properties inherent to the Universe itself are constants of nature. The speed of light in a vacuum, the strength of the gravitational force between two masses, and the constant inherent to the quantum nature of the Universe, Planck’s constant, are all examples of fundamental parameters that never change, regardless of circumstances. But it’s also plausible that some of the so-called constants aren’t exactly constant, but vary either across space or time, changing as the Universe and its properties also evolve.
Could that be the case for the so-called Hubble constant? In fact, does the idea that the expansion of the Universe could be constant even make sense in light of what we know today? That’s what Warren Chu wants to know, asking:
“If Perlmutter and [Riess], in 1998, discovered that the expansion is accelerating, how can [the Hubble constant] be a constant, when it is slower for more distant sources and faster for closer sources?”
If you’ve ever heard the phrase, “The expansion of the Universe is…
