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The Question

(Submitted May 28, 2010)

I heard that the Hubble constant varies every year because the universe expands. Is this true? If not why, and how can it be a constant if it varies? I am confused, please help.

The Answer

Thanks for the question. If you are on a train cruising between two stations, does the fact that the train is moving imply that its speed is changing? Of course not --- and the Hubble constant is like the speed of a train. The fact that the universe is expanding does not imply the Hubble "constant" must change. If the train accelerates or brakes, on the other hand, its speed will change, but that's a separate question from whether the train is moving or not.

In fact, the expansion of the universe is accelerating, but not so fast that we can detect the changes in the Hubble constant directly. If we wait 1 billion years, its value will have changed enough for us to be able to measure the difference. For a wait of 1 million years, we probably have no chance, with the accuracy of the techniques we use today. So, for all practical purposes, the Hubble constant is indeed a constant.

Best wishes,

Koji & Georgia
for "Ask an Astrophysicist"

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