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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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