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The Question

(Submitted October 19, 1997)

I've been trying to figure out exactly what the Hubble Constant is for quite a while. I know that it has to do with the expansion rate of the Universe and that it can also directly yield the distance scale and the age of the Universe. Could you possibly explain it to me in fairly simple terms. I'm in 11th grade and I've had about a month's worth of class on astronomy.

The Answer

Early in this century Edwin Hubble discovered that galaxies are moving rapidly away from us (this was the first evidence for the big bang theory for the creation of the Universe). He also discovered that more distant galaxies were moving away at a higher velocity and proposed the following relationship:

v = H*r

where v is the velocity of the galaxy, r is distance and H is Hubble's constant. Finding a more precise value for Hubble's constant is an area of very active research, since many questions of cosmology are tied up with this value. Here are a few URLs that are related to the Hubble Constant:

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970321d.html

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970326e.html

I hope this helps,

Jeff Silvis
For Ask an Astrophysicist

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