How fast are galaxies moving relative to us?

Recall that one can measure how fast a galaxy is moving away from us or toward us by measuring the Doppler shift of the spectral lines from the galaxy. Note that we determine in this way the relative motion of us and the distant galaxy: Recall also that one can measure, with some difficulty, how far a galaxy is from us. In the 1920's Hubble investigated the relation between distance to galaxies and their velocity toward or away from us. He found (as had been noted earlier) that most galaxies are moving away from us.

In a paper ppublished in 1929, Hubble reported a relation between distance and velocity, now known as the Hubble law.

Here is a graphical representation of the data for many galaxies

(This graph does not have real data on it, but gives the qualitative picture.)

From such graphs we learn

The constant H0 is the Hubble constant. Its value is controversial. The data on the graph above suggest a value

H0 ~ (30 km/s)/Mly
Taking this value we find Many astronomers would prefer a value
H0 ~ (15 km/s)/Mly.


The Hubble law has a profound consequence. How can we visualize the expansion?

When did it all start?

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA