# Blue shift galaxies and the expanding universe

We are told the universe is expanding because galaxies are red shifted and that it is a doppler effect. Do blue shifted galaxies contradict the idea of an expanding universe?

marcus
Gold Member
Dearly Missed
We are told the universe is expanding because galaxies are red shifted and that it is a doppler effect. Do blue shifted galaxies contradict the idea of an expanding universe?
No the blueshifts of some galaxies only correspond to small (few 100 km/s) random velocities---don't contradict the overall average pattern of recession rate.

If you really want to get to understand redshift and the standard cosmological model then a good thing to do is practice with this calculator
http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html

to get some experience of what recession rates (and distances) are associated with which redshifts.

To use it you first enter the three principal parameters in the three boxes over on the left labeled "matter fraction", "cosmo constant", and "Hubble". Standard numbers to use are .27, .73, and 71.

Then put in a redshift, like 8.5 (the redshift of a galaxy measured last year, since then some more distant galaxy may have been discovered) and press calculate.

It will give you the distances and recession rates.

You will see that the recession rates are mostly much larger than a few 100 km/s.

(Considering that 300 km/s is only 1/1000 of the speed of light!)

The distance definition commonly used by redshift calculators (like this and like Ned Wright's) is the same distance used in stating the Hubble Law v = H d
where v is the recession rate and d is the distance. It is the "freezeframe" distance you would measure by any ordinary means if you could freeze the expansion process, so that the distance would not change while you were trying to measure it.

Have a look at Morgan's "cosmos calculator" and see if works for you. Please let me know if you have any trouble.
The link is in my signature, or just google "cosmos calculator" and you'll get it.

Chronos