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xalos
2008-Dec-19, 02:55 AM
"The unit of redshift z used here is related to distance by approximately: d = 4300z Mpc for small z"
this is the equation for redshift given by
http://howdy.physics.nyu.edu/index.php/What_is_out_there
how does this work?!? isn't redshift dependent on the relative velocity of two objects?

Spaceman Spiff
2008-Dec-19, 03:19 AM
"The unit of redshift z used here is related to distance by approximately: d = 4300z Mpc for small z"
this is the equation for redshift given by
http://howdy.physics.nyu.edu/index.php/What_is_out_there
how does this work?!? isn't redshift dependent on the relative velocity of two objects?

Hubble's Law:

v(recession) = c x z = H_o x d(Mpc), c = speed of light (km/s), H_o is Hubble's parameter (units of km/s/Mpc) at the present time.

Do the algebra. I find (keeping a few more digits):

d(Mpc) = 4283 * (70/H_o) * z, good for z < 0.1, beyond which there really is no unique value of distance (http://www.atlasoftheuniverse.com/redshift.html) due to expansion of space-time and then eventually the effects of acceleration/deceleration.

Ken G
2008-Dec-19, 04:39 AM
And adding to that, what z really means is that the cosmologically redshifted wavelength you observe is the wavelength you'd otherwise expect times 1+z. Hence z=0 is what you get from the laser pointer in your hand, and large z means a whopping amount of redshift (expanded wavelength). As mentioned above, the connection to distance comes from the Hubble law, which says that larger z implies larger distance. Somewhat more naturally, you can imagine that z measures how much the universe has expanded since the light was emitted, so it's really more a measure of the history of the expansion than it is a distance, but the two are closely related by the fact that we know the speed of light (and hopefully, we know the expansion history, though that is a matter of somewhat more debate).