# Luminosity distance

• October 6th, 2011, 08:32 AM
PetTastic
Luminosity distance
Elsewhere, I am having a few slightly heated discussions about how to calculate the Luminosity distance as a function of z, for an alternative cosmology.
The Wikipedia page "Luminosity distance - Wikipedia, the free encyclopedia" gets a bit complicated very quickly for the BBT version.

I was assuming that if I have a function distance(z) and function photonRate(z) (photon rate decreases with z)
Then Luminosity distance is just: distance(z)/photonRate(z)

:confused:
• October 6th, 2011, 08:54 AM
PhysBang
Could you give an argument as to why you think this equation is appropriate?
• October 6th, 2011, 10:23 AM
PetTastic
Quote:

Originally Posted by PhysBang
Could you give an argument as to why you think this equation is appropriate?

Yes, but it does not work :oops:

I got a mail pointing out my silly mistake within minutes of posting.
If photonRate(z) return 0.25 it only makes the object look twice the distance not 4 times.

Edit---
Double oops, there is also photonEnergy(z) :oops::oops:
Basicaly I made a right mess of the post when trying to remove all the none standard cosmolgy crap into functions.
DL = distance(z)/((photonRate(z)*photonEnergy(z))^0.5)
The real question is does the angular size come into it?