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Thanks. You, of course, have much deeper knowledge of the subject, I hoped somebody with more knowledge will eventually comment and add some more details. For

by acqq 10y ago

Thanks. You, of course, have much deeper knowledge of the subject, I hoped somebody with more knowledge will eventually comment and add some more details. For those who want to know what you talk about (at the start) and I've avoided for simplicity, lacking some more exact link, I hope wikipedia is good enough: https://en.wikipedia.org/wiki/Comoving_distance https://en.wikipedia.org/wiki/Comoving_distance If I may ask, not doing astrophysics professionally, but being interested, I don't understand why you started with z = 0.076 and then calculated proper distance at z=0.078. I also don't know what (a=0.928) there stands for.

raattgift 10y ago
Sorry about the delay, I didn't notice the reply initially. > I don't understand why you started with z = 0.076 and then calculated proper distance at z=0.078. I also don't know what (a=0.928) there stands for a here is the FLRW scale factor: https://www.wikiwand.com/en/Scale_factor_(cosmology) https://www.wikiwand.com/en/Scale_factor_(cosmology) The two proper distances can be thought of as, for the larger, what a notional instantaneous (>> FTL) analogue of radar measurement would decide the distance to the galaxy we are observing is, and for the smaller, what we would read if we had optical telescopes that could read the display of such a measurement device showing a counterpart in the distant galaxy what the instantaneous distance to us is. (Or instead of an instant FTL analogue of radar, you could use a pair extremely long measuring tapes with the 0 end anchored on the observed party; if we look at a telescope at their tape's indicator, it would read 969.4 Mly, but looking at our own tape would read 1043 Mly.) The numbers are close because ~1 Gly is near enough that the metric expansion of space hasn't carried our galaxy cluster and theirs very far apart in the past billion years. They would see similar numbers looking at a galaxy at z=0.076 in a different part of their sky than where the Milky Way is; we would see a different and almost certainly higher redshift when looking at that galaxy from here.

by acqq 10y ago

raattgift 10y ago

Sorry about the delay, I didn't notice the reply initially. > I don't understand why you started with z = 0.076 and then calculated proper distance at z=0.078. I also don't know what (a=0.928) there stands for a here is the FLRW scale factor: https://www.wikiwand.com/en/Scale_factor_(cosmology) https://www.wikiwand.com/en/Scale_factor_(cosmology) The two proper distances can be thought of as, for the larger, what a notional instantaneous (>> FTL) analogue of radar measurement would decide the distance to the galaxy we are observing is, and for the smaller, what we would read if we had optical telescopes that could read the display of such a measurement device showing a counterpart in the distant galaxy what the instantaneous distance to us is. (Or instead of an instant FTL analogue of radar, you could use a pair extremely long measuring tapes with the 0 end anchored on the observed party; if we look at a telescope at their tape's indicator, it would read 969.4 Mly, but looking at our own tape would read 1043 Mly.) The numbers are close because ~1 Gly is near enough that the metric expansion of space hasn't carried our galaxy cluster and theirs very far apart in the past billion years. They would see similar numbers looking at a galaxy at z=0.076 in a different part of their sky than where the Milky Way is; we would see a different and almost certainly higher redshift when looking at that galaxy from here.