# If I look beyond (about same region, higher redshift) a galaxy, will I see its progenitors?

Is there a range of redhsift dz, such that if I look at z+dz from a massive object at redshift z, I will see its progenitors? (in a LCDM scenario of halo assembling, with smaller halos evolving towards larger ones across the cosmic time)

I am looking for a detailed explanation of this (maybe naif) question.

## 1 Answer

I think I understand what you mean and the answer is: specifically no; but generally yes.

If we look out into space, we can divide up what we can see into redshift slices of arbitrary thickness. The light we see from galaxies in each slice has been travelling for a certain amount of time, so we are seeing that "piece of space" at a particular cosmic epoch corresponding to its redshift.

If we look past that galaxy to a higher redshift slice, then what we are seeing there is a different patch of space at a younger cosmic epoch. i.e You cannot see the same galaxy as it was at a different time. We see each and every galaxy only once.

But in general you will be seeing younger galaxies at higher redshift. In an LCDM model then smaller galaxies build up into larger ones, so there will be a higher density of galaxies at higher redshifts (even after accounting for cosmic expansion), but the galaxies they will produce by merging etc. are not yet visible to us. It is possible that we will see them in the future (depending on the exact values of the cosmological parameters and what redshift we are talking about) as that distant patch of space recedes from us and becomes older (in cosmic terms).

• Rob, is this related to expansion? I mean in an hypothetical contracting universe could one observe the same object at two different epoch (z)? – Alchimista Dec 19 '17 at 20:34
• @Alchimista No. Each galaxy is observed at a particular epoch. – ProfRob Dec 19 '17 at 23:09
• That would mean a galaxy emitting and approaching us faster than light :) I reversed the universe too much :) – Alchimista Dec 19 '17 at 23:12
• @Alchimista With gravitational lensing we can get 2 (or more) signals that have traveled over slightly different paths, through different gravity wells, so there is a small but significant time difference between them. There's some info at The Hubble Constant From Gravitational Lens Time Delays by Kochanek & Schechter. – PM 2Ring Dec 20 '17 at 2:29
• PM 2Ring good point . The same could be with a close U. But I was thinking something different which implied a reverse Hubble flow with shrinking region at v > c. :) based on a reversal thinking done in a rush of enthusiasm. – Alchimista Dec 20 '17 at 8:38