<p>Separations go with the scale factor, so get smaller by <span class="math-container">$(1+z)^{-1}$</span>. Volumes scale as <span class="math-container">$(1+z)^{-3}$</span>.</p>
<p>At <span class="math-container">$z=3$</span>, that volume was 64 times smaller.</p>

CC BY-SA 4.0
2016-04-16T23:23:34.560Z
2024-12-23T13:27:48.813Z
2024-12-23T13:27:48.813Z
ProfRob
<p>If you were to divide the present-day
universe into cubes with sides 10 million light-years long, each
cube would contain, on average, about one galaxy similar in size
to the Milky Way. Now suppose you travel back in time, to an era
when the average distance between galaxies is one quarter of its
current value, corresponding to a cosmological redshift of z = 3.
How many galaxies similar in size to the Milky Way would you
expect to find, on average, in cubes of that same size? In order to
simplify the problem, assume that the total number of galaxies of
each type has not changed between then and now. Based on your
answer, would you expect collisions to be much more frequent at
that time or only moderately more frequent?</p>

<p>I am very confused with this question. From my intuition, I take that volume would equal to distance^3. They have given us that the past distance would be 1/4 of current. Therefore, do I cube 1/4distance? Also how does the redshift of 3 play a part in the solution. Thanks for any help! I do not need an answer, just how to start.</p>

<fundamental-astronomy>
Cosmological redshift and comparing past galaxy sizes
mathenator2015
# Cosmological redshift and comparing past galaxy sizes

(Asked by: username_0 on 2016-04-16)

If you were to divide the present-day
universe into cubes with sides 10 million light-years long, each
cube would contain, on average, about one galaxy similar in size
to the Milky Way. Now suppose you travel back in time, to an era
when the average distance between galaxies is one quarter of its
current value, corresponding to a cosmological redshift of z = 3.
How many galaxies similar in size to the Milky Way would you
expect to find, on average, in cubes of that same size? In order to
simplify the problem, assume that the total number of galaxies of
each type has not changed between then and now. Based on your
answer, would you expect collisions to be much more frequent at
that time or only moderately more frequent?

I am very confused with this question. From my intuition, I take that volume would equal to distance^3. They have given us that the past distance would be 1/4 of current. Therefore, do I cube 1/4distance? Also how does the redshift of 3 play a part in the solution. Thanks for any help! I do not need an answer, just how to start.

**Tags:** fundamental-astronomy


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## 1 answers

### Answer by username_1 on 2016-04-16. Score: 1 | Accepted answer

Separations go with the scale factor, so get smaller by $(1+z)^{-1}$. Volumes scale as $(1+z)^{-3}$.

At $z=3$, that volume was 64 times smaller.


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