• #### Is higher mass-density at higher Z detectable with current probes?

From stargene@21:1/5 to All on Mon Nov 19 09:11:07 2018
A question regarding the expansion of the universe:
As the Hyperphysics page @ http://hyperphysics.phy-astr.gsu. edu/hbase/Astro/redshf.html shows, the mass density rho(M@z)
= rho(Mz=0)x(1+z)^3, for the universe at any value of z.
Eg: At a z = 4, the mass density of the universe at that point in its
expansion (at about 13.8 Gly (z=0) - 12.77 Gly (z=4) = ~1.03 Gly),
would be roughly 5^3 or 125 times its current mass density. So--

Would any of the deep galactic survey programs, like the Hubble
Ultra-Deep Field, actually be capable of detecting such an increase
in both the average mass density, and perhaps even the number-
density of galaxies, possibly correlating with such a higher mass
density?

[[Mod. note -- I doubt it. It's already rather difficult to measure
the mean mass density of the local universe (z=0), and such a measurement
would be vastly harder at high redshift where we can only observe the high-luminosity tail of the luminosity function. (That is, at high
redshift we can only observe objects (e.g., galaxies) which are
intrinsically much more luminous highly luminous than the average
object at that redshift.) Measuring the number density is even harder,
because it's more affected by intrinsically-faint objects (galaxies).
-- jt]]

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• From Phillip Helbig (undress to reply)@21:1/5 to stargene on Mon Nov 19 23:35:49 2018
stargene <stargene@sbcglobal.net> writes:

rho(M@z)= rho(Mz=0)x(1+z)^3, for the universe at any value of z.
Eg: At a z = 4, the mass density of the universe at that point in its expansion (at about 13.8 Gly (z=0) - 12.77 Gly (z=4) = ~1.03 Gly),
would be roughly 5^3 or 125 times its current mass density.

Not "roughly"; pretty much exactly. Of course, this is the average
density.

Would any of the deep galactic survey programs, like the Hubble
Ultra-Deep Field, actually be capable of detecting such an increase
in both the average mass density, and perhaps even the number-
density of galaxies, possibly correlating with such a higher mass
density?

[[Mod. note -- I doubt it. It's already rather difficult to measure
the mean mass density of the local universe (z=0),

The radiation density goes like (1+z)^4 (photons are diluted by (1+z)^3
like non-relativistic particles, and additionally they are redshifted by (1+z)). This CAN be detected.

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