Problem interpreting Mpc/h in maps of DM

[BillSaltLake]

Problem interpreting Mpc/h in "maps" of DM

In charts of ρ distribution, such as the z = 0 image
http://www.mpa-garching.mpg.de/galform/virgo/millennium/seqF_063a_half.jpg

(taken from http://www.mpa-garching.mpg.de/galform/virgo/millennium/ ),
the distance scale is usually expressed in units of Mpc/h. In this image, there is a scale of 31.25 Mpc/h, although I think this is a misnomer-- it should be written [31.25/h] Mpc, because h is decreasing. At present, that scale then represents 31.25/0.71= 44 Mpc. Does that seem correct?
There may be another problem; I expect the distances to scale with the scale factor "a", not with h ($\propto$ $\dot{a}$/a). For example, from 1 Gyr to the present, h has changed by ratio ~9, whereas a has changed by ratio ~6.5, and I would think that the scale in the t = 1 Gyr image
http://www.mpa-garching.mpg.de/galform/virgo/millennium/seqF_019a_half.jpg
should represent 1/6.5 (not 1/9) of the scale in the z = 0 image. Does this seem correct?

 

[Jorrie]

Mpc/h and [31.25/h] Mpc are basically the same thing. This is comoving distance, which does not change with the evolution of h (or a), because it is present proper distance. I think the h is there to cater for the uncertainty in h, which may soon be revised upwards to around 0.74, yet they do not have to alter the scale of their simulation.

 

[BillSaltLake]

Note that h ($\propto$ H) and a have different time evolution. Therefore a parameter that scales with h does not scale with a, and comoving distance scales with a. I think that the scale in the image should read "44/(1+z) Mpc", noting that 1/(1+z) $\propto$ a.

 

[Jorrie]

... Therefore a parameter that scales with h does not scale with a, and comoving distance scales with a. I think that the scale in the image should read "44/(1+z) Mpc", noting that 1/(1+z) $\propto$ a.

The picture is for z=0 (today), so it could have read either 44 Mpc (h=0.71), or 31.25 Mpc/h. The latter value remains constant when h (= H0/100) is refined by observation. Cosmologists use this so that their reports do not have to be updated as better values of H0 become accepted. AFAIK, h refers only to H0 and hence does not evolve with a. The parameter evolving with a is usually referred to as H(t).

BTW, although not applicable here, I thought it is proper distance that scales with a and comoving distance is constant for a given galaxy.

 

[sardar]

From my understanding, there are two main issues with interpreting Mpc/h in maps of dark matter (DM). The first issue is that the use of h in the unit can be misleading. As the author pointed out, h is not a fixed value and is dependent on the expansion rate of the universe. Therefore, using Mpc/h as a unit of distance can lead to confusion and incorrect interpretations of the scale in the maps.

The second issue is that the distances in the maps should be scaled with the scale factor "a" rather than h. This is because the expansion of the universe affects the distance between objects, not the value of h. Using h as a scaling factor can lead to incorrect interpretations of the distances between objects in the maps.

To address these problems, it would be more accurate to use the unit Mpc (megaparsec) without h in maps of DM. This would avoid confusion and allow for a more accurate interpretation of the distances between objects. Additionally, the distances in the maps should be scaled with the scale factor "a" to properly represent the expansion of the universe. This would provide a more accurate depiction of the distribution of dark matter in the universe.

In conclusion, the use of Mpc/h in maps of DM can be misleading and may lead to incorrect interpretations of the scale and distances between objects. It would be more accurate to use Mpc without h and scale the distances with the scale factor "a" to properly represent the distribution of dark matter in the universe.