# Does the recent news of “ten times more galaxies” imply that there is correspondingly less dark matter?

Headlines sometimes oversimplify. But if this is really true, that there seems to be about ten times more galaxies than had been generally assumed, does this impact other assumptions? Does this imply the universe is ten times more massive, or just that there is less dark matter than was previously thought.

• I hadn't heard this amazing (even if sensationalized) news - thanks for bringing it to the fore here! – Fattie Oct 14 '16 at 10:40
• Just to point out, it is better practice to leave questions open for a longer period. Accepting the first answer, even if it is a good one, usually discourages others from answering. You might get more/better responses by waiting a bit longer before accepting. – zephyr Oct 14 '16 at 13:45
• @zephyr I currently have more than several question still open within the greater stackexchange ecosystem, many have one or more answer posted but none are good enough in my opinion. The answer here is a particularly good answer for me, and it continues to be improved even after being accepted. If someone has a good additional answer and chooses to withhold it from us because it might not be the accepted answer, that would be unfortunate. Plenty of people have no problem posting more answers even if one is accepted. Those answers ALSO get upvoted based on merit. – uhoh Oct 14 '16 at 14:08
• Didn't mean to offend. I was only pointing out that it is encouraged to leave questions open for a bit. I fully understand that Rob's answer is a very good one and that he often gives good answers. I only wanted to point out that you discourage others from answering by accepting an answer. – zephyr Oct 14 '16 at 14:15
• Recent update, New Horizons observations cut galaxy estimate back down to $2\times 10^{11}$ from $2\times 10^{12}$: nasa.gov/feature/… – alexchandel Jan 15 at 20:14

All Conselice et al. (2016) appear to suggest is that when you look at something like the Hubble deep field, there are many faint (and presumably low mass) galaxies that are not seen. This has absolutely no effect on the need for dark matter.

The main results are: (i) as you look back in time, the overall (co-moving) density of galaxies (more massive than a million times that of the Sun) increases. (ii) But the density of more massive galaxies actually decreases. This is consistent with hierarchical merger picture where small galaxies merge to become larger galaxies. This really doesn't have any influence on the need for dark matter.

First, the presence of dark matter is inferred from many different observations. Some of these (e.g. galaxy rotation curves) are not influenced at all if there are lots of extra galaxies.

Second, the "missing" galaxies are at high redshift, not (or not all) in the present day universe, so they cannot significantly affect a calculation of how much normal matter there is in the universe today. Presumably, many of these small galaxies then merge to become larger galaxies and the total mass is conserved.

Third, just because there are lots of them does not mean they contain much mass anyway. The "mass function" (number density as a function of mass) of galaxies goes roughly as $\phi(M) \propto M^{-1}$ at low masses. This means the mass contained in any interval is $$M_{\text{tot}}\propto \int^{M_2}_{M_1} M\phi \ \mathrm{d}M \ = M_2 - M_1$$ So although, low mass galaxies may be ten times more frequent, they are ten times less massive and so don't change the total mass very much. I will need to read the paper more carefully to see if the authors are suggesting that low mass galaxies are much more common in the early universe than was already supposed.

Fourth, primordial nucleosynthesis calculations tell us that only 4 per cent (as a fraction of the critical density) of the energy density of the universe is in the form of baryonic mass. Observations of gravitational lensing, cluster dynamics and the cosmic microwave background tell us that the mass density is actually around 30 per cent of the critical density. Thus most of the dark matter is non-baryonic and cannot be in the form of missing faint galaxies, or any other form of normal baryonic matter.

• My understanding is that the number might actually be lower than was expected for the early universe. – zibadawa timmy Oct 14 '16 at 7:49
• @uhoh The study is a study of the galaxy luminosity/mass function as a function of redshift. The total baryonic mass of the universe is conserved. The idea is that there were many more small galaxies in the past than we see today. – ProfRob Oct 14 '16 at 8:56