If there were enough primordial black holes (with the right mass) they could explain dark matter. Could they explain the gargantuan monster holes in the center of galaxies?
2 Answers
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Yes. One of the most important open problems regarding supermassive black is their early formation. We have observed supermassive black holes at redshift $z>6$, when the universe was less than 1 billion years old. One example is SDSS J0100+2802 (Wu et al. 2015), a black hole of $1.2 \times 10^{10} M_\odot$ at $z=6.3$. We see these kind of SMBH in the form of quasars, i.e. we see the extremely bright light that is emitted by the matter infalling in the black hole. This means that these black holes are accreting a lot of mass, they are growing fast, so they must have started as much smaller black hole, but when did they form and how massive where their progenitors (usually called seeds)?
It turns out that there is a limit on how fast a black hole can accrete mass (Eddington Limit), so if they have reached $10^{10}M_\odot$ already before $z=6$, they must either have started from already quite massive seeds or they must have started accreting very early in the history of the universe, before the formation of the first stars.
Primordial black holes could explain both scenarios, because they are thought to form at the time of inflation, so much less than one second after the big bang and they also could have a very wide range of mass, depending on who you ask. Therefore primordial black holes have been seriously considered as a possible source of super massive black hole seeds.
Could supermassive black holes be quintessential primordial black holes?
Primordial seeds of supermassive black holes
Massive Primordial Black Holes from Hybrid Inflation as Dark Matter and the seeds of Galaxies
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Prallax's answer deals with whether primordial black holes could be the seeds for supermassive black holes, and I have no argument with that answer. However, it doesn't deal with the second part of the question, or rather the statement - which is could those seeds, which would need to be $>10^4M_\odot$ to make much difference to the "Eddington accretion timescale problem", form all or a significant part of dark matter. The answer to that is currently thought to be NO.
The reasons are reviewed in section 6 of Villanueva-Domingo et al. (2021), (see also Carr & Kuhnel 2022), who show this plot - of the fraction of dark matter that could be in the form of primordial black holes, as a function of the black hole mass. Big swathes of black hole mass parameter space are ruled out as being significant contributors to dark matter by (i) microlensing observations that fail to find significant populations of $10^{-10}< M/M_\odot < 100$ compact objects in the halo of our Galaxy; (ii) the lack of evidence for perturbation of wide binary stars by a significant population of $> 10^3 M_\odot$ black holes; (iii) accretion onto a significant population of primordial back holes would modify the CMB spectrum and its anisotropies.
Constraints on primordial black holes as a fraction of the required dark matter. The shaded regions are ruled out by various labelled observational constraints.
The overall conclusion is that primordial black holes with masses $>10^{-10}M_\odot$ are not a major contributor to dark matter.
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$\begingroup$ That's wonderful news. That means the black holes have about 1/10000 of an Earth mass. $\endgroup$– FeliciaCommented Jun 22, 2022 at 16:54
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$\begingroup$ @Felicia I don't follow your logic. The plot shows that primordial black holes with masses between $10^{17}$g and $10^{22}$g may still be significant contributors to dark matter. It doesn't show that they are. Such hypothetical objects have nothing to do with the growth of supermassive black holes. $\endgroup$– ProfRobCommented Jun 22, 2022 at 20:15
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$\begingroup$ But is black holes with a mass bigger than 10exp-10 times of the Sun can't be a major contributor can holes with a smaller mass be a major part. $\endgroup$– FeliciaCommented Jun 22, 2022 at 21:26
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$\begingroup$ @Felicia that is exactly what I said. They could be. $\endgroup$– ProfRobCommented Jun 23, 2022 at 7:27
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$\begingroup$ That's why I said it's wonderful news. $\endgroup$– FeliciaCommented Jun 23, 2022 at 7:40