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What portion of the universe is black holes? Is it possible to estimate the percent of all mass that is in the black holes?

Qmechanic
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Euphorbium
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  • Related: https://physics.stackexchange.com/q/749978/2451 and links therein. – Qmechanic Feb 16 '23 at 16:57
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    This review gives an estimate of the number of galaxies in the observable universe with all the large uncertainties involved, https://www.skyatnightmagazine.com/space-science/how-many-galaxies-in-universe/ . see also this https://science.nasa.gov/astrophysics/focus-areas/black-holes – anna v Feb 16 '23 at 18:24
  • Answers here and here may be useful. – g s Feb 17 '23 at 04:19

3 Answers3

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The cosmic inventory of the mass of various types of object is discussed in a well-known review by Fukugita & Peebles (2004).

They estimate that the fraction of the total matter in the universe that is made up of stellar-mass black holes (the final states of massive stars) is about 0.00025 with about a 30% uncertainty. A further fraction of about $10^{-5}$ is in the form of supermassive black holes at the centres of galaxies.

NB. These fractions are of the total (including dark matter). If you want the fraction of "normal", baryonic mass, them multiply these fractions by about 4.5.

As another answer correctly points out, a candidate for dark matter is primordial black holes, formed in the very early universe. Since dark matter makes up about 82% of the matter in the universe and no dark matter candidates have yet been identified, then it is possible that primordial black holes make up 82% of the matter in the universe.

ProfRob
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This is an open question. We don't know. It's somewhere between 0% of the critical energy density (negligible) and 27% (all of dark matter).

At the risk of giving an answer way more complicated than you are looking for, check:

  • Wikipedia on dark matter. Black holes would be a form of dark matter.
  • There are ways to detect black holes. Naturally the most massive black holes are the easiest to detect, but smaller ones (so-called MACHOs, or Massive Compact Halo Object) can still be detected with microlensing.
  • But this is imperfect because at some mass black holes become too small to detect. For example, an asteroid-mass black hole at the distance of Neptune from the Sun would be practically impossible to detect - the mass & radius are just too small.
  • But in that case, it becomes possible that all of dark matter are tiny black holes. The major challenge would be "how did these little black holes form?", and the only real explanation is that they are primordial, i.e. they formed from density perturbations in the very early universe.
  • But we have no idea how massive primordial black holes are, or if there are even any around. The theory behind density perturbations in the very early universe is not sufficiently well developed. Maybe primordial black holes exist and account for all the dark matter, in which case the answer to your question is 27%. Or maybe there are no primordial black holes and dark matter is made of WIMPs or axions or something else. In this case the answer is approximately 0%.

So the tl; dr is: we don't know, and the possible range is very wide.

Allure
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  • Careful, in a particle physics and cosmology context, black holes (e.g. from a supernova) are NOT considered dark matter. Astronomers might beg to differ. – rfl Feb 17 '23 at 11:58
  • @rfl I do think that once everyone is on the same page they will agree if black holes are / are not dark matter. Problem with astrophysical black holes is that they were previously baryonic matter, and there are cosmological constraints on that abundance. But once the star goes supernova, the resulting black hole can be dark matter (in fact it would be a MACHO), it's just that dark matter can't be made up completely of such black holes. – Allure Feb 20 '23 at 01:03
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As a point of interest, there is a theory that proposes that the extra mass in galaxies we call "dark matter" is actually small primordial black holes. This theory has not yet been entirely excluded by experimental evidence, but it's quite difficult to prove since the hypothetical black holes would be smaller than the size of the moon and not densely distributed throughout the galaxy.

klippo
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  • Could you cite a reference that would specify why the size of earth's moon would represent the upper limit of primordial black holes? I failed to pick that up in a skimming of the Wikipedia article's current version. – Edouard Feb 19 '23 at 20:53
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    In the Wikipedia article, observational limits are given in terms of the masses, which can be turned into radius limits by applying the formula for the schwarzschild radius. The article says depending on the model, primordial black holes could be in the range of $10^{-8}kg$ to thousands of solar masses. A 1000 solar mass black hole would have a radius on the same order as the moon. – klippo Feb 19 '23 at 22:02
  • I appreciate your helpful reply, but another problem with my understanding your answer is my impression that the Schwarzschild radius assumes a lack of rotation, and is consequently a sort of training model for the mathematically complicated Kerr and Kerr-Newman BH's. If you feel I'm mistaken about that, please send another reply, or mention a mode of BH formation that would avoid that oversimplification. – Edouard Feb 20 '23 at 03:00
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    Here is a relevant review paper. I'm not an expert on this by any means so I found this very helpful: https://www.frontiersin.org/articles/10.3389/fspas.2021.681084/full – klippo Feb 20 '23 at 04:08
  • This is very confused. There are observational constraints from microlensing etc. and limits imposed by Hawking radiation that mean the window of possible primordial black hole masses is much narrower than you suggest – ProfRob Feb 20 '23 at 09:02