Astronuc said:
https://phys.org/news/2021-11-lack-massive-black-holes-telescope.html
I wonder how this affects the 'known', or rather, 'observed', mass in the galaxy and universe.
Probably not much.
Black holes, collectively, account for a pretty small share of the total mass of the universe.
Stellar black holes were estimated,
as of 2014, at about 0.1% of the total mass of a typical galaxy and supermassive black holes account for about 0.01% of the total mass of a typical galaxy.
Intermediate black holes would probably have an aggregate mass intermediate between that of supermassive black holes and stellar black holes. So, the impact is probably less than 0.1% of mass in galaxies. The difference is less than the uncertainty in the aggregate mass estimates.
More directly, the
mass of the Milky Way galaxy is on the order of 10
12 stellar masses. There are up to
an estimated 46,000 intermediate sized black holes in its vicinity with masses of 10
3 to 10
5 stellar masses each. So the aggregate mass of these 46,000 intermediate sized black holes is less than 4.6*10
9, and probably more like a factor of five or ten less than that (given the distribution of masses of observed intermediate sized black holes), call it 10
9. So, intermediate sized black holes by this alternative measure also make up 0.1% or less of ordinary matter in galaxies, confirming my estimate above by a different methodology.
The impact might be comparable in magnitude to the aggregate mass of planets and sub-planet objects like asteroids and comets. Most "ordinary matter" is believed to be in the interstellar medium although even estimates of the share of ordinary matter which is found in stars and galaxies isn't very precise
at 6%-10% of the total. According to
this source:
The census of known baryons in the universe tallied to around 60% of total baryons until the resolution of the missing baryon problem. This is in distinction from composition of the entire universe which includes dark energy and dark matter of which baryonic matter composes only 5%. Around 7% of baryons exists in stars and galaxies, while most of it exists around galaxies or galaxy clusters. The
Lyman-alpha forest contains around 28% of the baryons. The warm phase of the WHIM was detected by soft X-ray absorption in 2012 to establish 15% of total baryon content. The
intracluster medium (ICM) accounts for around 4% of total baryon content. It is composed of mostly ionized hydrogen and is about 10% of a galaxy cluster's total mass; the rest being dark matter. The ICM is low density with around 10
−3 particles per cm
3. The circum-galactic medium (CGM) was confirmed in 2003 by
Chandra and
Xmm-Newton. The CGM is a large sphere surrounding galaxies with a radius > 70 - 200 kpc.The CGM accounts for 5% of total baryons in the universe.
Since galaxies only make up 7% plus or minus, of the total ordinary matter in the universe, you are talking about a share for intermediate sized black holes on the order of 0.007% of the total (interstellar material that hasn't condensed into stars is also highly unlikely to have condensed into black holes which require even more concentration of matter into a small space.)
Also, these estimates of "ordinary matter" don't include estimates of "dark matter" that are determined, to a significant extent, by methods like lensing and the dynamics of stars in galaxies and galaxy clusters that are sensitive to total inferred mass, but not to the composition of the mass detected. We estimate the dark matter proportion by determining total inferred mass estimates by these methods and backing out the ordinary matter that we can infer, mostly from star luminosity. So, any increase in aggregate "ordinary matter" from intermediate sized black holes merely slightly reduces the total dark and ordinary matter estimate by reducing the dark matter estimate by a like amount.