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Message boards : Cosmology and Astronomy : How many black holes are there?

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PeterT
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Message 20745 - Posted: 23 Jan 2016, 1:04:40 UTC

Has any cosmological simulation worked out the possible mix of matter by the time the first galaxies formed at 500 million years (ie, gas, stars, dust, rocks, black holes) by mass?
How many non-massive black holes there could be now? Could it be 25% ?

Peter

Profile Benjamin Wandelt
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Message 20746 - Posted: 23 Jan 2016, 19:31:54 UTC - in response to Message 20745.

Hi Peter -
This is a question that touches on current research themes... in other words we don't know the answer very precisely at this time. It's only been in the last year or two that cosmological simulations have had the ability to resove individual galaxies and include mechanisms for the formation of black holes (so-called sub-grid models, see below). Examples are the Horizon-AGN simulation run that was performed by my colleagues here at the Institut d'Astrophysique de Paris (one of the first such runs). Another is the EAGLE run done by a consortium of other groups. The links are worth checking out for impressive visualizations and movies of the simulation results.

The difficulty with these runs is that the detailed physics of black-hole formation works on much shorter time-scales and length scales than can be resolved in these large simulation volumes even with the largest current supercomputers.

For this reason the formation of black holes is not simulated from first principles but using prescriptions (called "sub-grid models") that compute the probability of black hole forming in a particular grid cell, and then randomly add a black hole based on this probability. The details of the black hole (ie the accretion disk physics etc) are also not simulated but the winds from the accretion disk etc are computed based on the density, temperature etc of the cell that contains the black hole).

As a result the number of black holes at any given time depend on the parameters of the sub-grid models. The approach is to run a few simulations with different plausible values of these parameters and then to compare the results with observations of the real universe.

So to answer your question: yes there are cosmological simulations that compute the fraction of total mass in gas, stars, black holes and even the abundances of various chemical elements etc. But the results do not just depend on fundamental laws of physics but on approximate "recipes" and particular choices of the parameters of these recipes. The results are difficult to validate since this is really frontier research.

For now comparison to deep astronomical observations are our best bet at validating the sub-grid models.

Hope this helps!

Best,
Ben
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PeterT
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Message 20756 - Posted: 29 Jan 2016, 4:44:41 UTC

Thanks Ben
It does help, very much. I am considering going back to study / research to look into the formation of black holes from the population III supernovae. With a maths/physics degree and computing career behind me I now have time.
Peter

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Message 21173 - Posted: 21 Sep 2016, 20:41:43 UTC - in response to Message 20756.

Hi Peter,
Did you go back to study and research? I am a brand new C@H subscriber and thought I'd put my first message out to you. I am curious: I hope you did go into studying black holes, and wonder whether you did.

As for me, I went back to school after 8 years off, to take up physics. I do not know which branch I will settle on, but cosmology, like all branches of physics, is fascinating. For now, I am keeping an open mind.

Good luck in whatever you have decided to do!

Jeff
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Message boards : Cosmology and Astronomy : How many black holes are there?