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Tom Mazanec
2010-Nov-20, 01:39 PM
Within the Hubble radius, what is the formation rate of black holes? One way of estimating might be the frequency of gamma ray bursts. Short ones may be neutron star collisions, which should create a black hole. Long ones may be more complicated...the rate may be higher (if a significant number of GRBs are missed from being unaligned with Earth) or lower (if a significant number of GRBs are merely neutron star formations). Another way of estimating may be the number of stars more massive than N solar masses, where N is what ever the cutoff between neutron stat and BH formation is, in the galaxy, divided by the lifespan of such stars, multiplied by the number of galaxies in the visible universe. Yet another way of estimating might be the number of BHs in the Milky Way (do we have an estimate of how many are known compared to how many are overlooked?) divided by age and multiplied by # of galaxies.
Has anyone made these estimates? Do they agree? What are the statistics?

antoniseb
2010-Nov-20, 03:09 PM
Within the Hubble radius, what is the formation rate of black holes? One way of estimating might be the frequency of gamma ray bursts. Short ones may be neutron star collisions, which should create a black hole. Long ones may be more complicated...the rate may be higher (if a significant number of GRBs are missed from being unaligned with Earth) or lower (if a significant number of GRBs are merely neutron star formations). Another way of estimating may be the number of stars more massive than N solar masses, where N is what ever the cutoff between neutron stat and BH formation is, in the galaxy, divided by the lifespan of such stars, multiplied by the number of galaxies in the visible universe. Yet another way of estimating might be the number of BHs in the Milky Way (do we have an estimate of how many are known compared to how many are overlooked?) divided by age and multiplied by # of galaxies.
Has anyone made these estimates? Do they agree? What are the statistics?

There are a lot of factors involved, and your question is a little ambiguous in a few places... nonetheless it is an interesting question. Generally, we are seeing about one GRB per day. It is possible that with more sensitive and all-sky coverage we'd see lots more, but this gives an order of magnitude for the observed number. However, your question specifically mentions out to the Hubble Radius, and there we have some trouble because we don't necessarily observe GRBs out beyond z=5 or 6 except for large unusual ones.

Depending on what you want to know (i.e. how many black holes should there be present today?) you might do better to ask about Black Hole formation rates in each of the various epochs, and then do your summing up. The rate of formation has changed during the life of the universe.

neilzero
2010-Nov-20, 06:52 PM
There were perhaps 10E20 stars when the Universe became transparent. 10% = 10E19 had enough mass to become black holes went super nova after 1.37 billion years on the average. We've had ten generations for these massive stars, so we are back to 10E20, for the present number of black holes minus a very few which have collided. This may be offset by a larger number of neutron stars which have collided , thus becoming black holes. Perhaps 10E18 of these are still in the visible Universe = one million times a trillion black holes = one million times one million times one million. Likely there are more stars than 10E20 stars in recent years, but this is offset by average stars being less massive than in the early Universe, plus there were likely some black holes when the Universe became transparent. 10E18 divided by ten billion years = 10E8 = 100 million new black holes per year. Using 13.7 billion years = 73 million new black holes per year. One per year for our galaxy may be about right. Can someone supply better numbers? Neil

Tom Mazanec
2010-Nov-20, 07:48 PM
IIUC, almost all of these formed in the first tenth of the Universe's life, when stars were more massive. Is this correct? And now we have gone from 73 million BHs a year to maybe 365?

antoniseb
2010-Nov-20, 08:32 PM
IIUC, almost all of these formed in the first tenth of the Universe's life, when stars were more massive. Is this correct? And now we have gone from 73 million BHs a year to maybe 365?

Someone closer to published ideas can tell you more certainly, but I had the impression that the first stars did not typically form black holes... exploding completely with electron-positron annihilation. ... I suspect that your 73 million black holes/year number is probably high by five or six orders of magnitude (my guess).