[ExI] Has the mystery of Dark Matter been solved?

Robin D Hanson rhanson at gmu.edu
Mon May 23 18:30:40 UTC 2016

Yes, this seems a very reasonable guess to me as well.

On May 23, 2016, at 1:19 PM, John Clark <johnkclark at gmail.com<mailto:johnkclark at gmail.com>> wrote:

I would give  50% odds that the mystery of Dark Matter has been solved and it will turn out not to be some new particle but will consist of Primordial Black Holes. We know from the percentage of the  elements Hydrogen, Deuterium, Helium and  Lithium  how much regular matter was around one minute after the Big Bang when nucleosynthesis cooked up these elements, and there is no room for Dark Matter. So the Black Holes that form the bulk of the Dark Matter can't have come from the corpses of dead stars made of regular matter; but maybe Black Holes formed long before nucleosynthesis occurred when the universe was much less than one minute old and things were too hot for even protons to exist much less elements.

Stephen Hawking proposed this explanation for Dark Matter some years ago but the idea had fallen out of favor because it was largely (but not entirely) ruled out by the data. We know that to account for all the Dark Matter the Black Holes can't be larger than 100  solar masses because there would be more gravitational microlensing than we observe. And we know that to account for all the Dark Matter the Black Holes can't be smaller than 10 solar masses because we'd see Black Hole explosions /evaporations (if they were REALLY small) and the orbits of widely spaced binary stars would be disrupted, but we don't see any of that.

There is still a window for Primordial Black Holes being Dark Matter that the data hasn't excluded and it's between 10 and 100 solar masses, and during its short engineering run that's just what LIGO discovered. It found a 29 solar mass Black Hole merging with a 36 solar mass Black Hole in a fifth of a second producing a 62 solar mass black hole and 3 solar masses of energy in the form of Gravitational Waves.  Everybody was amazed they found something that good so quickly when the instrument hadn't even reached its design sensitivity yet, everybody thought it would take years of observing to detect a thing like that. Maybe they just got extraordinarily lucky, or maybe Black Holes are far far more common than had been previously thought. Maybe 85% of all the matter in the universe is in the form of Primordial Black Holes. The two LIGO detectors will get back online in September and with greatly improved sensitivity and will be joined by a third detector, VIRGO near Pisa in Italy. So we should know pretty soon if Dark Matter and Black Holes are the same thing, if they are then the second greatest mystery in physics will have been solved, but we'll still have the mystery of Dark Energy.

 John K Clark

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Robin Hanson rhanson at gmu.edu<mailto:rhanson at gmu.edu>
Future of Humanity Inst., Oxford University
Assoc. Prof. Economics, George Mason University
See my new book: http://ageofem.com

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