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

Anders anders at aleph.se
Wed May 25 08:33:08 UTC 2016

Likely just to annoy John, there is a recent paper ( 
http://arxiv.org/abs/1603.08522 ) arguing the early supermassive holes 
could be due to direct collapse of gas clouds. I cannot judge the 
likelihood of this, but it will be interesting to see how it turns out.

On 2016-05-23 20:30, Robin D Hanson wrote:
> 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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