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

John Clark johnkclark at gmail.com
Mon May 23 17:19:41 UTC 2016

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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