
Physicists propose new theories of black holes from the very early universe - dnetesn
https://phys.org/news/2017-09-physicists-theories-black-holes-early.html
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QAPereo
_When that happens, Kusenko said, the primordial black hole consumes the
neutron star from the inside, a process that takes about 10,000 years. As the
neutron star shrinks, it spins even faster, eventually causing small fragments
to detach and fly off. Those fragments of neutron-rich material may be the
sites in which neutrons fuse into heavier and heavier elements, Kusenko said.
However, the probability of a neutron star capturing a black hole is rather
low, said Kusenko, which is consistent with observations of only some galaxies
being enriched in heavy elements. The theory that primordial black holes
collide with neutron stars to create heavy elements also explains the observed
lack of neutron stars in the center of the Milky Way galaxy, a long-standing
mystery in astrophysics._

Very interesting, and amazing when you consider the odds of something as
minuscule as a neuton star and primordial black hole colliding. Without the
papers, it's hard to tell if that aspect of the conjecture is valid, could
occur with the needed frequency to match observations.

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jessriedel
For what it's worth, there are friction-like processes that allow black hole
and neutron stars to get captured into each other's orbits and then spiral in
toward each other. BH and NS mergers (such as seen by LIGO) are _not_ direct
head-on chance collisions between freely propagating objects in the Galaxy.

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wyager
Are these processes gravitationally mediated, like tidal energy transfer?

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AlanSE
Binary systems radiate out gravitational waves. Google image search for
"binary system gravitational waves" and you will pretty easily identify the
canonical graphical representation. While there are lots of problems with the
"sheet" analogy for general relativity, there is still value in considering
gravitational radiation to be waves in the sheet. From a layman perspective,
the analogy is irresistible.

The mental picture of the spiraling-in process is pretty easy to grasp, but
the process of gravitational capture is a little more difficult. Gravitational
radiation is extremely dependent on the distance of the mutual orbit, so the
inward spiral progresses rapidly at short distances, but can take a very long
time at greater distances. So I imagine the research comes down to the general
statistics of the formation of binary systems, how often they will involve a
neutron star with a black hole, and how often they form at distances close
enough to create the collision.

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jessriedel
This only works when the bodies are very close together, right before they
coalesce. For most of the lifetime of binary BHs and NSs, the gravitational
radiation is negligible.

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rbanffy
They could at least link to the papers they mention...

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Cozumel
[https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11...](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.061101#fulltext)

[https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11...](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.031103#fulltext)

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andrewflnr
I don't see how this process clears the center of the galaxy of neutron stars,
especially given the fact that only some galaxies have heavy elements. How
does our galaxy get cleaned out while others are left alone?

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ajross
This was poorly explained. My assumption is that it changes the population
spreads of early stars that would have been expected to populate the galactic
center: we now need fewer supernovae to produce the heavy element distribution
observed, so we'd expect to see fewer supernova remnants.

Which then begs the question of why there is a dearth of neutron stars in the
galactic center in particular. This effect would reduce the number of old
neutron stars everywhere, which I guess we don't observe? Or maybe it's just
that very old neutron stars are hard to find in isolation (i.e. once their
nebulae have dispersed) and that in the center we could see them due to
gravitation because of the star density there?

