
Clandestine black hole may represent new population - dnetesn
http://phys.org/news/2016-06-clandestine-black-hole-population.html
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misja111
Black holes seem to be much more abundant than previously thought. Earlier
this year there was the discovery of the gravitational wave from the collision
of two black holes with masses of about 30 times the sun, later this year
another such collision was detected in the data. And all of this during an
experiment that took only about a month.

And now this discovery again seems to indicate that there must be many small
sized black holes and that they have been very easily overlooked.

Maybe it is too obvious, but I'm wondering: could dark matter be made up of
all those undiscovered black holes?

~~~
antognini
As other commenters have noted, the idea that dark matter could be composed of
undiscovered black holes has been proposed before. The idea is that there
might be somewhat more generally "massive compact halo objects" (MACHOs),
which would be at least as massive as planets, but would not emit light. A
black hole would be one kind of MACHO, but neutron stars and free-floating
planets would also be MACHOs as well.

The issue is that these MACHOs would be detected through microlensing. If the
conditions as right, as they pass in front of more distant stars, their
gravity focuses the light from the background star, leading to an apparent
spike in the star's brightness. By monitoring the sky for these microlensing
events, astronomers have built up quite strong statistics about the presence
of MACHOs in the Galaxy and have found that they are far too rare to comprise
a significant component of dark matter.

~~~
T-A
As pointed out in [1]: There is a window for PBHs to be DM if the BH mass is
in the range 20M_sun <~ M <~ 100M_sun. Lower masses are excluded by
microlensing surveys. Higher masses would disrupt wide binaries.

[1] [https://arxiv.org/abs/1603.00464](https://arxiv.org/abs/1603.00464)

~~~
ISL
The surprising thing to me, as a practicing professional physicist, with
astrophysics connections, is that the MACHO hunts stopped without closing that
window. It's such a clean explanation, and so many resources have been devoted
to the hunt for particle dark matter while MACHOs still had a chance.

In the late 90's/early 2000's, the paradigm was "WIMPS vs MACHOs", and then
the MACHOs dropped off the radar following the lensing studies. When the paper
you've cited appeared, my reply to the friend that pointed it out was, "Oh,
there's all those lensing studies that convincingly sunk the MACHOs more than
a decade ago, can't be true." When I went back, read the lensing papers, and
realized that there was still a window, I felt really let down by the
astrophysics community at large.

What existing physics only gravitates and is intrinsically invisible? A black
hole. They may not be the dark matter, but we owe it to ourselves to be real
sure that they're not.

~~~
jessaustin
Is this a case of searching under the lamppost? As in, since lensing didn't
demonstrate MACHOs, and there wasn't another obvious way to find them, maybe
we should just look for something else? If so, that seems defensible on
practical grounds?

~~~
ISL
As I understand it, the reason that the MACHO surveys weren't continued was
because the event rate was low (you expect 1% the event rate with 100 M_sun
MACHOs than 1 M_sun MACHOs). Looking longer would solve the problem.

The thing I'm really bummed about is that MACHOs dropped out of the
conversation ten years ago, so nobody new to the field would have known
without digging that MACHOs still had a shot.

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pmontra
Nearly off topic, but the sight of a black hole close to Earth reminded me of
this novellette of Geoffrey A. Landis: "Approaching Perimelasma"
[http://www.infinityplus.co.uk/stories/perimelasma.htm](http://www.infinityplus.co.uk/stories/perimelasma.htm)

Hard sci-fi, it's a dive inside the event horizon of a much closer black hole:
57 ly vs 7,200.

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elorant
Is there any way we could build a miniature black hole in a lab?

~~~
HCIdivision17
Yes, but it's not terribly useful. It was theorized that the LHC could
possibly generate a micro black hole that caused a bit of brouhaha, but it
turns out they evaporate exponentially faster as they get smaller. (This is
due to the surface area being more domininant as the volume shrinks.) So the
damn things just evaporate before they get to any detecting equipment.

Of course, if we could make bigger ones, well, that'd take some significant
mass, and at that point you'll have some rather impressive innovations in
crushing stuff. (After all, particles are darn near point masses already, but
crushing matter down past neutronium is gonna be _hard_.)

~~~
mtgx
It might be just what we need to power an Alcubierre drive. I'd probably want
them to research this kind of energy source for at least 200 years before
trying something like that, though. We wouldn't want to wipe out the solar
system _by accident_.

~~~
adrianN
[https://en.wikipedia.org/wiki/Black_hole_starship](https://en.wikipedia.org/wiki/Black_hole_starship)

Potentially useful even if Alcubierre drives turn out to be impossible after
all.

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HCIdivision17
How... how in the name of Science are you supposed to keep track of an
attometer-wide object?! The dang thing would be so easy to lose I imagine that
misplacing the drive would be the biggest problem such a ship would have! We
always imagine spaceships with these brilliant, impressive reactors and
drives, and here's one that is so underwhelming it may as well not be there.
(Save for the petawatt energy flowing off it...)

~~~
sevenless
Well, it's not that hard to keep track of an atomic nucleus. I suppose you
charge the thing up and manipulate it with electromagnetic fields.

