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Clandestine black hole may represent new population

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47 pointsdnetesn28 comments

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?

It has been suggested: http://www.extremetech.com/extreme/230715-why-gravitational-...

arXiv preprints:

https://arxiv.org/abs/1603.00464

https://arxiv.org/abs/1605.04023

> Black holes seem to be much more abundant than previously thought.

Not really. The event rate seen by LIGO is well within the accepted range of uncertainty prior to turning LIGO on. The uncertainty range only tightened modestly, because there's been so little data, and only went upward a bit. I'd guess it was a "surprise", given a reasonable choice of priors of the community, at something like the 1-sigma level. Which is about the chance of it raining on any given day in San Fransisco -- not that surprising.

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

This has been considered long ago, and generally disfavored for a few reasons, most notably being the lack of observed microlensing.

https://en.wikipedia.org/wiki/Massive_compact_halo_object

The new data from LIGO is weak, and should not much change your mind about the likelihood of black hole being the dominant dark matter. (That is, the only people who should believe in BH DM are those who already did, of which there are a few.)

This is an optimistic take:

http://resonaances.blogspot.ca/2016/06/black-hole-dark-matte...

one of the proposed physics theories on how the EM Drive works actually rids us of the need to even have dark matter/energy as a concept. of course it had yet to be proven/accepted, and may in fact be disproven at some point.

http://arxiv.org/pdf/1604.03449v1.pdf

The paper was too technical for me. How might it explain dark matter?

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.

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

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.

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?

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.

"Could dark matter be made up of all those undiscovered black holes?"

IANAP(I am not a physicist). I have a pet theory relating to this that goes something like this, and please forgive the simplification: normal matter is "seeking" to return to its original state in the other/dark universe membrane which can only be done through black holes, so dark matter wouldn't be made of black holes, but black holes are the portal through which matter becomes dark matter. In my theory Gravity is actually only a second teir side effect of normal matter seeking to return to dark matter to some degree. I would guess a way to test this would be to measure dark matter emittance near black holes. I suspect it will be highly concentrated in the area.

Keep in mind that these were at distances of around a billion light-years. How many galaxies are there within a billion light-years of Earth? A lot. Millions in fact. Millions of galaxies all with hundreds of billions of stars in them, adding up to hundreds of quadrillions of stars in that volume.

https://4.bp.blogspot.com/-jnY74NBc7ic/V2UcuCswL-I/AAAAAAAAB...

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

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

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

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.)

A particularly off-the-wall paper proposed firing a lot of X-ray lasers simultaneously into the same point to generate black holes. Photons being bosons, all you need is several million tons mass-energy equivalent occupying the same few cubic femtometers...

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.

"We wouldn't want to wipe out the solar system by accident."

I'm totally with you there. If we are going to do it we should be doing it on purpose.

https://en.wikipedia.org/wiki/Black_hole_starship

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

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...)

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.

It might always be a lot easier to find one and two it, than it will be to manufacture one in terms of total energy output. Imagine a far future with mature gravitational and neutrino astronomy, and good AI. You could send a fleet of drones to search for black holes that met your criteria, establish orbit, then send for help from a "tug".

The biggest use for a black hole would be dropping garbage into it and harvesting some of the energy.

> It was theorized that the LHC could possibly generate a micro black hole that caused a bit of brouhaha...

Reminded me of this:

http://www.hasthelhcdestroyedtheearth.com

Reminds me of this: https://www.youtube.com/watch?v=9JYkMhQ9gf8

Hahaha! Oh, shit. I'd forgotten all about that clip. A masterpiece.

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