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Comment by ifdefdebug | Hacker News Reader

From the article: «The discovery of a silent, invisible black hole in HR 6819 provides clues about where the many hidden black holes in the Milky Way might be. “There must be hundreds of millions of black holes out there, but we know about only very few. Knowing what to look for should put us in a better position to find them,” says Rivinius. Baade adds that finding a black hole in a triple system so close by indicates that we are seeing just “the tip of an exciting iceberg.”»

As a layman I wonder if this could be the solution for dark matter ... but experts probably already checked that idea.

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We have!

A constraint on dark matter is that it needs to be around at very early times. Before the time of the CMB (400 000 years after the Big Bang). If there wasn't dark matter at this time, the under/overdensities of baryons alone are not large enough to produce the large scale structure (galaxy groups/clusters) that we see today.

What this means is that black holes, formed in the conventional process (stars dying), cannot be dark matter. The first stars only formed much later. However, primordial black holes (formed at very early times, before the CMB) were still a possibility.

This possibility has mostly been ruled out though. The main way we have done that is through microlensing. If there were lots of reasonably sized black holes floating around, they would magnify background stars as the passed in front of them. It's a pretty cool effect. Here's a nature paper from a couple of years ago that investigated it [1]. The abstract is very readable and figure 5 shows how people have been slowly ruling out black holes as a major component of DM.

[1] https://arxiv.org/pdf/1701.02151.pdf

They have, and it has a great name: MACHO

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

One of my favorite things about astronomers/space science in general is that they let their geek flag fly when they name things.

My favourite example comes from the development of the modern picture of the formation of the solar system, when it was suggested that giant planets often swapped places during the early days of this and other planetary systems. This idea (now widely accepted as probably true) was known as the Jumping Jupiter hypothesis.

And of course the main competing hypothesis—which appears to be winning—is WIMPs:

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

(Note that confusingly, while both are called "massive", the mass of an average MACHO would be somewhere between 50 and 60 orders of magnitude greater than that of a WIMP!)

I'm under the impression that WIMP uses "massive" in the sense of "not-massless"?

Indeed, though I believe any candidate WIMPs would also have a fairly high mass as far as fundamental particles go, at least on the order of tens of GeV (which is to say tens of times the mass of proton).

I think they're "massive" in relation to the weakly-interacting-and-very-much-not-massive neutrinos, which were another dark matter candidate.

Named by astronomers, who think carbon is a metal, so what do you expect.

Close enough. Neon, on the other hand…

Pretty much anything heavier than hydrogen and helium is a "metal" in astronomy. They work with a lot of strange things so I guess they should have the freedom to use strange naming conventions.

The idea that dark matter is hidden inside black holes has been ruled out via observations looking for gravitational micro-lensing† which is basically looking for distortions in our views of the sky. Although we cant easily locate specific black holes, we know there is not enough micro lensing going on for the explanation for dark matter to possibly be black holes (the total mass of the black holes would have to be many times more than the amount of visible matter in the universe)

† https://en.wikipedia.org/wiki/Gravitational_microlensing

That’s making a fundamental flawed assumption. A small black hole on it’s own multiple light years from a star would likely produce lending on the order of seconds not hours.

In such a situation, the lens will pass by the source in a reasonable amount of time, seconds to years instead of millions of years.

Current survey’s have looked at leasing across longer time frames as that’s much easier to deftest though also much rarer. It’s possible to model this and make useful estimates, but such models are dependent on a huge range of assumptions.

That does not matter because you aren't looking to find every specific black hole, the amount of black holes required for MACHOs to be the answer to dark matter would have orders of magnitude more micro-lensing than we see, pretty much no matter what, there is only so much space inside a galaxy.

More transient events are also orders of magnitude more common. If a black hole’s geometry from you lenses’s light from a volume of space that could contain a star for X time. Them it would lens an equivalent volume of space 10x as far from it for 1/10th as long, and but that volume of space is 10^3 times as large. (Ignoring the edge of the Milky Way etc.)

If you consider just how exact the alignments need to be for hour+ events the difference becomes even more pronounced.

PS: To be clear 2048’s point is still correct, black holes are a small fraction of the Milky Way’s total mass and a long way from filling the dark mater gap. It’s the actual experiment which is generally misinterpreted.

Going further it would be interesting to see a galactic model with potential distributions of the previously unknown and hiding blackholes that would explain galaxy rotation we observe. I would assume there must be an unusual distribution of these blackholes to have the effect on galaxies that we observe but can't explain without dark matter.

The other question/thought I have is would it be possible that so many previously unknown blackholes could actually explain our observations and could it be possible our observations are not consistent with reality rather these newly found blackholes cause the slowdown/bending of light escaping the outer portions of galaxies which simply give an illusion in how we observe the speeds of galaxies. In other words could the inner portion of galaxies be spinning faster than the outter parts as explained by gravity, but we observe a consistent speed across the entire galaxy because the light escapes at different speeds from the center out (consistent with blackhole distribution) giving us the appearance of consistent speeds across the galaxy.

The study of galactic spin rates is based on other galaxies, not our own, since we can't actually see how fast our arm is spinning.

So black holes would not explain why galaxies rotate like a wheel rather than a swirling drain.

> since we can't actually see how fast our arm is spinning.

It's tougher, but we can. We know that the Sun has an orbital speed of approximately 220 km/s relative to the center of the galaxy. And we can measure the relative speeds of stars and gas clouds inside the Milky Way to derive a rotation curve for the Milky Way. This is discussed some here: https://ned.ipac.caltech.edu/level5/March01/Battaner/node9.h...

A side note that "speed of an arm" is potentially ambiguous. The arms in spiral galaxies are thought to be at least partly a reflection of density waves moving through disks. Those density waves can in principle have (pattern) speeds which are different than the orbital speeds of the gas and stars themselves. Talking about the speed of an arm could in principle refer to either the pattern speed of a density wave or to the velocities of the mass components which are overlapping with that density wave.

There are pretty consistent new astronomy discoveries these days, it seems, but I think this one is significant. Being able to study this one closely could lead to discoveries of many more black holes that are relatively close to us.

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