ESO Instrument Finds Closest Black Hole to Earth | Hacker News Reader

ESO Instrument Finds Closest Black Hole to Earth

eso.org

131 pointsoriggm46 comments

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.

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.

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.

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.

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.

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

This is a wonderful discovery. I had built up a picture of black hole environments always being an accretion disk plus lots of powerful radiation, which would make most unexplorable without some incredibly survivable craft.

A quiet non-accreting hole would be something you could actually visit and be curious about without getting fried by radiation or sandblasted by disk material. That's awesome.

One of my favorite astronomy hypotheses is that Planet 9 (not Pluto, the one that's maybe mucking up the orbit of Kuiper belt objects) not only exists but is a primordial black hole. That would just be the most awesome thing to have a black hole accessible within our lifetimes. We would have to stop every other space mission and send up a mission to test quantum gravity on it.

I could get on board with leaving Moon, Mars, and most other planetary missions on hold for some serious replanning and some serious test missions.

Which turns into the science fiction hypothesis it is the wormhole / ancient alien artifact connecting Sol to the rest of the galaxy.

The paper: https://www.aanda.org/articles/aa/full_html/2020/05/aa38020-...

I wonder at what point we can spot the SMBHs/supernovae that had given rise to our own solar system, opening a whole new chapter in pre-solar history. I guess it should be possible to estimate the age of the other two (visible) stars in this system.

> I wonder at what point we can spot the SMBHs/supernovae that had given rise to our own solar system

Likely never, if you mean an actual supernova remnant that released the enriched material into the interstellar medium. Partly because the formation of the Solar System requires that the enriched material mixes with the ambient gas and cools. That means the supernova remnant would most probably not be identifiable as a distinct entity. The other reason is that there almost certainly isn't a single supernovae that gave rise to the enriched material that formed the Sun and Solar System. The enrichment probably happened through multiple generations of star formation and supernova enrichment cycles.

Given a conservative estimate of a quarter of a billion years for the Milky Way to rotate once, and let's imagine two or three supernovae, you'd be looking for a pattern of intersecting ripples in some already scarce interstellar matter smeared out over forty or so galactic revolutions.

My guess is that you'd need a pretty good survey of local stellar fragments (black holes, neutron stars, or white dwarfs) and a very, very good survey of the density of local interstellar media. Modeling that "rewinding" would be interesting.

I have no idea if we have the data now to accomplish it but that is interesting to think about.

They say that the system can be seen with the unaided eye. If I were to point my telescope directly at the black hole, what would I see? Nothing? Or a lensing effect of the stars behind it?

Based on the article it's completely invisible, they only deduced it's there because of the orbit of the two companion stars.

They say that the stars orbiting it can be seen but the hole itself is hidden.

I'm curious why there's no lensing, though. Or is it just that the size makes the lensing weak enough to not be noticable?

They say it has a mass of about 4 suns, which translates to an event horizon diameter of about 15 miles, so I think the lensing would be imperceptible unless you were right up close to it or it happened to perfectly occlude a visible star right when you were watching.

It is very hard to observe lensing around a small object. A light source needs to pass directly behind it, and stay behind it for long enough that your camera exposure can capture it (which for distant objects, can take days or months or years).

That's why gravitational lensing has only been observed for

1. very bright stars behind the Sun (event that lasts a second detectable with sub-second exposure)

2. Galaxies behind galaxies (event that lasts millions of years detectable with hours-long exposure)

3. an SMBH warping the light generated by its own disk (permanent event detectable with years-long exposure)

Those are not the only ways that gravitational lensing has been observed. Gravitational lensing has also been detected for stars in the Galaxy lensing background stars further away in the Galaxy. While the shift in position can't be observed, the lens increases the apparent brightness of the background star as the two come into alignment over the course of a few hours or so.

This phenomenon is known as microlensing [1] and it's one of the ways that exoplanets have been discovered because the exoplanets also lens the background star and produce a characteristic spike on top of the light curve.

That said, microlensing is sort of statistical by nature. It's very unlikely that any particular object will be aligned with a background object just right to produce noticeable microlensing. But with a large field of stars and enough observations it becomes possible to observe a microlensing event every now and again.

[1]: https://en.wikipedia.org/wiki/Gravitational_microlensing

If lensing is so hard to observe, why is lack of lensing considered enough to reject "dark matter==black holes"?

Because if the galactic halo was so full of black holes that it accounted for all the dark matter in the galaxy, then lensing would be extremely easy to detect. If the MACHO hypothesis was correct, there would be lensing everywhere you look because the whole galaxy is totally surrounded by enormous black holes. A lensing effect could be constantly seen viewing any object outside of the galaxy, because there's guaranteed to be a black hole in front of it. It would be difficult to get a good picture of any but the closest galaxies because of all the lensing artifacts from the black holes all over the place. Lensing around black holes is hard to detect precisely because black holes are so rare and don't frequently cross in front of very bright objects.

You won't see the black hole in your telescope, because it's black, and there isn't any stuff currently falling into it (which is one way that black holes can make themselves very visible).

Also, using "your telescope", you won't see the orbits of the black hole's companions with sufficient precision to deduce that the black hole exists and that it's a black hole.

Article says finds black hole.

There has never been direct observational evidence for one.

Hmmmmm...

If you want to be pedantic, you cannot observe direct evidence of anything. After all, when you look at your hand you don't see the hand itself, just the photons that happened to interact with it a short moment ago. Actually you don't see those photons, you only perceive how those photons interact with the cells in your eyes.

Evidence for black holes has been shown in the way they bend light from distant galaxies and cause stars to orbit around them. There's not much stronger evidence needed to prove their existence.

That’s...an interesting way of looking at it.

A black hole is made up of an infinitely dense point-mass singularity and an event horizon. Nobody has every found either. So nobody has found a black hole.

What's your alternative theory for this object?

This is astronomy, we have only photons or the lack thereof and our interpretations of these photons and their lack. We haven't even sent a probe yet to the surface of the Sun (we have gotten eleven million miles away). So, by "direct observation" standards, we don't even know the Sun exists.

For your standard, we would need to send ships to each star, including the event horizon of a black hole. These aren't standards we can do anything with.

They're already playing definition games at the second reply. Don't feed the troll.

Yes, that's what the article says. The research paper says:

"Several dozen optical echelle spectra demonstrate that HR 6819 is a hierarchical triple. A classical Be star is in a wide orbit with an unconstrained period around an inner 40 d binary consisting of a B3 III star and an unseen companion in a circular orbit. The radial-velocity semi-amplitude of 61.3 km/s of the inner star and its minimum (probable) mass of 5.0 M (6.3 ± 0.7 M ) imply a mass of the unseen object of ≥ 4.2 M (≥ 5.0 ± 0.4 M ), that is, a black hole (BH)."

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