
ESO Instrument Finds Closest Black Hole to Earth - origgm
https://www.eso.org/public/news/eso2007/
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ifdefdebug
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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didgeoridoo
They have, and it has a great name: MACHO

[https://en.m.wikipedia.org/wiki/Massive_compact_halo_object](https://en.m.wikipedia.org/wiki/Massive_compact_halo_object)

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

[https://en.wikipedia.org/wiki/Weakly_interacting_massive_par...](https://en.wikipedia.org/wiki/Weakly_interacting_massive_particles)

(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!)

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ThrowawayR2
I'm under the impression that WIMP uses "massive" in the sense of "not-
massless"?

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

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zentiggr
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.

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andrewflnr
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.

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zentiggr
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.

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abecedarius
The paper:
[https://www.aanda.org/articles/aa/full_html/2020/05/aa38020-...](https://www.aanda.org/articles/aa/full_html/2020/05/aa38020-20/aa38020-20.html)

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tannhaeuser
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.

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privong
> 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.

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cevn
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?

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ebg13
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?

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hannasanarion
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)

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nytgop77
If lensing is so hard to observe, why is lack of lensing considered enough to
reject "dark matter==black holes"?

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hannasanarion
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.

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mangix
Article says finds black hole.

There has never been direct observational evidence for one.

Hmmmmm...

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spuz
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.

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mangix
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.

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at_a_remove
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.

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

