
More Evidence for Coming Black Hole Collision - dr_
http://www.nytimes.com/2015/09/22/science/space/more-evidence-for-coming-black-hole-collision.html?hpw&rref=science&action=click&pgtype=Homepage&module=well-region&region=bottom-well&WT.nav=bottom-well&_r=0
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antognini
The article alludes to an open question in astronomy known as the "last parsec
problem," but doesn't quite state it outright. The issue is that we observe
that all galaxies have supermassive black holes at their centers and that
galaxies merge. Due to a phenomenon called dynamical friction, as the
supermassive black hole of one galaxy passes through the stellar field of the
other galaxy, its velocity slows until it comes nearly to rest with respect to
the common center of mass of the two galaxies. This leads both supermassive
black holes to sink to the center of the newly merged galaxy and come into
orbit with each other over a relatively short timescale (I want to say ~100
million years at most).

Over time, the orbit of these two supermassive black holes shrinks because
stars wandering through the orbit get kicked out and take energy with them.
However, once the orbit shrinks to about a parsec, this process stalls. All
the stars that could wander through the orbit have already been kicked out and
there's nothing left to shrink the orbit. Furthermore, gravitational wave
radiation is still too weak to make much of a difference at that distance.

Now, supermassive black hole binaries are rare, so clearly they end up merging
somehow. It's just not known how this happens. For a while it was thought that
the last parsec problem could be solved by taking into account the triaxiality
of the galactic potential [1] (and the references therein). The idea was that
new stars from further out in the galaxy would wander in as quickly as the
black holes could kick them out and the orbit would continue to shrink.
However, new work seems to be casting doubt on this and whether or not
triaxiality solves the final parsec problem is still being debated [2].

[1]:
[http://adsabs.harvard.edu/abs/2013ApJ...773..100K](http://adsabs.harvard.edu/abs/2013ApJ...773..100K)

[2]:
[http://adsabs.harvard.edu/abs/2014ApJ...785..163V](http://adsabs.harvard.edu/abs/2014ApJ...785..163V)

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mikeash
When you say that supermassive black hole binaries are rare, I understand this
to mean that lots of galaxies have been surveyed and they almost all have a
single supermassive black hole in the center, which indicates that something
allows them to overcome this problem and merge. Is that right?

How do you detect the existence of two supermassive black holes one parsec
apart in a galaxy millions of light years away?

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antognini
The main way is through the presence of active galactic nuclei (AGN). If the
black hole accretes gas rapidly enough, it can become very luminous, sometimes
brighter than all the stars of the galaxy combined. One thing astronomers have
looked for has been two AGN in the center of a galaxy (known as dual AGN). I
think the main way that they are detected is in the X-ray since the two AGN
generally have different spectra, so it's possible to distinguish them. (I'm
not an X-ray astronomer, though, so I may be way off base.) But these searches
for dual AGN have found that they are very rare relative to single AGN.

~~~
mikeash
Very interesting, thank you!

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scott_s
The best description of our understanding of what would happen when two black
holes collide is from a reddit poster, RobotRollCall:
[https://www.reddit.com/r/science/comments/f78wo/would_a_blac...](https://www.reddit.com/r/science/comments/f78wo/would_a_black_hole_be_able_to_suck_in_another/c1dtbzh?context=1)

She no longer posts, but was thought to be a physics professor.

More black hole fun from RobotRollCall:
[https://www.reddit.com/comments/f1lgu/what_would_happen_if_t...](https://www.reddit.com/comments/f1lgu/what_would_happen_if_the_event_horizons_of_two/)

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eps

      - Should I be worried?
      - Only in the broadest, existential sense.
    

[1]
[https://blog.pinboard.in/2014/11/new_pricing_policy/](https://blog.pinboard.in/2014/11/new_pricing_policy/)

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Terr_
I don't understand your link. Are you saying Pinboard's new pricing policy
sucks like a black hole?

~~~
eps
I'm saying that two black holes collapsing may cause some people ask if they
should be worried. They should, but only in existential sense. This also
happens to be a question from Pinboard's FAQ on the price change, so quoting
it from _there_ is a joke in itself.

Joke explained is a joke ruined though. Happy downvoting!

