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More Evidence for Coming Black Hole Collision
- eps 11y ago- 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/
- Terr_ 11y agoI don't understand your link. Are you saying Pinboard's new pricing policy sucks like a black hole?
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- trentnelson 11y agoWhat an elaborate, peculiar burn.
- andrewflnr 11y agoMaybe they just thought it was an appropriate quote and wanted to source it properly?
- eps 11y agoI'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!
- antognini 11y agoThe 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
- AnimalMuppet 11y agoDynamic friction with dark matter? Interaction with the accretion disk?
- antognini 11y agoWith stars and dark matter in the galaxy. A picture explains it better than anything else: http://burro.astr.cwru.edu/JavaLab/cannibal/CannibalDynaFric.html http://burro.astr.cwru.edu/JavaLab/cannibal/CannibalDynaFric... or http://abyss.uoregon.edu/~js/lectures/cannibalism/cannibalism_7.html http://abyss.uoregon.edu/~js/lectures/cannibalism/cannibalis...
- AnimalMuppet 11y agoSo, if I understand correctly, if there's no stars in there, but there is a dark matter halo, the black holes could be slowed by dynamic friction with the black hole halo, correct? Could that solve the last parsec problem?
- antognini 11y agoYes, a dark matter halo can lead to dynamical friction, though I think that stars are more important through most of the process. But by the time you get to a ~few parsec orbit the orbital velocity is too fast and the dark matter density is too low to provide a substantial drag.
- darkmighty 11y agoAs an aside, is this dynamical friction completely analogous to friction in gases? Do stars reach "thermal equilibrium" in their velocities, and can be somehow talk about things like entropy w.r.t to stars instead of gases? Found it fascinating for some reason...
- antognini 11y agoIt's very similar, but not completely analogous to friction in gasses. But the long-term result is the same. An object moving through a gas will slow down until it comes to rest with respect to the gas. Similarly a massive object moving through a medium of stars will slow down until it comes to rest with respect to the stars. There are many parallels between thermodynamics and orbital mechanics. In fact, a collection of stars can be thought of as a self-gravitating gas where the particles don't collide (they're so far apart relative to their size that collisions are very, very rare). The analogous concept to thermal equilibrium in gravity is called "virial equilibrium." One of the very interesting things about a self-gravitating system in virial equilibrium (like a cluster of stars) is that it has negative specific heat! So if it collapses it loses energy, but the stars move faster so it heats up!
- scott_s 11y agoThe 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_black_hole_be_able_to_suck_in_another/c1dtbzh?context=1 https://www.reddit.com/r/science/comments/f78wo/would_a_blac... 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_the_event_horizons_of_two/ https://www.reddit.com/comments/f1lgu/what_would_happen_if_t...
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