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Thread: Why do black holes merge ?

  1. #1
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    Why do black holes merge ?

    This a quote from : http://www.universetoday.com/2009/03...-cosmic-dance/

    "If two black holes are present, they would orbit each other before merging and would have a characteristic dual signature in their emission lines. This signature has now been found."

    Please , can somebody explain me why these BHs orbiting each other are due to merge.

    Because , you know , with ordinary celestial bodies like Earth and the Moon or the Sun and the planets , the merging never happen. So why do BHs merge ?

    Has the cause something to do with the accretion disks around the BHs ? Or some magic relativity trick ?

    Thanks in advance.

    Galacsi

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    In the standard model, they will merge because they are losing orbital energy to the gravitational waves they are radiating by orbiting.
    Forming opinions as we speak

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    Stars can and do merge. IIRC there's a type of binary called ER Vulpeculae binaries that are known to be closing in and ultimately merge into a single star.
    Last edited by EDG; 2009-Mar-04 at 09:04 PM.

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    Similar article although it states that they would continue to orbit each other until "pushed" together.

    Once the black holes have reached the gravitational center of the merged galaxy and come as close as they can through interactions with the surrounding stars, they need an extra push to fully merge. If gas later collects at the center of the galaxy, it could absorb some of the remaining orbital momentum of the black holes and provide that extra oomph to push them into a merger.
    http://www.livescience.com/space/090...ack-holes.html

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    "Push," huh?

    Hmm... www.live"science".com?

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    That comment by oilismastery is funny... he's gotta be a poe.

    BTW galacsi, the answer to your question can be found here.
    Last edited by slang; 2009-Mar-04 at 09:45 PM. Reason: Nobel prize 'n stuff
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  7. #7
    rommel543 and mugaliens: The "push" in that sentence is the loss of orbital angular momentum of the pair due to the surrounding gas.

    galsci: the answer to your question depends on how far apart we put the black holes initially, as the forces that drive them together are different at different times.

    1. The galaxies start very far apart (more than a million light years, generally). As the two galaxies merge, the black holes at their centers will eventually get close together, where "close" is a thousand light years or so.
    2. The black holes fall in toward the center of the galactic mess (see T.J. Cox's simulation movies for some examples of "galactic mess") and eventually get close together, where "close" is now defined as a few light years.
    3. As the black holes accrete material from the surrounding galactic mess, they lose orbital angular momentum and get very close together, where "close" is now a few astronomical units.
    4. Now general relativistic "magic" takes over, and the radiating gravity waves sap away the last of the orbital angular momentum, until the black holes finally merge.

    Until recently, we didn't have the computing power to perform the necessary calculations to explore that last step (and we can still only approximate the first three, although the details aren't as important for those). This paper by Tamara Bogdanović, Christopher Reynolds and M. Coleman Miller is a recent example describing some of the odd things that can occur during that "relativistic magic," including the resultant black hole being "kicked" entirely out of the galaxy.

    Oh, and as a side note, I hope to have something useful to say about the exact system discussed in the UT article. At least, assuming my data is useable...

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    Quote Originally Posted by parejkoj View Post
    rommel543 and mugaliens: The "push" in that sentence is the loss of orbital angular momentum of the pair due to the surrounding gas.

    galsci: the answer to your question depends on how far apart we put the black holes initially, as the forces that drive them together are different at different times.

    1. The galaxies start very far apart (more than a million light years, generally). As the two galaxies merge, the black holes at their centers will eventually get close together, where "close" is a thousand light years or so.
    2. The black holes fall in toward the center of the galactic mess (see T.J. Cox's simulation movies for some examples of "galactic mess") and eventually get close together, where "close" is now defined as a few light years.
    3. As the black holes accrete material from the surrounding galactic mess, they lose orbital angular momentum and get very close together, where "close" is now a few astronomical units.
    4. Now general relativistic "magic" takes over, and the radiating gravity waves sap away the last of the orbital angular momentum, until the black holes finally merge.

    Until recently, we didn't have the computing power to perform the necessary calculations to explore that last step (and we can still only approximate the first three, although the details aren't as important for those). This paper by Tamara Bogdanović, Christopher Reynolds and M. Coleman Miller is a recent example describing some of the odd things that can occur during that "relativistic magic," including the resultant black hole being "kicked" entirely out of the galaxy.

    Oh, and as a side note, I hope to have something useful to say about the exact system discussed in the UT article. At least, assuming my data is useable...
    Thanks Parejkoj and other for your answers.

    So the radiating of gravitational waves causes the decay of the orbits of the BHs and the final merging. this is only true with BHs and only if they got near each other.

    BTW , I don't think it will work with ordinary stars.For these I think other causes must be taken into account.

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    Quote Originally Posted by galacsi View Post
    So the radiating of gravitational waves causes the decay of the orbits of the BHs and the final merging. this is only true with BHs and only if they got near each other.
    No, not just BH's. The Nobel prize that I linked to was awarded for a discovery involving a binary pulsar, which is a neutron star. IIRC the companion is a white dwarf, but I'm not sure.
    ____________
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    Quote Originally Posted by slang View Post
    No, not just BH's. The Nobel prize that I linked to was awarded for a discovery involving a binary pulsar, which is a neutron star. IIRC the companion is a white dwarf, but I'm not sure.
    slang. Yep.Not just BH's is right. There is nothing unique to the laws of physics involving BH mergers, ...all objects in orbit around each other emit gravitational waves. Satellites. Moons. Asteroids in belts.Galaxies. You dancing with your dance partner. It's just that they're strong enough in BH mergers and binary pulsars to make the time scales short enough to be interesting in that you can see the effects during a lifetime or less.

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    Quote Originally Posted by parejkoj View Post
    ... including the resultant black hole being "kicked" entirely out of the galaxy.
    So there's a chance that a galactic core sized BH could be flying though space with no way of seeing it. That could make a mess of another galaxy if it punched through. So what would happen to the remaining galaxy? Would it just turn into a big mess or fall apart?

  12. #12
    Quote Originally Posted by rommel543 View Post
    So there's a chance that a galactic core sized BH could be flying though space with no way of seeing it. That could make a mess of another galaxy if it punched through. So what would happen to the remaining galaxy? Would it just turn into a big mess or fall apart?
    The black hole doesn't really contribute much to the total galactic gravitational field. Galaxies get by just fine without a central black hole (see M33 for a local example), so if the black hole gets kicked out, the galaxy will just go on about its buisness.

    But yes, it appears that there can be giant black holes (a few million solar masses or more) rocketing through space (more than a thousand kilometers per second) all by their lonesomes. Of course, the particular situations that produce this situation don't happen much in the current universe, so there probably aren't any such black holes in our vicinity. No need for black hole insurance.

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    Quote Originally Posted by slang View Post
    No, not just BH's. The Nobel prize that I linked to was awarded for a discovery involving a binary pulsar, which is a neutron star. IIRC the companion is a white dwarf, but I'm not sure.
    Yes you are right , my mistake . .

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    Why should the gas push the black holes together? Cannot the gas also push the black holes apart?

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    Quote Originally Posted by chornedsnorkack View Post
    Why should the gas push the black holes together? Cannot the gas also push the black holes apart?
    If I am not mistaken, I read in Sky and Telescope a few years ago that the dynamics of in-rushing gas could indeed stall the normal tendency of a pair of black holes to spiral in and merge. It will take a while to search my collection to find the article. Perhaps others could help in the meantime.

  16. #16
    Quote Originally Posted by chornedsnorkack View Post
    Why should the gas push the black holes together? Cannot the gas also push the black holes apart?
    It's not so much that the gas is pushing the black holes around, but rather the black holes lose orbital angular momentum to the gas as they accrete it. Think of it rather like friction, slowing them down (that analogy isn't really correct, but I can't think of a better one right now).

    Hornblower: I suspect the article you are thinking of was talking about the accretion disks in black holes that are orbiting much more tightly. When they are further apart, they are interacting with the galactic gas which has a lot more random motion and they are still forming large accretion disks.

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    Just look at the overly complicated answers being offered here.. The question is simple enough. " Why do black holes merge.?" and my first thought on that is... If the radial velocity is less than the gravity force then a merger is immanent. How and why is this so complicated,?

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    Quote Originally Posted by astromark View Post
    Just look at the overly complicated answers being offered here.. The question is simple enough. " Why do black holes merge.?" and my first thought on that is... If the radial velocity is less than the gravity force then a merger is immanent. How and why is this so complicated,?
    Because there is more than one dimension.

    Very few planets, asteroids or comets merge with the Sun. They have to move very precisely towards the Sun to merge. Otherwise they make an elliptic, parabolic or hyperbolic bypass, and fly away again. The same applies to black holes.

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    Quote Originally Posted by parejkoj View Post
    rommel543 and mugaliens: The "push" in that sentence is the loss of orbital angular momentum of the pair due to the surrounding gas.
    Gotcha. Then it should be "aerobraking," not "pushed."

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    But if the gas is able to receive angular momentum from the smaller black hole, should it not then be falling directly into the bigger black hole?

    Now suppose that infalling gas has angular momentum relative to the big black hole. In the absence of the small black hole, it would make a parabolic bypass and transfer no mass or angular momentum.

    Now place a small black hole at a circular orbit around the big black hole, tangent to the infalling gas at its periapse.

    The speed of gas at parabolic periapse is square root of 2 times the speed of the small black hole. So, as the gas falls into the small black hole, its speed should increase, and its orbit ought to expand!

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    Quote Originally Posted by parejkoj View Post
    It's not so much that the gas is pushing the black holes around, but rather the black holes lose orbital angular momentum to the gas as they accrete it. Think of it rather like friction, slowing them down (that analogy isn't really correct, but I can't think of a better one right now).

    Hornblower: I suspect the article you are thinking of was talking about the accretion disks in black holes that are orbiting much more tightly. When they are further apart, they are interacting with the galactic gas which has a lot more random motion and they are still forming large accretion disks.
    I found the article in question, which was about computer simulations of the impending merger of the Milky Way and M31. The author, University of Toronto astronomer John Dubinski, mentioned a brief stalling of the otherwise inexorable migration of the two black holes toward a merger, but unfortunately he did not go into any detail. It may have been lost in the editing. My educated guess is that gas which was spiraling into the central region temporarily transferred some angular momentum to the black holes. The author estimated that this action would last only a few million years, out of the several billion of the entire event.

    The long term trend in the absence of fluid dynamics from gas is migration of the heavier objects, including the two black holes, toward the center. Low-mass stars migrate toward the halo, and many are ejected. This is the result of complex exchanges of angular momentum and kinetic energy by means of gravitational interactions. When the black holes get really close together, gravitational radiation becomes significant and hastens the spiraling-in action.

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    Quote Originally Posted by Hornblower View Post
    The long term trend in the absence of fluid dynamics from gas is migration of the heavier objects, including the two black holes, toward the center. Low-mass stars migrate toward the halo, and many are ejected. This is the result of complex exchanges of angular momentum and kinetic energy by means of gravitational interactions.
    Is the Earth spiralling out so that Jupiter as the most massive planet could get closer in?

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    ... what ?

    Quote Originally Posted by chornedsnorkack View Post
    Because there is more than one dimension.

    Very few planets, asteroids or comets merge with the Sun. They have to move very precisely toward the Sun to merge. Otherwise they make an elliptic, parabolic or hyperbolic bypass, and fly away again. The same applies to black holes.
    Let me suggest that the stability we see is a result of billions of years of evolutionary instability. The Earth is not spiraling out as Jupiter is not spiraling inward. Two black holes could orbit with apparent stability. The very mechanism of a black hole is why it would attract mass and undo that stability and a merge is on. Remembering that billions of years can pass before change is detected. If gravity force exceeds radial velocity then a merger is immanent.

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    Quote Originally Posted by astromark View Post
    Two black holes could orbit with apparent stability. The very mechanism of a black hole is why it would attract mass and undo that stability
    The effects of black hole on distant objects do not differ from the effects of extended objects of the same mass.

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    Quote Originally Posted by chornedsnorkack View Post
    Is the Earth spiralling out so that Jupiter as the most massive planet could get closer in?
    That is a good question, and the answer is no, at least for billions of years in the foreseeable future.

    The Solar System is a very different animal from the core of a galaxy or a globular cluster. In the former, the small inner planets are tightly bound to the Sun, which has about 1,000 times Jupiter's mass. For the time being, these planets do not get close enough to Jupiter to enable the type of close encounter which would yield an ejection and a corresponding inward migration of Jupiter. Some recent computer simulations have suggested the possibility that far in the future, the recurring small perturbations by Jupiter of Mercury's orbit could increase Mercury's eccentricity to the point of destabilizing the inner planets, in which case all bets are off.

    In the case of two galaxy cores in a merger, we have millions of stars in a helter-skelter collection of eccentric, crossing orbits leading to complex energy exchanges beyond my ability to analyze. I must yield to Dr. Dubinski and his colleagues in the galaxy dynamics field.

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