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B Questions on this discovery

  1. Apr 6, 2016 #1
  2. jcsd
  3. Apr 6, 2016 #2

    PeterDonis

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    None. BB theory is perfectly consistent with the existence of supermassive black holes.

    No. A black hole by itself does not emit gravitational waves. What LIGO detected was a merger of two black holes.

    I would say it's too early to tell. This looks more like the end state of a galaxy than an early state.
     
  4. Apr 6, 2016 #3

    phinds

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    None, as far as I can see.

    detect WHAT? It's not merging with anything so not likely giving off gravitational waves like what you get when black holes merge.

    Don't see why. It's still an open question about the relationship between SMBH formation and galaxy formation but one more big one hardly adds anything.

    EDIT: I see Peter's beat me to the punch again :smile: Only time I ever beat HIM to the punch is when it has rum in it.
     
  5. Apr 6, 2016 #4

    mfb

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    The title doesn't seem to match the news text. They discovered a new supermassive black hole. Okay, great, but on a large scale: so what? Keep looking, and you'll find more.
     
  6. Apr 6, 2016 #5
    I thought ejecting the equivalent of the milky way would cause a detectable signal.
     
  7. Apr 6, 2016 #6

    Orodruin

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    You are not reading the article properly. It is talking about having ejected that amount of stars from its vicinity. LIGO does not detect stars, it detects gravitational waves.
     
  8. Apr 6, 2016 #7

    ohwilleke

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    Any mass in motion should create a gravitational wave in principle. The problem is not that gravitational waves are not created at all, but that the gravitational waves created by ordinary activity are so faint that a detector can't see it. The entire solar system creates gravitational waves with a combined power of about 100 watts. The event LIGO observed has power greater than the EM output of every star in the universe at once. The merger of two substantial sized black holes is a powerful signal. Stars can generate gravitational waves too, and in tightly bound binary systems the magnitude of the waves might even be possible to detect. Of course, not only must you have a sufficiently powerful wave to detect at a great distance from its source, you must also have a gravitational wave with a frequency that your detector is tuned to hear, and you must, of course be looking in the right place at the right time.
     
  9. Apr 6, 2016 #8

    Vanadium 50

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    That simply isn't the case. (In its rest frame, how is it emitting radiation?)
     
  10. Apr 6, 2016 #9
    No expert but I doubt any LIGO is built in the rest frame of merging black holes.
     
  11. Apr 6, 2016 #10

    phinds

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    There IS no "rest frame of merging black holes". You can have a frame in which one of them is at rest but then the other is moving relative to it, so you have relative motion which causes gravity waves. A single star is just out there on its own.
     
  12. Apr 6, 2016 #11

    PeterDonis

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    No, this is not correct. In order to emit gravitational waves, a system must have a time-varying quadrupole moment. That's quite a bit more complicated than just "being in motion" (which, as Vanadium50 has pointed out, is frame-dependent anyway).
     
  13. Apr 6, 2016 #12
    It makes me wonder that if there more of these than previously thought, or more black holes in general then previously thought, could that be the elusive 'dark matter'?
     
  14. Apr 6, 2016 #13

    PeterDonis

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    Supermassive black holes could not be what we see as dark matter, because they would be much too compact--too much mass in too little space. The whole point of dark matter is that it is spread out fairly evenly throughout a galaxy; a supermassive black hole would have a substantial fraction of the galaxy's mass in a very small volume, relative to the galaxy's volume.

    As I understand it, the possibility that ordinary sized black holes (stellar mass or lighter) could be what we see as dark matter has been pretty well ruled out by gravitational microlensing observations--if the dark matter were black holes, it would have lensing effects on the light coming from the stars in galaxies that we do not in fact observe.
     
  15. Apr 7, 2016 #14

    mfb

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    Binary stars cannot radiate in the right frequency range for LIGO. Their orbits need more than 10 minutes even for contact binaries, where ground-based detectors have way too much seismic noise. eLISA with its million kilometer arm length has some chance to detect them if they are close enough.

    You don't have to look at the right place, interferometric gravitational wave detectors look at nearly the whole sky at the same time. They try to look all the time, experimental constraints allow to be sensitive about half of the time.
    Right. There is a small mass range for substellar black holes (which would have to come from the big bang) that could make up some relevant fraction of dark matter, but such a large amount of black holes with a narrow mass range has no good motivation, and it would still mean that most of dark matter has to be something else.
     
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