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Blackholes and expanding universe

  1. Jan 16, 2008 #1
    Dear all,

    i'm very new to cosmology/astronomy branches of physics.
    i've one simple question.
    it's known that the universe is expanding and i suppose the measurement was made by hubble for which he won the nobel prize.
    but also, i recently learnt that there're blackholes at the centre of galaxies like milky way.
    if that's true, the blackholes shud be 'sucking' in the other celestial bodies, wont they?
    in this case, considering there are many blackholes spread across the universe, isnt it cotradictory to the fact of 'expanding universe' ?
    could someone explain?

    Last edited: Jan 16, 2008
  2. jcsd
  3. Jan 17, 2008 #2


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    Blackholes at galaxy centers are sucking in nearby matter. There is no contradiction to the universe expansion, since that is an expansion of space, observable between galaxies. The black hole gravity falls off by the inverse square law, so its sucking is only on objects that are closeby within the galaxy.
  4. Jan 21, 2008 #3

    Edwin Hubble was not awarded a Nobel Prize for his discovery of the expansion of the universe. By the time Nobel community realised that astronomy was a branch of physics it was too late.
  5. Jan 22, 2008 #4
    oh i see.. but i got to know abt Hubble winning nobel prize through a documentary on the history channel in their 'Universe' series..
    may be they were wrong..
  6. Jan 22, 2008 #5
    You should see this.

    The following site quotes:

    'While Hubble never did win the Nobel Prize (there was no category for astronomy at the time), his legacy reaches far beyond such temporal tokens of recognition. Stephen Hawking, noted cosmologist and author of A Brief History of Time, has hailed Hubble's expanding-universe discovery as "one of the great intellectual revolutions in the twentieth century." '

    Last edited: Jan 22, 2008
  7. Jan 25, 2008 #6
    The reason black holes are not sucking in all the planets is because of a boundary known as the event horizon, it's the point towards a black hole where there is no chance of possible escape. Any matter outside of the event horizon is not affected.

    Think of the event horizon as a shadow, anything outside of the shadow is mainly unaffected. By unaffected though I mean they are not inevitably sucked into the black hole, however due to the mass of a black hole it still has gravitational effects even on objects outside of the event horizon, which is why some are orbited by solar systems and such.

    For more information read chapters 6 and 7 within Steven Hawking's book A Brief History of Time.
    Last edited: Jan 25, 2008
  8. Jan 29, 2008 #7
  9. Jan 29, 2008 #8
    Hi folks,

    There seems to be a misconception in this discussion. Supermassive black holes in galactic centers are very concentrated gravitational bodies, but they are no Godzillas sucking in everything in the Galaxy. For example, Sagittarius A*, the supermassive black hole at the center of the Milky Way, has an estimated mass of 3.6 million solar masses, and a diameter of 20 million kms, about equivalent size to the orbit of Venus. I read somewhere (I forget where), that black holes in (non-active) galactic centers like ours are estimated to suck in less than 1 star per year on average. (Don't hold me to an exact number). By comparison, there are about 3.6 million stars concentrated in our galactic center within a 1 parsec radius from Sagittarius A*. The central part of our galaxy has a radius of about 10 parsecs, and its crowded with stars. Inside 1 parsec, each star actually orbits Sgr A*, as if they were the only two objects in the gravitational system.

    Here's a paragraph from the Black Hole Encyclopedia online:

    "From a distance, a black hole's gravitational pull is no different from that of any other star or other object of the same mass. If our Sun were suddenly replaced with a black hole of the same mass, for example, there would be no change in Earth's orbit -- the planet would not be "sucked in" by the black hole. A black hole's surface gravity is stronger than that of a normal star because all of its mass has been squeezed into an almost infinitely small point. But from comparable distances in space, if you judged by gravitational pull alone, you couldn't tell the difference between a black hole and any other object of the same mass."

    Also, a black hole's event horizon does not in any way inhibit the infall of objects or radiation. Once something has fallen inside, the super-high gravity extending out to the radius of the event horizon simply prevents it from getting back out.

    The largest known supermassive black hole is in the M84 galaxy. It is estimated at 1.5 billion solar masses and has a diameter equal to Neptune's orbit. Oops, M87's black hole is estimated at 3B solar masses, and a diameter larger than Uranus'.

    Last edited: Jan 29, 2008
  10. Jan 31, 2008 #9
    nothing can be said about the blackhole because it lies at the center of the milkyway which is surrounded by many stars. in that case blackhole can produce many fluctuations. these fulctuations are responsible for emission of HAWKING RADIATION that a blackhole emits. if this emission is further continued the event horizon decreses due to loss of energy and thus the balck hole could evaporate !!!
  11. Jan 21, 2012 #10
    The black holes at the centers of galaxies suck in matter around them while those black holes move away from each other due to the expansion of the universe. These two phenomena happen simultaneously. They have a negligible effect on each other because of the difference in scale. The expansion of the universe is a much larger and slower phenomenon than the effects of individual black holes.
  12. Jan 21, 2012 #11


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    I suggest that you read up a bit more on black holes.
  13. Jan 29, 2012 #12
    As far as i know;

    - Even if it is a black hole, the gravitational force decrease exponentially as the distance increases.. Well maybe can affect matter in another galaxy, as much as an electron affects our planet by its gravitational force.

    - You can think of the dark matter as the web the galaxies are placed onto.. Its actually what really kept the expansion of the universe at bay so far, but Nobel Prize winners proved that this web has failed. And the universe is doomed to expand until everything freezes in Absolute Cold

    - The doom of our universe is the Dark Energy. It expands space, and with the failure of the Dark Matter, keeping this force at bay by its gravitational affects up until five billion years ago. The dark energy has won, and thus Hubble made his discovery of the expanding universe..

    This caused Einstein to call his constant, his biggest blunder. I find it funny he ever said that, he almost sounds like he miscalculated something at the creation of this universe and failed to create a static universe.. Well, his biggest blunder(as he puts it) is a nobel winning discovery now. Go figure...
  14. Jan 30, 2012 #13


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    Gravitational force decrease with distance by inverse square, not exponential.
  15. Jan 31, 2012 #14
    Ah true, my english calculus kinda failed miserably there :)
  16. Jan 31, 2012 #15
    What? If BH's didn't affect anything outside of their EH then how would they have any influence on a galaxy?
  17. Jan 31, 2012 #16


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    Read his entire post. His statements are correct.
  18. Feb 7, 2012 #17
    Black hole effect outside the event horizon is defined on the size of the black hole...........if a black hole is small in size then it will be having high gravitational pull beyond event horizon, as in case of a small black hole it will tear everything apart before even entering the black hole..........on the other hand larger black holes have that gravitational pull inside the event horizon............let's take the case of the galactic black hole at the centre of our milky way, the black hole at the centre is so huge in size that its gravitational pull bind the galaxy like it is..........if this black hole was not in the centre then due to the negative energy each object in our galaxy would have moved apart from each other.........
  19. Feb 7, 2012 #18


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    Uh ... just exactly WHAT "negative energy" would that be, pray tell?
  20. Feb 7, 2012 #19
  21. Feb 7, 2012 #20
    Phinds responded to my post on the previous page. I made the comment leading to yours without reading the entire post, and drew an incorrect conclusion. That statement taken out of context is untrue, but his entire post validated it.

    Your post however, confusing as it is, is referencing density as opposed to "size". It is estimated that some SMBH's have a density approaching water, if I understand correctly; their size however is on the order of a billion or more solar masses. The mechanism that binds galaxies in their current configuration is not understood and is believed to be Dark Matter, since the stars at the edge of at least elliptical galaxies are too far removed from the center to feel the pull from the respective SMBH's.

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