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Milkyway Black-hole vs Andromeda Collision

  1. Jan 12, 2013 #1
    Since black-holes are the catalyst of galaxies and black-holes nomnomnom galaxies; wouldn't it be true that the Milkyway galaxy is shrinking, taking earth (our solar-system) with it, NOT the holamola entire universe expanding?

    So, wouldn't the Milkyway galaxy implode before it would collide with the Andromeda galaxy?
  2. jcsd
  3. Jan 12, 2013 #2


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    Welcome to PF omar1,

    Your question stems from a common misconception about black holes. The gravity of a black hole behaves in exactly the same way as the gravity of any other object of the same mass. For instance, if you were to magically and instantaneously replace our Sun with a black hole of exactly the same mass, then the Earth and the other planets would continue on in the same orbits as though nothing had happened. The reason is because the strength of the gravity of an object depends only on the mass of that object, and your distance from it. This is as true for black holes as it is for anything else.

    What's different about black holes is that they are very compact. All the matter of a black hole is compressed down to a single, infinitely dense point. So, you can get much closer to a black hole than you can to an ordinary object. The closer you get, the stronger gravity gets. Eventually, you will reach a distance at which the gravity is so strong, that nothing can escape, not even light. This boundary (defined by this distance) is known as the Event Horizon. Ordinary objects like stars and planets don't have event horizons because they are not so compact. So you can't get that close to them: not before touching their surfaces. ONLY once you get within the event horizon of a black hole is it true that you cannot escape. Outside the event horizon, you can escape if you have enough speed (just like how a spacecraft can escape the pull of Earth's gravity).

    For this reason, the central black hole of the Milky Way galaxy is NOT sucking everything in, and the Milky Way will NOT impode.

    One last point: the "holamola" entire universe is MUCH bigger than one galaxy, and the expansion of the universe is not affected by the gravity of just one single object.
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