In the recent news, scientists at NASA have found “unprecedented” black hole cluster near Andromeda’s central bulge. I wonder why doesn't all these black holes merge and such each other in until just 1 remains? Is that even possible? Can one black hole suck in another black hole?
[Physics] Can one black hole suck in another black hole
angular momentumblack-holesconservation-lawsgravity
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I'm not going to address the production mechanism,1 just the nature of the "sound" in this case.
What you think of as the hard vacuum of outer space could just as well be seen as a very, very, very diffuse, somewhat ionized gas. That gas can support sound waves as long as the wavelength is considerably longer than the mean free path of the atoms on the gas.
As for the tone, there is a simple relationship between the tone of the same name in different octaves, so once they know the dominant frequency they can figure its place on the scale.
1 Though it won't be happening inside the event horizon -- which is where "not even light can escape" holds -- but in the region around the hole proper where it accumulates gas and dust and the magnetic fields from the hole play merry havoc with the ionized components of the accumulated stuff.
Warp drives are not allowed by the basic laws of physics, in particular the theory of relativity prohibits any superluminal motion or superluminal propagation of usable information. So whatever "exotic matter" or other wordings are proposed to justify the superluminal warp drives is banned, too. The typical "exotic matter" needed for warp drives would need to admit a negative energy-mass density. If Nature allowed the energy density to go negative, the vacuum itself would be unstable.
The laws of physics also ban the transfer of an object from a universe to a different universe through a black hole, whether it is primordial or not. Wormholes allowed by the laws of physics, if there are any, have to be non-traversable. It means that by jumping into such a black hole, he is guaranteed to end up in the singularity. At most, the singularity where he ends may be shared between a pair (or perhaps a higher number) of black holes. Such a pair of black holes is known as the non-traversable wormhole or the Einstein-Rosen bridge (at least the simplest one). But whoever falls into a black hole can never escape to the "liberated space" outside it, in any Universe, by the very definition of a black hole. Incidentally, the technical reason why a traversable wormhole can't be built is the very same why the warp drives are impossible: negative energy density would be needed for them, too.
Sagittarius A* is in no way the nearest black hole to the Earth. For example, V404 Cygni has a black hole in it that
is 7,800 light years away, and there are probably many closer ones, too.
It wasn't my plan to correct statements in the question but I have to. While an idealized empty large black hole is indeed mildly curved and one may survive the fall behind the horizon, that's not true for realistic black holes, especially Sagittarius A*. The latter is surrounded by plasma, matter that the black hole devours. A good description of the plasma around Sgr A* is the two-temperature plasma. The temperature of the electron component of the plasma is about 100 billion kelvins. No material that could be used to build a spaceship could survive these conditions. We would need a much more isolated black hole to avoid this thermal hell but it isn't clear whether such astrophysical black holes exist.
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It's a very common misconception that black holes suck in matter. Outside the event horizon the gravitational field from a black hole is just like the gravitational field from normal matter with the same mass. The black hole can't suck in matter because matter will orbit it just like matter orbits a normal star.
It's likely there are a lot of black holes in Andromeda's central bulge simply because there are lots of old massive stars in Andromeda's central bulge and this type of star tends to form a black hole as it ages. The black holes orbiting the galaxy core won't fall into it unless there is some way they can lower their angular momentum. This can happen if they pass close to other stars. Interactions between two orbiting bodies can lower the angular momentum of one body and raise the angular momentum of the other. In a galaxy this is known as dynamical friction, and it tends to make heavy objects move inwards and light objects move outwards. Eventually the effect will cause many of the black holes to merge with the central black hole, but the timescale is much longer than the current age of the galaxy.
See Are galactic stars spiraling inwards? for some more info on this effect.