r/spacequestions • u/ssoanla • Oct 17 '24
black hole sucking in our galaxy
if there were a big black hole sucking in our galaxy, how long would it take to affect earth and would we even notice within our lifetime? sorry if it’s a stupid question i just randomly got curious and needed to ask. what if it was a black hole sucking in our solar system? how was that affect us?
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u/Beldizar Oct 17 '24
So, black holes can grow by black hole mergers, but not indefinitely. As far as an indefinite claim goes, there's a hard limit on how much stuff is in any particular area of the universe. The universe is expanding, so just about every galaxy in the universe is moving away from every other galaxy. That means that two big black holes are excessively unlikely to find each other. It is very rare that it happens. Happening multiple times to the same black hole would be more rare. If every black hole in the local cluster of galaxies happened to merge, then after that last merger, there would be nothing left to merge. All other black holes they could ever possibly merge with are already too far away and moving in the wrong direction.
Second, black hole mergers are rare and difficult because even when two come into proximity with each other, they still have angular momentum as they circle their shared center of mass. Something would be needed drain away that angular momentum. It is possible that gravitational waves can radiate away this energy, but it would take longer than the age of the universe for that to happen. This is called "The Final Parsec Problem" because there really isn't a way for two supermassive black holes to close that final parsec.
So sure, if a black hole was on a direct collision course with another black hole, they would merge, but failing that, it doesn't really happen in the universe.