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If two super massive black hole are 10 l.y away and completely still relative to one another... They will start moving towards each other due gravity but they will never reach each other due universal expansion, in this case they will accelerate for an infinite amount of time, what will prevent them from reaching the speed of light once they reach 99,999...% light speed (and still accelerating)?

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You are making a wrong assumption that the space expansion "pulls" matter apart with a force. It does not. When two heavy objects are attracted to each other with a gravitational force, it is the only force that acts on them. The space expansion does not apply a counteracting force. In other words, in the universe expanding without acceleration, the space expansion is manifested by the recession speed, but not by force or acceleration. Therefore, in your example, nothing would "pull apart" the black holes, other than their initial speed.

At a 10-ly distance, the recession speed due to the space expansion is approximately 2.4 cm/sec, not a significant amount. However, the gravitational acceleration of a black hole of 2 solar masses at this distance is 0.000,000,000,000,03 m/s^2. It would take approximately 25,000 years for the black holes to stop moving apart and start moving toward each other with no effect from the space expansion.

According to the recent data, our universe may be expanding with a small acceleration. In this case the scenario would be slightly different. However, the acceleration is so small, that it would not produce a significant effect. So the overall picture would remain the same.

You can find more details here: Why does space expansion not expand matter?

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