In my video on gravitational waves I mention that they might be something we could use to communicate with and detect parallel universes, if such things exist. Question is, what makes gravity so special?
Firstly, there are two common types of parallel universe theory, one is the many worlds interpretation of quantum mechanics suggested by Hugh Everett in which Universes are constantly diverging and decohering from one another. In this model the Universes are utterly separate and no communication would be allowed between them. The other common parallel universe idea is the multiverse hypothesis of M-theory which suggests that there are many, many universes all inhabiting a higher-dimensional space called "the bulk".
OK, so where does gravity come in? Well, the problem with gravity is that it's remarkably weak. It seems strong when we're standing on the Earth because there's a lot of mass there, so a lot of gravitational force. But when you rub a balloon and stick it to a wall, you've just beaten the entire Earth's gravitational pull with a bit of static. Gravity is actually the weakest of the known forces in nature, of which there are four: Strong, Electromagnetic, Weak and Gravity.
But it's not just the fact that it's the weakest force, it's weakest by a very peculiar amount. If we took the Strong force to be 1, the electromagnetic would be 0.01, the weak would be 0.0000001 and the gravity force would be 0.00000000000000000000000000000000000001. That's not just weaker, that's a bizarre amount of weaker.
The solution some physicists have proposed is that the reason gravity appears so much weaker is that it is the only force with the ability to leak out of our universe. And, by contrast, gravity can leak into our universe from other universes. If this were true then any time we tried to measure gravity it would seem a lot weaker than it should be (a lot is missing) and we would also find a mysterious amount of apparent extra gravity in the universe with us (which we do).
To be explicit: we don't know for definite if gravity can escape our universe, we don't even know if there is an "outside" for it to escape into. But, now that we know gravity waves are a real thing, we can start studying them and probing to see if these other universes do exist. It's kind of like when we ask if there's extra-terrestrial life in our solar system. We don't know, but if we had to place bets on where to look, Europa and Mars would be good candidates. Likewise, we don't know if parallel worlds really do exist but if so, gravity waves are our best chance of detecting them!