Is there one or more parallel universes? There are many ways to measure that which is almost beyond measure. Here are a few.
If there is just one parallel universe one could measure light, energy, waves, particles, motion, distance and more. Another theory put forth is that of the Multi verse. If we consider just one parallel universe we would focus on just finding one more data set that mirrors everything we see in our own Universe.
An old fashioned science fiction movie depicted the idea of a parallel earth directly on the other side of our sun. It would have been orbiting in the exact same fashion, and in line with exact order of the planets in our own system. But such a system would surely show signs that astrophysicists, and other scientists could easily have measured by now.
The greatest case put forward for the idea that we are just one of a multitude of Universes is the Anthropic Principal. Imagine a pond in which the fish cannot conceive of the idea of life above the water. They swim in their environment, not even skimming the surface for food, or other necessity. Their eyes are downward facing, they do not know other beings, or life, could exist elsewhere because all the sense abilities are based on their limitations. The fish know only one Universe, but others exist beyond them.
Similarly, the Anthropic principle, meaning literally; Human centered, is based on the observation that people have limited ability to sense any other dimensions. It is also partly based on string theory, which connects what is known of particles, matter, and their interactions. String theory is based on 11 dimensions of linking, and connecting by various oscillations and vibrations all forces, and all matter. The tiniest electron and the largest galaxy are thus all strung together in a theory of Everything. .
The Mulitiverse theory states that we could be in one bubble of one universe, and perhaps by worm holes, or other connecting tunnels through black holes, other Universes are beyond our own, expanding, and containing life that can contemplate itself. An infinite number of such bubbles could continually be born in on-going genesis, and still allow “stability” and permanence, as we know it.
Gravitational waves, as detected in vast space, are one tool astrophysicists hope to utilize to gain more information about the possibility of Multiverses. The new Laser Interferometer Space Antenna, LISA, will use lasers to detect gravitational waves by their effects as witnessed by interference between three distant points in space.
The greatest problem with the Mulitiverse theory is that anything based on something we cannot comprehend, sense, or verify is by definition neither provable, nor falsifiable.