Causal isolation is the reason they're called separate universes. It means they cannot influence each other; no signal from either can reach the other.
In a universe where spacetime itself is expanding, the further apart two galaxies are, the faster they will be receding from each other. At some distance, they will be separating faster than light, and beyond that distance (roughly), no signal from one will ever be able to reach the other. The point beyond which no signal can ever reach you is called the
cosmic event horizon. Volumes delimited by cosmic event horizons can be thought of as separate universes because they're causally isolated. A spatially infinite universe would contain an infinite number of these 'universes'.
You're welcome.
OT: The speed of light. I have heard from many sources, that a solid can never reach the speed of light. I have to assume they mean there, the speed of light relative to some point in spacetime.
I revere science and even respect scientists enough to suppose that there is some concept that I just don't get in all this, since scientists make so many claims that are hard to make sense of, so I ask the following questions, maybe to help me understand.
I have also heard that when an object reaches the speed of light, it becomes energy. (To me, that doesn't quite make sense if E=MCsquared, but anyway). If that is true, when object #1 is approaching the speed of light, compared to location A, and object #2 is traveling an opposite direction, say at nearly the speed of light, then they each to the other would seem to be energy. I wonder then, is all matter actually energy, since it is obviously receding from some point past the speed of light? You seem to imply that no perception of one to the other would be possible, so the question is moot. Yet it seems to me that perception is not the definition of reality. (I.e. the fact we don't see that object receding from us does not mean it is not). We see all objects as cohesive reality; in the less abstract mind, for that object to become energy, how can if remain cohesive, so that as it slows it becomes the same object again?
I ask all that to introduce the following question: Is all energy matter sped up, or the potential to be matter if slowed down? If so, would the waveform of that energy define the matter it becomes? and vice-versa?
I understand all this seems to assume that the whole object reaches the speed of light at the same time, which is not proven, nor does it reference any relation of the action of the atoms and their parts within the object to the speed of the object.
The sonic boom is relative to the speed of the air in which the soundwave is compressed and unable to escape forward. If one was able to travel the speed of light, I must assume that can only be relative to a point, since it seems there is no static medium to travel through.
If a "universe", then, is separating from this one faster than the speed of light, there must exist spacetime between the two, by which the difference can be actual, no? If the direction from the one could be known, could not a theoretical spaceship, traveling from the edge of this "universe" approaching the speed of light send information at the speed of light relative to that ship toward the departing "universe" faster than that departure? (I put "universe" in quotes, since these seem more like galaxies to me than universes, since I have heard that a universe is defined by spacetime --there being no reality (spacetime) outside of it).