Alastair Reynolds is a hard scifi author with a PhD in astrophysics, and most of his works deal with this in some fashion. One is Pushing Ice (SPOILER IN REST O

Alastair Reynolds is a hard scifi author with a PhD in astrophysics, and most of his works deal with this in some fashion. One is Pushing Ice (SPOILER IN REST OF POST), which tackles your question about civilizations not really having the ability to communicate with each other, not only because the galaxy 100,000 light years across, but also because it's billions of years old. That is, you could have lots of civilizations rise and fall, even in the same neighborhood of the galaxy. So the big idea revealed at the end of the book (most of it is structured as "what is going on!?") is that one of the early civilizations planted lots of moons around the galaxy that would lure in and trap ships and then accelerate off somewhere. At high accelerations, for various small amounts of subjective time, millions or billions of years could pass. By calibrating all the moon traps then, lots of different civlizations from different corners of the galaxy, each originating in different eras, would be arriving at about the same time/place.