[willis936]

The problem with option 3 is that even a small drop below light speed becomes a large multiplier when looking at galactic and intergalactic distances. Let's say you can manage 0.5 c (pushing far beyond any current physical understanding of what is possible), that means we would have up to a 40,000 year heads up on an approaching galactic civilization. Even if it was 0.9c we would have up to 8,000 years notice. Even with something crazy like direct antimatter - matter conversion the amount of energy to bring a ship to that kind of speed would be a gigantic beacon in the night sky. Barring science fiction we can be relatively confident none are on their way right now.

[pfdietz]

Given that the universe is 13.8 billion years old, 40,000 years is nothing.

The presumption that other putative galactic civilizations start at nearly exactly the same time as us is implausible, especially considering more than 9 billion years passed before the solar system even formed.

[willis936]

Sure, but we'd see the markers on timescales relevant to us. The gap between the light cone and actual velocity is the critical difference between a kugelblitz and an invasion: you could conceivably conceal the former but not the latter.

[pfdietz]

I'm pointing out that any scenario that requires this synchronization is inherently implausible. So if we don't see the markers, trying to say it's because there are lots of civilizations but they just happened to pop up in synchrony with us is not a plausible theory.