It doesn’t really need to be that fast, provided that the expansion front is deep. Seed probes that act as a nucleus for construction could be almost impossible to see, and the parent civilization might be very distant.
Even if the parent civilization did megaengineering of a galaxy (e.g. enclosing all the stars in Dyson swarms or outright disassembling them), we’d probably see that as a natural phenomenon. We can’t tell what would otherwise have been there instead, and such large-scale changes probably do still take a long time to carry out even with advanced technology.
There are in fact a great many observations in astronomy where we don’t really know what’s happening. Obviously nobody is claiming “aliens did it”, especially after the pulsar debacle last century. There are moderately plausible natural hypotheses. But if aliens were doing it, we probably couldn’t conclusively say so.
It doesn’t really need to be that fast, provided that the expansion front is deep. Seed probes that act as a nucleus for construction could be almost impossible to see, and the parent civilization might be very distant.
Even if the parent civilization did megaengineering of a galaxy (e.g. enclosing all the stars in Dyson swarms or outright disassembling them), we’d probably see that as a natural phenomenon. We can’t tell what would otherwise have been there instead, and such large-scale changes probably do still take a long time to carry out even with advanced technology.
There are in fact a great many observations in astronomy where we don’t really know what’s happening. Obviously nobody is claiming “aliens did it”, especially after the pulsar debacle last century. There are moderately plausible natural hypotheses. But if aliens were doing it, we probably couldn’t conclusively say so.