Wrong frequency shift. Magnitude too large. It'll probably be several decades before we can construct a Death Star early warning system. Even an upgraded eLISA is probably only good for detecting binary neutron stars and the birth of Galactus.
Galactus is the least of our problems, just give him a foot massage and back and shoulder rub, let him take a bite out of the moon and otherwise agree not to eat planets people live on.
I'm hoping someone gets the reference.
on the note of the death star, if earth is any mirror of life in the universe then our warheads = their death stars, they've probably got like 40 of them per galactic governance or more and only let the superpowers actually use them, who knows maybe they contribute to universal darkening (the mirror of global warming) point is there are probably going to be no death star attacks because they've got a multi-galactic governance agreement not to use them and have death star defense fields to protect themselves.
plus we currently are like austrailia, WTF mate? (I hope some of you get that reference). no one in the galaxy cares about us, no one is going to bother destroying us, at this point we aren't even considered a target worthy of performing a test because we don't have any defensive systems.
the only thing such a universe need fear is the repercussions of Galactus having severe gastrointestinal problems.