I've sort of decided one of the possible solutions for gamma ray bursts is just starships exiting to realspace.
What would happen to radio transmissions from an Alcubierre drive fitted space ship while in transit?
I'm not an expert on the physics involved, but I imagine they would probably just get reflected back as Hawking radiation or something.
The point of the bubble is to separate an object entirely from our own universe, because while there is a limit to information travelling at the speed of light, and relativity makes it impossible for an object with mass to approach lightspeed, there are no such limitations on spacetime moving through spacetime.
If anything can cross the threshold and get into or out of the bubble in transit, then there is some degree of quantum entanglement and information transfer and it still counts as a "thing" in our universe, so I would assume the trick only works if the bubble is essentially a black box.
speculatory edit: this also seems to suggest that a ship in an Alcubierre bubble would be blind and unable to actually steer. I don't know if the bubble itself would decay (possible due to collisions with interstellar matter or some inherent instability) or would have to be collapsed from the outside. If the latter, then you need an existing infrastructure of stargate-like things already in place (either moved by conventional means or constructed) and just have to hope they're working and where you're aiming when you set off. There might be models that overcome these issues, I don't know.
If it turns out that Alcubierre drives are the only viable means of FTL, then one possible solution to the Fermi Paradox might lie in the complications inherent to using them - the civilizations that survive tend to be the ones that don't progress beyond simple rockets and radio telescopes, and FTL civilizations are still limited to expansion at below relativistic speeds, and even then the inherent complexity and risk means FTL travel isn't likely to be common.
I'm not sure even having stargates/mass relays would be enough if the bit about collected stray particles turning into omni-directional death-rays upon exiting the bubble is true. You'd need to somehow actually carve a clear path in space-time between any two points.
Alternatively, you might be able to create some sort of limiter and only do "short" hops that limit how much energy gets ejected upon exiting. If you're building up energy inside the bubble though, wouldn't it be putting some kind of strain or pressure on whatever has to maintain the bubble? Seems like that alone would create an upper limit on how much you can "charge" it up.
And since heat and radiation may be factors inside the field, available shielding might also be a limiting factor.
And then I would solve the particle problem by saying the field somehow converts real particles into virtual particles and then put bars on my windows to protect against the bricks thrown by physicists ;)