I'm a follower of the "Quantized Gravity" flavor of modified gravity, as proposed by Mike McCulloch[1]. I fully realize it's a fringe theory, but there should be experimental evidence from an actual thruster flown in space, launching in less than 90 days..[2] I hope he's right... but the only way to know is to wait and see.
Reactionless thrust would be a handy way to get around the cosmos.