Positive-Energy Warp Drive in a De Sitter Universe

Generalizing the result of H. Ellis who embedded a warp bubble in the background of a black hole, we introduce a warp bubble in a de Sitter universe. We show that under certain conditions (namely, that the bubble is moving in the radial direction at a velocity equal to the speed of the expansion of the universe), it is possible for the bubble to have strictly non--negative Eulerian energy density and satisfy the averaged weak and null energy conditions (though they are violated locally). We also prove a more generic theorem that if perturbations of vacuum energy produce at least some underdense regions in all reference frames, they always result in local violations of NEC and WEC. We discuss the implications of these results and their possible applications to models of dark energy like "dark fluid" and quintessence, as well as to physical systems like Casimir cavities and analogue gravity setups.