This is my semi-educated guess as to what's going on here. Beware that semi-educated guesses are sometimes worse than uneducated ones, since they are more plausible and not necessarily more correct.<p>The Alcubierre warp drive requires both positive and negative curvatures. Positive curvatures are "easy" to make, since mass and energy bend space positively. Negative curvatures are typically said to require negative energy to produce, but the full story is thankfully more complex.<p>The universe, despite being filled with mass and energy, is roughly flat. That means that the natural bending of spacetime, without any stuff inside it, has to be negative - otherwise all the mass would make it extra positive. If you believe the consequence of vacuum energy in QED, that the fields filling the universe are VERY heavy, even when no particles are around, then you must believe the natural curvature to be very negative indeed.<p>So, it stands to reason, if you want to bend the universe backwards, take the weights off of it - <i>really</i> off of it, in this case using metal plates to forbid certain vacuum fluctuations, and let it unbend itself.