> space expansion and the speed faster than c as a result is possible. Without exotic matter, etc. Except that in order to get the observables right, dark ene

> space expansion and the speed faster than c as a result is possible. Without exotic matter, etc. Except that in order to get the observables right, dark energy is required to keep the galaxy clusters running away from each other faster over time. It's a heavy price that no physical cosmologist is really happy about, but the concordance cosmology gets its name from how it is explicitly constructed to match ("concord with") observation (and it is consequently updated from time to time). It is not really surprising that some energy is required to keep the warp bubble from dissipating away from its compact region of spacetime, particularly since the Alcubierre model is a vacuum spacetime, so there's no mass in the middle to keep the bubble local to it (or source it, or however you want to look at it). He says so at the end of his paper[1]. It's important to remember that there is no observational evidence supporting Alcubierre's metric; it concords with nothing. However, one can still attempt to answer the questions he himself raised, and Lobo & Visser do just that in their 2004 paper[2], where they note that the geometry of the Alcubierre metric requires an even-more-unusual-than-dark-energy stabilizing term even at arbitrarily low warp-bubble velocities. - -- [1] https://doi.org/10.1088/0264-9381/11/5/001 https://doi.org/10.1088/0264-9381/11/5/001 https://arxiv.org/abs/gr-qc/0009013 https://arxiv.org/abs/gr-qc/0009013 (short, not especially difficult, fewer than 20 equations, 4 refs) See the text bracketing eqns (18) & (19) [2] https://doi.org/10.1088/0264-9381/21/24/011 https://doi.org/10.1088/0264-9381/21/24/011 https://arxiv.org/abs/gr-qc/0406083 https://arxiv.org/abs/gr-qc/0406083 (twice as long, but mostly developed through more than 120 equations, 38 refs incl. [1])