Ultraviolet stable, Lorentz-violating dark energy with transient phantom era

Phantom fields with negative kinetic energy are often plagued by vacuum quantum instability in the ultraviolet region. We present a Lorentzviolating dark energy model free from this problem and show that the crossing of the cosmological constant boundary omega = - 1 to the phantom equation of state is realized before reaching a de Sitter attractor. Another interesting feature is a peculiar time dependence of the effective Newton's constant; the magnitude of this effect is naturally small but may be close to experimental limits. We also derive momentum scales of instabilities at which tachyons or ghosts appear in the infrared region around the present Hubble scale and clarify the conditions under which tachyonic instabilities do not spoil the homogeneity of the present/ future Universe.