Applicability of weak-coupling results in high density QCD

Quark matter at an asymptotically high baryon chemical potential is in a color superconducting state characterized by a gap Delta. We demonstrate that although present weak-coupling calculations of Delta are formally correct for mu --> infinity, the contributions which have to this point been neglected are large enough that present results can only be trusted for mu>>mu (c) similar to 10(8) MeV. We make this argument by using the gauge dependence of the present calculation as a diagnostic tool. It is known that the present calculation yields a gauge invariant result for mu --> infinity; we show, however, that the gauge dependence of this result only begins to decrease for mu greater than or similar to mu (c), and conclude that the result can certainly not be trusted for mu<(c). In an appendix, we set up the calculation of the influence of the Meissner effect on the magnitude of the gap. This contribution to Delta is, however, much smaller than the neglected contributions whose absence we detect via the resulting gauge dependence.