I have read often that the vacuum energy density in a Casimir vacuum is negative, but I do not understand this. My understanding is the following. The Hamiltonian can be written as a sum over all spatial modes. Calculating the energy density for the vacuum state H|0> = ∫dV ρ_o |0>, leaves an integral ρ_o ~ ∫ w dk, which is divergent. As far as I know one has two options now. The first option is to take ρ_o as a normalization constant and assume the ground state energy is zero. In my oppinion this is only justified for cases where vacuum energy is negligible or not observable. The second option is to calculate the integral with some cut-off for low wavelengths. This procedure is used for cases where the vacuum energy density has to be taken into account, like cosmology (additionally, other assumptions are made in order to make the intergral smaler to fit with the cosmological constant). Now, to calculate the value of the energy density in a Casimir vacuum, one takes the same integral but ignores some modes, which are not possible due to geometrical constraints. Thus, for cosmological or gravitational purposes the Casimir vacuum should have positive energy density according to my reasoning above. But reading about metric engineering based on negative energy density (Alcubierre warp drive and so on,...) the Casimir vacuum is often mentioned. Obviously I am missing something. Regards.