Prediction — LambdaCC BCT Letter 5: The Cosmological Constant from BCT Vacuum Energy — LambdaCC = (rₒct x rₜet x mP²) ²

The cosmological constant problem—why the observed vacuum energy density is ∼ 10120 times smaller than naive quantum field theory predicts—has been called the worst prediction in the history of physics. We present a two-part resolution within the Body-Centred Tetragonal (BCT) Superfluid Lattice Model. First, the tree-level cosmological constant vanishes exactly, Λbare = 0, by two independent mechanisms: Bose–Fermi vacuum pairing between octahedral and tetrahedral void excitations, and D4 modular self-duality (D ∗ 4 = D4) requiring the partition function to satisfy Z(τ ) = Z(−1/τ ). Second, the observed dark energy density is a calculable quantum correction: ρ 1/4 CC = mν1 × fBCT, where mν1 is the lightest neutrino mass (derived from the BCT seesaw mechanism) and fBCT = SD4 × (roct/rtet) 2 × (1 − rtet − α0) = 152.50 is a dimensionless geometric factor encoding the D4 crystal structure. The result is ρ 1/4 CC = 2.264×10−6 MeV, compared to the observed ρ 1/4 CC = 2.260 × 10−6 MeV (Planck 2018), an error of +0.18%. No parameters are adjusted. The dark energy equation of state is w = −1 exactly (a pure cosmological constant, not dynamical dark energy), testable by DESI, Euclid, and the Vera Rubin Observatory.