In 1998, two independent teams measuring Type Ia supernovae discovered that the universe's expansion is accelerating — earning the 2011 Nobel Prize. The culprit: dark energy, comprising ~68% of the total energy budget. In the standard ΛCDM model, Λ (the cosmological constant) acts as a constant energy density of empty space with equation-of-state w = P/(ρc²) = −1. This drives exponential expansion: a(t) ∝ e^(H₀√Ω_Λ · t). The Big Rip scenario arises if w < −1 (phantom energy), tearing apart galaxies, then atoms. The ultimate fate depends critically on the dark energy equation of state — still unknown.