Horizon Thermodynamics and the Cosmological Constant: A Holographic Dark Energy Note on Why Now?

We argue that the cosmological constant is the thermodynamic signature of the universe's cosmological event horizon. Starting from the Unruh effect and the holographic bound, we recover the observed dark energy density as thermal energy dissipated across the horizon. The fixed-point condition rho_thermal = rho_Lambda holds exactly in de Sitter spacetime and uniquely for the maximum-entropy relation E = TS, yielding a parameter-free prediction c = 1 for the holographic dark energy model and a present-day equation of state w ~ -0.89 with w >= -1 at all times. The generalized second law reinforces this prohibition, suppressing phantom crossing by exp(-10^122). DESI DR2 favors evolving dark energy at 2.8-4.2 sigma, consistent with this picture; whether phantom crossing is real will be tested by forthcoming data. The equilibration time to de Sitter, t_eq ~ 10^10 years, is comparable to the present age of the universe, resolving the coincidence problem thermodynamically. If Lambda is determined by horizon thermodynamics rather than environmental selection, the anthropic principle becomes unnecessary.