What have electrons, black holes and Clifford Algebra ever done for us?

  Yes , you can define an effective Poisson ratio for the horizon—but only in a very specific, emergent, hydrodynamic sense—and it takes its extremal value. The value you get is not arbitrary; it is forced on you by horizon incompressibility, maximal entropy, and diffusion physics. 1. What a Poisson ratio really measures The Poisson ratio (ν) is defined (in ordinary elasticity) as $$\nu = - \frac{\text{transverse strain}}{\text{longitudinal strain}}.$$ Equivalently, in terms of elastic moduli: $$\nu = \frac{3K - 2\mu}{2(3K + \mu)} \quad \text{(3D bulk)},$$ or $$\nu_{2D} = \frac{K_{2D} - \mu_{2D}}{K_{2D} + \mu_{2D}} \quad \text{(membrane)}.$$ So to even ask for a Poisson ratio, we need: a bulk (or area) modulus (K), a shear modulus (μ). This is where horizons are special. 2. Horizon fluids: what moduli exist? From the membrane paradigm, AdS/CFT, and your own derivations: Horizons have: finite shear viscosity (η), no elastic shear modulus ((μ = 0)), negative bulk ...

  De Sitter space as a thermodynamic equilibrium During both inflation and late-time cosmic acceleration, the Universe is well-approximated by a de Sitter (dS) spacetime with nearly constant curvature radius ($\ell_\Lambda $). Our present Universe may therefore be regarded as a quasi–de Sitter state , possessing a cosmic event horizon (CEH) associated with its vacuum energy density (cosmological constant). A defining feature of de Sitter space is that the cosmological horizon is not merely a causal boundary but a thermodynamic object, endowed with temperature, entropy, and energy. In this context, the total bare (rest) energy associated with the horizon, defined via the Brown–York quasilocal energy, can be written as: \begin{equation} E_0 = 2\, k_B T_{dS} S_{dS} = 2\, m_{CEH} c^2 = 2E_H  \end{equation} This relation is a horizon version of the entanglement first law , from which the Einstein equations themselves can be derived. Here: $S_{dS}$ is the de Sitter entropy (d...