Hubble Tension as an Effect of Horizon Entanglement Nonequilibrium

We propose an infrared mechanism for alleviating the Hubble constant tension, based on a small departure from entanglement equilibrium at the cosmological apparent horizon. If the horizon entanglement entropy falls slightly below the Bekenstein-Hawking value, we parametrize the shortfall by a fractional deficit $δ(a)$ evolving with the FLRW scale factor $a$. The associated equipartition deficit at the Gibbons-Hawking temperature then sources a smooth, homogeneous component whose density scales as $H^{2}/G$, with a dimensionless coefficient $c_{e}^{2}(a)$ of order unity times $δ(a)$. Because this component tracks $H^{2}$, it is negligible at early times but can activate at redshifts $z\lesssim 1$, raising the late time expansion rate by a few percent without affecting recombination or the sound horizon. We present a minimal three parameter activation model for $c_{e}^{2}(a)$ and derive its impact on the background expansion, effective equation of state, and linear growth for a smooth entanglement sector. The framework predicts a small boost in $H(z)$, a mild suppression of $fσ_{8}(z)$, and a corresponding modification of the low-$z$ distance-redshift relation. We test these predictions against current low-redshift data sets, including SN~Ia distance moduli, baryon acoustic oscillation distance measurements, cosmic chronometer $H(z)$ data, and redshift space distortion constraints, and discuss whether the $H_0$ tension can be consistently interpreted as a late-time, horizon-scale information deficit rather than an early universe modification.

💡 Research Summary

The authors introduce a novel infrared (IR) mechanism, dubbed Horizon Entanglement Equipartition Deficit (HEED), to address the persistent Hubble‑constant (H₀) tension. The central hypothesis is that the entanglement entropy S_ent associated with quantum fields across the apparent (Hubble) horizon is slightly smaller than the Bekenstein–Hawking entropy S_BH = A/(4G). This shortfall is quantified by a fractional deficit δ(a)=1−S_ent/S_BH, which may evolve with the scale factor a. By treating the horizon as an entanglement screen at the Gibbons–Hawking temperature T_dS = H/(2π) and invoking the equipartition relation E = (1/2) N T for the surface degrees of freedom N_surf = A/G = 4 S_BH, the missing energy ΔE_surf = (1/2) δ N_surf T translates into a homogeneous bulk energy density

ρ_HEED(a) = (3/8πG) c_e²(a) H²(a), c_e²(a) ≡ 2 δ(a).

Thus the new component scales as H²/G, a familiar holographic scaling, but its coefficient is tied to an IR entanglement deficit rather than a UV area‑law term. Because ρ_HEED ∝ H², it is negligible during radiation and matter domination, preserving the sound horizon and CMB acoustic physics. At late times (z ≲ 1) the authors posit a smooth activation of the coefficient through a three‑parameter switch function

c_e²(a) = c_e0² g(a; a_t, k), g(a) = 1 + (a_t/a)^k /