The 'Participatory Horizon': Causal Limits and the CMB Low Power Anomaly

Several anomalies in the Cosmic Microwave Background (CMB) appear concentrated at the largest angular scales, most notably an anomalous suppression of temperature correlations for separations $\theta \gtrsim 60^\circ$. Curiously, the scale at which these correlations vanish corresponds to a comoving distance comparable to the radius of the observable universe. While conventional explanations invoke statistics or tuned primordial initial conditions, we explore an alternative hypothesis motivated by Wheeler’s participatory view of quantum measurement. We propose that classical cosmological records are instantiated only to the extent that correlations can be resolved and irreversibly registered within an observer’s causal domain. In this stance, the CMB is treated as a relational reference frame rather than a fixed fossil record, with correlations on scales comparable to the 'participatory horizon' remaining under-instantiated as classical records, effectively filtering their contribution to the realised sky.

We explore this concept phenomenologically by applying a smooth horizon-scale infrared suppression to a fiducial $\Lambda$CDM spectrum while leaving the background cosmology unchanged. Fitting to Planck TT bandpowers over $2 \leq \ell \leq 30$ yields a preferred cutoff scale $k_{\rm cut} = \alpha (\pi / \eta_0)$ with $\alpha \simeq 0.69$ (for a representative sharpness $p=6$), corresponding to $k_{\rm cut} \simeq 1.53 \times 10^{-4} \, \text{Mpc}^{-1}$. This result confirms a preference for a suppression scale anchored to the observer's present horizon. The fit is improved by $\Delta \chi^2 \simeq -5.7$ and the large-angle statistic $S_{1/2}$ is reduced by $\sim 85\%$ relative to fiducial $\Lambda$CDM, while the acoustic peak structure at $\ell \gtrsim 50$ remains unchanged.

The model makes testable, falsifiable predictions for the low-$\ell$ $EE$-mode spectrum and temperature-polarisation cross-correlations. These exploratory results suggest that the low power anomaly might be feasibly reframed as a dynamic participatory effect, providing a quantitatively viable overlay to the $\Lambda$CDM model.