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

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Standard $\Lambda$CDM cosmology generally provides an excellent fit to the observed Cosmic Microwave Background (CMB), yet anomalies persist at the largest angular scales. One of the most persistent is the near vanishing of the two-point correlation function of CMB temperature fluctuations for angular separations $\theta \gtrsim 60^\circ$, as captured by the $S_{1/2}$ statistic. Curiously, the angular scale at which the correlation function crosses zero corresponds, in comoving units, approximately to the size of the present observable universe.

Motivated by Wheeler's `participatory universe', this work explores a conceptual shift in which the CMB is treated not only as a fossil record, but also as a relational record conditioned by the detector that registers it. Adopting a strict definition of measurement as `irreversible amplification', distinct from environmental decoherence, we take as an explicit assumption that decoherence alone may be insufficient, on its own, to select a unique realised history for modes that do not terminate in an accessible macroscopic record. Within this interpretive stance, we hypothesize that the low power anomaly could reflect an apparent loss of realised long range correlations because the largest scale modes remain effectively unmeasured relative to the detector, lying beyond a detector dependent `participatory horizon'.

As an initial phenomenological proof of principle, this hypothesis is implemented by modelling the local causal horizon as an effective information aperture that suppresses sensitivity to modes with wavelengths comparable to, or larger than, the present conformal horizon. This is encoded as a smooth, scale dependent filter applied to the primordial scalar power spectrum. Using Planck 2018 temperature data and a simplified low multipole $\chi^{2}$ diagnostic (with baseline $\Lambda$CDM parameters held fixed), the model favours a cutoff parameter near $\alpha \simeq 1.5$, corresponding to a suppression scale of order the observer's horizon. In this diagnostic, the filtered spectrum improves the fit to the lowest multipole temperature anisotropies ($\Delta\chi^{2} \simeq -5$) and reduces the $S_{1/2}$ statistic by approximately $80\%$ relative to the Planck $\Lambda$CDM best fit, moving it toward the value reconstructed from the measured sky.

If the filter acts at the level of primordial scalar perturbations (rather than as a temperature only selection effect), it should also imply a correlated suppression in the largest scale $E$-mode polarization pattern, offering a prospective observational test for future missions such as LiteBIRD.