O'Grady, Gregrory (2025) The ‘Participatory Horizon’: Causal Limits and the CMB Low Power Anomaly. [Preprint]

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Abstract

Standard ΛCDM 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 θ≳ 60◦, as captured by the S1/2 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 introduces the conceptual shift that the CMB should not be treated as a passive fossil record of a fixed past, but as a relative reference frame dynamically coupled to the detector. Adopting a strict definition of measurement as ‘irreversible amplification’, distinct from environmental decoherence, it is argued that decoherence alone is insufficient to select a single realised history for modes that do not terminate in a record. Consequently, it is proposed that the low power anomaly arises because correlations on the largest scales remain in unmeasured superposition, effectively filtered out because they lie beyond the detector’s ‘participatory horizon’.

A first demonstrative implementation of this hypothesis is presented, phenomenologically modelling the local causal horizon as an intrinsic information aperture that limits the resolution of long wavelength modes collapsing into distinct classical records. This results in a horizon scale modification of the primordial scalar power spectrum, implemented as a smooth, scale dependent filter. On evaluation with Planck 2018 temperature data, the model yields a preferred cutoff parameter α ≃1.5, indicating that the data naturally prefers a suppression scale commensurate with the observer’s causal horizon, without requiring fine-tuned early-universe dynamics. The filtered spectrum improves the fit to the lowest multipole temperature anisotropies (∆χ2 ≃−5) and reduces the S1/2 statistic by approximately 80% relative to the Planck ΛCDM best fit, bringing it into close agreement with the measured sky.

The model predicts that this horizon effect must imply a correlated suppression in the largest scale E-mode polarization pattern, offering a falsifiability test addressable in future missions such as LiteBIRD. Possible extensions toward anisotropic filters relevant to quadrupole–octopole alignments and rest-frame tensions are briefly discussed.

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Item Type:	Preprint
Creators:	Creators Email ORCID O'Grady, Gregrory greg.ogrady@auckland.ac.nz 0000-0002-5998-1080
Keywords:	John Archibald Wheeler, Participatory Universe, Cosmic Microwave Background, Low Power Anomaly, Planck, Quantum Cosmology
Subjects:	Specific Sciences > Physics > Astrophysics Specific Sciences > Physics > Cosmology Specific Sciences > Physics > Quantum Gravity Specific Sciences > Physics > Quantum Mechanics General Issues > Realism/Anti-realism Specific Sciences > Physics > Relativity Theory
Depositing User:	Prof. Gregory O'Grady
Date Deposited:	12 Dec 2025 18:36
Last Modified:	12 Dec 2025 18:36
Item ID:	27451
Subjects:	Specific Sciences > Physics > Astrophysics Specific Sciences > Physics > Cosmology Specific Sciences > Physics > Quantum Gravity Specific Sciences > Physics > Quantum Mechanics General Issues > Realism/Anti-realism Specific Sciences > Physics > Relativity Theory
Date:	12 December 2025
URI:	https://philsci-archive.pitt.edu/id/eprint/27451

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