Observer-Conditioned Cosmology: Cosmic-Web Environment as a Bias Functional in Precision Parameter Inference

Modern precision cosmology generally treats the observer as a statistically typical pointin a universe that is homogeneous and isotropic after sufficient averaging. This assumptionis operationally powerful, but it is not itself a directly measured property of the observer’slocal cosmic environment. The Dark Energy Spectroscopic Instrument (DESI), together withpreceding redshift surveys, makes visually and quantitatively unavoidable the fact that theobserved universe is not a smooth distribution on accessible scales, but a foam-like network ofvoids, walls, filaments, and nodes. The standard interpretation is that this web is compatiblewith statistical homogeneity above a transition scale. In this paper we formulate a sharperquestion: even if the cosmological principle is valid in the ensemble or large-volume sense, arecosmological parameters inferred by embedded observers conditionally biased by the cosmic-webcell in which those observers reside?We propose an observer-conditioned inference framework in which locally inferred cosmological parameters are written asˆθobs = θglobal + BθδR, ∇i∇jΦ, Rvoid, Cweb, W(z, ˆ n) + ϵ,where θglobal denotes the idealized FLRW or ensemble parameter, Bθ is an environment-dependentbias functional, δR is the smoothed local density contrast, ∇i∇jΦ is the local tidal field, Rvoidcharacterizes the observer’s host void or underdense cell, Cweb encodes void-wall-filament-nodemorphology, and W(z, ˆ n) is the actual survey window. The central claim is not that visualinspection of DESI proves a failure of ΛCDM, nor that a local void alone explains the Hubble tension. Rather, the claim is methodological and testable: precision cosmological inferenceshould condition on the observer’s cosmic-web environment and should quantify whether inferredparameters flow with observer location, survey depth, sky direction, and density morphology.We derive the leading perturbative expectation for local expansion bias, formulate angularand redshift-dependent residual tests for H0, BAO, and dark-energy inference, and propose asimulation program in which mock observers are placed in void centers, void edges, walls, filaments, and nodes. The hypothesis is falsified if environment-conditioned mock observers do notexhibit statistically significant parameter shifts beyond known cosmic variance, peculiar-velocitycorrections, and survey-selection effects. It is supported if inferred H0, BAO dilation parametersα∥,α⊥, or dark-energy parameters (w0,wa) show reproducible dependence on observer environment after all standard corrections. The deeper implication is that the cosmological principlemay remain valid as an ensemble statement while being insufficient as a measurement principlefor a single embedded observer. If so, cosmological parameters are not only properties of theuniverse; they are also conditional observables of an observer, a survey, and a cosmic-web cell.