D18 Fundamental Cosmological Parameters

Topic

Central to this model is the complete set of fundamental cosmological parameters; measuring them requires combining several independent probes, each sensitive to a different epoch and scale of the universe: one for the early universe via the cosmic microwave background; another for baryon acoustic oscillations across a wide redshift range; another for weak gravitational lensing and structure growth at intermediate redshifts; another for the earliest galaxies ever observed; and gravitational waves as a distance indicator completely independent of all the others.

The strength of this approach lies in the fact that different probes carry ambiguities pointing in different directions within the parameter space; combining them resolves ambiguities that no single probe could untangle on its own.

The most remarkable aspect of this joint analysis is precisely that complementarity: the cosmic background is sensitive to the universe's geometry and its earliest epoch but harbors its own internal parameter ambiguities; baryon acoustic oscillations, measured at lower redshifts, break those very ambiguities; and weak lensing measures structure growth independently of the cosmic background itself.

The persistent tension between the clustering amplitude inferred from the cosmic background and that directly measured by weak lensing—combined with the Hubble tension—collectively suggests that the standard model may be incomplete; furthermore, the latest data hinting that the dark energy equation of state might not be perfectly constant currently offer the strongest evidence for new physics within the cosmic vacuum itself. Thus, from the set of fundamental cosmological parameters with which this document begins to the hint of new physics with which it concludes—encompassing the cosmic microwave background, baryon acoustic oscillations, and weak gravitational lensing—the joint analysis of independent probes ensures that no single ambiguity persists for long against the combined evidence of the others, and that the very tensions no isolated probe could explain ultimately point, in unison, in the same direction.

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