= Solution
Separate the observables before combining their parameter constraints. The <baryon> fraction is included within total matter: for the benchmark, $\Omega_{\rm CDM}=\Omega_m-\Omega_b=0.255$, rather than adding $\Omega_b$ to $\Omega_m$ a second time. With $h=H_0/(100\ {\rm km\,s^{-1}\,Mpc^{-1}})=0.7$, the physical density combinations are
$$
\Omega_bh^2=0.02205,\qquad \Omega_mh^2=0.147.
$$
For <baryons>, <Big Bang nucleosynthesis> predicts primordial light-element abundances as a function of the <baryon-to-photon ratio>; deuterium is especially sensitive. Independently, <baryon> inertia changes the relative heights of compressional and rarefaction peaks in the <cosmic microwave background> through the <baryon loading parameter>. Agreement of these two physical-density estimates supports a <baryon> abundance near the quoted value. For example, the https://arxiv.org/abs/astro-ph/0603449[three-year WMAP cosmological analysis] found $\Omega_bh^2=0.02229\pm0.00073$ in its fitted model.
For total matter, <matter-radiation equality> affects the microwave-background peak pattern and fixes a characteristic turnover in the <matter power spectrum>. <galaxy> clustering therefore constrains <matter density>, with corrections for <galaxy bias> and peculiar-velocity distortions. Cluster <baryon> fractions combine observed gas and stars with total <masses> inferred dynamically or by <gravitational lensing>; comparison with the <cosmic baryon fraction> also constrains $\Omega_m$. Galactic rotation, cluster dynamics and <gravitational lensing> give independent evidence that visible <baryons> alone cannot supply the gravitating <mass>. The precise $\Omega_m$ estimate comes from quantitative joint fits, rather than from a <rotation curve> by itself. Different 2006 data combinations gave somewhat different best fits, so $0.3$ is a rounded benchmark rather than a universal exact fit.
For a <cosmological constant>, standardized high-<redshift> <Type Ia supernovae> measure the shape of the luminosity-distance relation. In a flat matter-vacuum model,
$$
d_L(z)=\frac{c(1+z)}{H_0}\int_0^z
\frac{du}{\sqrt{\Omega_m(1+u)^3+\Omega_\Lambda}}.
$$
The observed supernova dimming relative to simple decelerating matter models favours late accelerated expansion. The original evidence is given by https://arxiv.org/abs/astro-ph/9805201[Riess and collaborators' 1998 supernova analysis]. Combining the distance curve with matter constraints selects a positive vacuum fraction close to the quoted benchmark; it gives $q_0=\Omega_m/2-\Omega_\Lambda=-0.55$.
For curvature, the angular scale of the <Cosmic microwave background acoustic peaks> compares the <sound horizon> with the <angular diameter distance> to last scattering. This strongly constrains spatial geometry, but curvature and late-time expansion parameters can partly compensate one another. Independent distances, notably <baryon acoustic oscillations> and supernova distances, help break that degeneracy. The https://arxiv.org/abs/astro-ph/0501171[2005 SDSS acoustic-peak detection] supplies a low-<redshift> standard ruler complementary to the microwave-background ruler. In combination, these observations support near-flat geometry and the closure relation $\Omega_m+\Omega_\Lambda+\Omega_k\simeq1$.
\b[The benchmark is supported by mutually constraining abundance, clustering, angular-scale and distance measurements.] Its uncertainties are correlated and depend on model assumptions. An exactly zero curvature parameter has no meaningful percentage error; observational evidence establishes consistency with zero and an absolute uncertainty, not a literal fractional accuracy better than ten per cent on zero.
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