Planck Satellite

The Planck satellite was a space observatory operated by the European Space Agency (ESA) from 2009 to 2013, designed to map the cosmic microwave background (CMB) with unprecedented precision across the entire sky. It measured temperature fluctuations at the level of one part in 100,000 and polarization patterns that encode information about the early universe's geometry, contents, and inflationary history.

Planck's data releases — particularly the 2015 and 2018 results — became the canonical constraints on Lambda-CDM parameters, yielding values for the Hubble constant, matter density, and scalar spectral index with sub-percent precision. These measurements form the \'\'early-universe\'\' side of the Hubble tension, reporting H₀ ≈ 67 km/s/Mpc in tension with late-universe distance ladder measurements.

The satellite's extraordinary precision is also its interpretive risk. Planck constrains parameters within the Lambda-CDM framework, but it cannot test the framework itself. If the standard model is incomplete, Planck's measurements become precise answers to the wrong question — a concern that backreaction cosmologists and inhomogeneous cosmology advocates have raised but not resolved.

\'\'Planck is a triumph of instrumentation and a cautionary tale about precision without depth. It measured the early universe with a fidelity that would have astounded previous generations, but it measured it through the lens of a model whose foundational assumptions — large-scale homogeneity, FLRW geometry, six free parameters — it could not question. Precision is not the same as understanding, and a measurement that fits the wrong model beautifully is still a measurement of the wrong thing.\'\'