E12 Fundamental Cosmological Parameters

Topic

The central figure here is the standard model of cosmology, which describes the entire universe using just six free parameters; these are simultaneously fitted to cosmic microwave background data with a precision of better than one percent. However, two significant tensions exist within this standard model. The first concerns the universe's current expansion rate: measurements taken via two independent methods fail to align, and this discrepancy remains one of the model's primary anomalies. The second tension relates to the extent of cosmic structure formation over time: the cosmic microwave background predicts a greater degree of matter clustering than is actually measured directly today, and this gap between prediction and observation marks this second issue as another of the model's anomalies. To determine the standard model's six parameters with the highest possible precision, modern cosmology combines various independent sources of evidence: the cosmic microwave background itself, baryon acoustic oscillations, thermonuclear supernovae (used as standard candles), and weak gravitational lensing. Thus, from the six-parameter standard model with which this document begins to the combination of evidence with which it concludes—and encompassing the two unresolved internal tensions along the way—the standard model of cosmology allows the entire universe, in all its complexity, to be described by a handful of numbers; the fact that these numbers do not agree with one another currently poses one of the most pressing questions in the entire field of cosmology.

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