The uncertainties in measuring cosmic expansion

Ninety years after Edwin Hubble discovered the systematic motions of galaxies and George Lemaitre explained them as cosmic expansion from a point using Einstein's equations of relativity, observational cosmology today is facing a challenge. Values deduced from the two primary methodologies--the properties of galaxies and the cosmic microwave background radiation (CMBR)--disagree with each other at roughly the ten percent level, yet each one is precise at the level of a few percent. Uncorrected o...