Part 7/11:
Conversely, measurements based on the cosmic microwave background—particularly from the Planck satellite—suggest a lower value around 67.4 km/sec per megaparsec. These estimates rely on extrapolating the universe's expansion back to its infancy, assuming the ΛCDM model, which combines dark energy, dark matter, and general relativity.
The Hubble Tension
The stark difference between these values—often exceeding the combined uncertainties—has become known as the Hubble Tension. Rather than narrowing with improved data, the gap has only widened, raising profound questions. It’s as if our model of the universe, built on sound physics and robust data, is missing a critical piece.