Certainly! The two different measurements of the Hubble expansion refer to the values obtained for the Hubble constant, which describes the rate at which the universe is expanding.
CMB Measurements: The Cosmic Microwave Background (CMB) is the residual radiation from the Big Bang, which permeates the entire universe. Scientists have extensively studied the CMB to measure the Hubble constant indirectly. This involves analyzing the patterns in the CMB fluctuations and how they have evolved over time. The most notable experiment for CMB measurements is the Planck satellite. The latest Planck data, combined with other CMB measurements, has provided a value for the Hubble constant.
Local Measurements: Local measurements of the Hubble constant rely on more direct techniques involving observations of nearby galaxies and their distances. One of the primary methods used is called the "cosmic distance ladder." It involves measuring the distances to nearby galaxies using various techniques, such as parallax, Cepheid variables, and Type Ia supernovae. By calibrating these distance indicators, astronomers can estimate the Hubble constant.
The difference between the two measurements arises from the fact that they employ different techniques and data sets. The CMB measurements provide a value based on the analysis of the early universe, while the local measurements focus on nearby galaxies and their distances.
Currently, there is a discrepancy between the two measurements. The CMB measurements suggest a Hubble constant value of around 67.4 kilometers per second per megaparsec (km/s/Mpc), with a small uncertainty. On the other hand, the local measurements yield a higher value of around 73.4 km/s/Mpc, with a similar level of uncertainty. This difference of about 6 to 7 km/s/Mpc has become known as the "Hubble tension."
At present, it is challenging to determine which measurement is favored. Both methods have their strengths and uncertainties, and the tension between them has spurred significant interest and ongoing research. Scientists are investigating potential sources of systematic errors and looking for new ways to refine the measurements further. Resolving the Hubble tension remains an active area of study in cosmology.