The Hubble tension refers to the growing discrepancy between different measurements of the Hubble constant—the rate at which the universe is expanding. This constant is essential for understanding the size, age, and evolution of the cosmos.
There are two main methods of measuring it:
- Local measurements: These use observations of nearby objects like Cepheid variable stars and Type Ia supernovae to directly calculate the expansion rate. This method, championed by teams like the SH0ES collaboration, gives a higher value for the Hubble constant.
- Cosmic microwave background (CMB) analysis: This involves observing the early universe, particularly the afterglow of the Big Bang, and using a model of cosmic evolution (ΛCDM) to infer the expansion rate. This method, based on data from missions like Planck, yields a lower value.
The fact that these two precise and independent methods do not agree is what creates the Hubble tension. The disagreement is now statistically significant, suggesting that it may not just be due to experimental error.
This tension could point to new physics beyond the standard cosmological model, such as unknown particles, changes in dark energy, or modifications to general relativity. Solving the Hubble tension is one of the biggest challenges in modern cosmology.