To understand the Hubble Tension, we must first understand the Hubble Constant. The Hubble constant (H0) is a value that measures the rate of expansion of the universe. As we know, the universe is constantly expanding. However, what it is expanding into is still an open question that, perhaps, may never be convincingly answered by our current knowledge of astrophysics.

Two values are generally observed for the Hubble constant. The first value is ~67 kilometers per second per megaparsec, which is determined by analyzing the cosmic microwave background, and modeling the present expansion rate based on those computations. This approach is called Lambda-Cold-Dark-Matter (or ΛCDM) modeling. The second value is ~73 kilometers per second per megaparsec, which is achieved by a method called the ‘Distance Ladder’ that involves measuring the distances to nearby galaxies and stars (especially a particular category of stars called cepheids.)

This discrepancy between the two values is statistically significant, and is known as the Hubble Tension, or the 4.2σ disagreement.

Scientists have been working on innovative approaches to address this discrepancy (e.g., using gravitational waves) but with limited success. The problem seems to be getting more complicated. A few studies also suggest that our current understanding of astrophysics (on which we build these models) may be inadequate, or even inaccurate – consequently, the models themselves may be inherently flawed.