What is the Hubble tension, and why can't cosmologists agree on how fast the universe is expanding?

The Hubble tension is a major disagreement in modern cosmology regarding the value of the Hubble constant ($H_0$), which measures the rate at which the universe is expanding.

The Two Ways to Measure

The tension exists because two primary, independent methods for measuring the Hubble constant yield different results that do not overlap:

1. The "Early Universe" Method (Cosmic Microwave Background): Scientists look at the afterglow of the Big Bang (using data from the Planck satellite). By applying the standard model of cosmology—which describes how gravity, dark matter, and dark energy interact—they predict the expansion rate we should see today. This method suggests an expansion rate of about 67 km/s/Mpc.
2. The "Late Universe" Method (The Distance Ladder): Astronomers look at nearby stars and galaxies directly. They use "standard candles," such as Cepheid variable stars and Type Ia supernovae, to calculate the actual distance to galaxies and how fast they are moving away from us. This method consistently measures a higher rate, around 73 km/s/Mpc.

Why They Can't Agree

The disagreement is considered a "tension" rather than a simple error because both sides have refined their measurements to be incredibly precise. If the discrepancy were merely a matter of measurement error, it should have disappeared as data improved; instead, the gap has persisted and grown more significant.

There are three main theories for why this is happening:

• Systematic Errors: Some believe there may be subtle, hidden biases in the measurement techniques. For example, perhaps our understanding of how Cepheid stars behave or the calibration of the instruments measuring them is slightly off.
• "New Physics": Many cosmologists suspect the issue isn't the measurements, but our model of the universe. If the early universe behaved differently than we currently understand—perhaps due to the influence of "early dark energy" or new types of subatomic particles that aren't in our current models—it could explain why the extrapolated data from the Big Bang doesn't match the modern, direct observations.
• The Unknown: It is possible that we are missing a fundamental piece of the puzzle regarding how dark energy or dark matter evolved over the last 13.8 billion years.

Currently, cosmologists cannot agree because neither side has provided evidence of a measurement flaw, but neither side can account for the other's results without introducing complex new theories that have yet to be proven. It remains one of the most exciting mysteries in physics.