What does this equation mean?

That number sits roughly five standard deviations away from Planck’s CMB-inferred value near 67.4. Both measurements have, individually, become far more precise over the past decade — which is exactly why the disagreement between them, once comfortably inside the error bars, is now a real discrepancy rather than noise. This is the Hubble tension, and it is the field’s most important open problem for one specific reason: it pits two independent, mature, heavily cross-checked measurement chains against each other, rather than one shaky number against a stable one. It is worth being precise about what has and has not been ruled out. Distance-ladder systematics — the Cepheid period- luminosity calibration, supernova standardization, photometric zero points — have been checked repeatedly, including by comparing SH0ES’s Cepheid calibration against an independent one using the tip of the red-giant branch, with only partial convergence. On the early-universe side, the acoustic scale itself is not in question, but the assumed pre-recombination physics used to convert it into $H_0$ could in principle be incomplete — an early form of dark energy, extra relativistic species, or modified matter behavior before recombination could each shift the inferred $r_*$ and thus $H_0$ without Planck’s raw measurement being wrong. No single proposed fix currently resolves the tension without introducing tension with some other, independent dataset. As of this writing the honest summary is not “here is the answer” but “the disagreement is real, well-measured on both sides, and unexplained.”