The ‘raisin bread’ model of the expanding Universe, where relative distances increase as the space (dough) expands. The farther away any two raisins are from one another, the greater the observed redshift will be by the time the light is received. The redshift-distance relation predicted by the expanding Universe is borne out in observations, but different methods of measuring the cosmic expansion yield different, incompatible results. (Credit: Ben Gibson/Big Think; Adobe Stock)
The big question isn’t whether the Universe is expanding at 67 or 73 km/s/Mpc. It’s why different methods yield such different answers.
One of the most puzzling facts about our Universe is that even though we have many different methods of measuring how fast the space between galaxies expands, two broad classes of measurements disagree significantly. One of them, known as “distance ladder” measurements,
starts by measuring the distance to nearby stars within the Milky Way of a specific type,
next finding stars of the same type in nearby galaxies,
then measuring some brilliant property (such as type Ia supernovae) that occur in those same galaxies,
and finally linking those bright signals (e.g., supernovae) to ones found at even great cosmic distances.
That method, regardless of the specific indicator or star type used, seems to lead to a value of H0, the Hubble constant, that’s around 73 km/s/Mpc, with only a 1% uncertainty.
But if you instead begin with a signal imprinted onto the cosmos in the early Universe, such as the acoustic scale (resulting from the interactions between normal matter and…
