In science, different methods of measuring the same properties should yield the same results.
But when it comes to the expanding Universe, two sets of groupsget consistently different outcomes.
Signals from the early Universe yield expansion rates of 67 km/s/Mpc, while late-time signals yield systematically larger values.
However, every individual measurement is subject to errors and uncertainties inherent to the method used.
Of all the late-time signals, the one with the smallest uncertainty is based on the cosmic distance ladder.
From just three measurements — parallax, Cepheids, and Type Ia Supernovae — they derive a value with just a 2% uncertainty.
But connecting these disparate measurements together requires finding galaxies with both Cepheids and supernovae.
Despite enormous surveys and decades of careful observations, only 19 known galaxies have had both.
This small sample could be inherently biased, a legitimate worry among astronomers in the field.
There are two mechanisms that create Type Ia supernovae, and Cepheid-rich regions likely contain both: another potential bias.
Scientist Lucas Macri noted, in similar historic cases, “the universe was trying to tell you that you didn’t have the whole picture.”
With more examples and improved data, cosmologists hope to finally resolve this conundrum.