The multiple clocks conjecture might explain why it is difficult to eliminate the Hubble tension by changing the cosmological model*

Attempts to eliminate the Hubble constant (H₀) tension by changing the cosmological model have revealed that it is difficult, perhaps surprisingly so, to do so while preserving good model fits to inferred CMB temperature and polarization power spectra and large-scale structure observables such as baryon acoustic oscillations. In this talk I introduce the "many clocks" conjecture as a potential explanation of this difficulty. The so-called clocks are the rates in the relevant Einstein-Boltzmann evolution equations. Chief among these are the gravitational free-fall rates associated with each component (CDM, neutrinos, etc.), the Hubble rate, and the Thomson scattering rate, all of which change with redshift. It is changes in ratios of rates that affect dimensionless observables (such as CMB anisotropies). Models can lead to reduced tension by increasing H₀ while also increasing some of the other rates as well so as to preserve at least some of the rate ratios. Or models can generate new effects that open up degeneracy directions that allow some rate ratios to change. The conjecture is that it is the high number of rate ratios and their diverse effects on observables that leads to the failure of most cosmological model changes to accommodate a high H₀. The difficulty provides valuable context for the empirical success of the ΛCDM model with CMB and large-scale structure observables and highlights how remarkable this success is. I will present evidence relevant to the conjecture, including the fact that the tension can be eliminated with adoption of a model explicitly designed to allow for preservation of all relevant rate ratios under H₀ variation.