Arguments against using $h^{-1}{\rm Mpc}$ units in observational cosmology

It is common to express cosmological measurements in units of $h^{-1}{\rm Mpc}$. Here, we review some of the complications that originate from this practice. A crucial problem caused by these units is related to the normalization of the matter power spectrum, which is commonly characterized in terms of the linear-theory rms mass fluctuation in spheres of radius $8\,h^{-1}{\rm Mpc}$, $\sigma_8$. This parameter does not correctly capture the impact of $h$ on the amplitude of density fluctuations. We show that the use of $\sigma_8$ has caused critical misconceptions for both the so-called $\sigma_8$ tension regarding the consistency between low-redshift probes and cosmic microwave background data, and the way in which growth-rate estimates inferred from redshift-space distortions are commonly expressed. We propose to abandon the use of $h^{-1}{\rm Mpc}$ units in cosmology and to characterize the amplitude of the matter power spectrum in terms of $\sigma_{12}$, defined as the mass fluctuation in spheres of radius $12\,{\rm Mpc}$, whose value is similar to the standard $\sigma_8$ for $h\sim 0.67$.