The observed distribution of galaxies on the sky provides a plethora of cosmological information, both on the evolution of the Universe and on large-scale structure. To interpret these observations however, we need to relate the observed galaxy sky positions and redshifts to the underlying space-time and matter distribution, a relation which is nontrivial even on very large scales, because of general relativity: the Universe is not exactly homogeneous and isotropic, and this deviation from an exact FLRW space-time distorts its own image. Light from galaxies is distorted (lensed, redshifted) by the large-scale structure – by the very distribution that we seek to observe.
The papers Ginat et al. (2021), JCAP12(2021)031 and Villey, Ginat et al., arXiv:2506.11260 (2025) seek to study these effects, with the help of the “cosmic ruler” formulation of large-scale structure observables. The second calculates the cosmic rulers explicitly at second order in cosmological perturbation theory – no mean feat!
