Covariant decomposition of the non-linear galaxy number counts and their monopole

Yonadav Barry Ginat, Vincent Desjacques, Donghui Jeong, Fabian Schmidt

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3 Scopus citations


We present a fully non-linear and relativistically covariant expression for the observed galaxy density contrast. Building on a null tetrad tailored to the cosmological observer's past light cone, we find a decomposition of the non-linear galaxy over-density into manifestly gauge-invariant quantities, each of which has a clear physical interpretation as a cosmological observable. This ensures that the monopole of the galaxy over-density field (the mean galaxy density as a function of observed redshift) is properly accounted for. We anticipate that this decomposition will be useful for future work on non-linearities in galaxy number counts, for example, deriving the relativistic expression for the galaxy bispectrum. We then specialise our results to conformal Newtonian gauge, with a Hubble parameter either defined globally or measured locally, illustrating the significance of the different contributions to the observed monopole of the galaxy density.

Original languageEnglish (US)
Article number031
JournalJournal of Cosmology and Astroparticle Physics
Issue number12
StatePublished - Dec 2021

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics


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