The relevance of the cosmic microwave background for cosmology

Abstract

Within the Hot Big Bang picture, as usually interpreted in the standard LCDM framework and its hierarchical scenario for elliptical galaxy assembly, the cosmic microwave background (CMB) is associated with photon decoupling near z = 1100, and the Planck angular power spectra are taken to constrain primordial 10^{-5}-level fluctuations that seeded later structure formation. We show that, in addition to the LCDM model, two cosmological models based on Milgromian gravitation are also consistent with the Planck CMB power spectra, such that the spectra do not uniquely favor dark matter. We then argue that the published CMB spectra are not direct observables, but reconstructed quantities obtained after foreground subtraction and corrections for weak lensing, photon-electron scattering, and gravitational energy redshifts along the line of sight. Because these steps depend on the assumed growth history of structure, the resulting spectra depend on the cosmological model adopted in the data reduction. We also note that the reported correlation between the hemispherical CMB asymmetry and the excess on-sky distribution of elliptical galaxies may point to matter inhomogeneities spanning large portions of the observable universe. Motivated by observational tensions in the hierarchical assembly of elliptical galaxies, we review the consequences of elliptical-galaxy formation on a downsizing timescale, which implies a significant extragalactic foreground released at 15<z<20. Under conservative assumptions, this foreground contributes at least 1.4 percent of the observed CMB energy density and has not been included in CMB analyses. It follows that the reported 10^{-5}-level fluctuations is introduced by overly aggressive foreground cleaning.