A neutrino model fit to the CMB power spectrum

The standard cosmological model, LCDM, provides an excellent fit to Cosmic Microwave Background (CMB) data. However, the model has well known problems. For example, the cosmological constant, is fine-tuned to 1 part in 10^100 and the cold dark matter (CDM) particle is not yet detected in the laboratory. Shanks previously investigated a model which assumed neither exotic particles nor a cosmological constant but instead postulated a low Hubble constant (H_0) to allow a baryon density compatible with inflation and zero spatial curvature. However, recent Planck results make it more difficult to reconcile such a model with CMB power spectra. Here we relax the previous assumptions to assess the effects of assuming three active neutrinos of mass ~5eV. If we assume a low H_0~45kms^-1Mpc^-1 then, compared to the previous purely baryonic model, we find a significantly improved fit to the first 3 peaks of the Planck power spectrum. Nevertheless, the goodness-of-fit is still significantly worse than for LCDM and would require appeal to unknown systematic effects for the fit ever to be considered acceptable. A further serious problem is that the amplitude of fluctuations is low (sigma_8~0.2) making it difficult to form galaxies by the present day. This might then require seeds, perhaps from a primordial magnetic field, to be invoked for galaxy formation. These and other problems demonstrate the difficulties faced by models other than LCDM in fitting ever more precise cosmological data.