Bumpy power spectra and ΔT/T

With the recent publication of the measurements of the radiation angular power spectrum from the BOOMERanG-98 Antarctic flight [1], it has become apparent that the currently favoured spatially-flat cold dark matter model (matter density parameter Ω_m = 0.3, flatness being restored by a cosmological constant Ω_Λ = 0.7, Hubble parameter h = 0.65, baryon density parameter Ω_bh² = 0.02) no longer provides a good fit to the data. We describe a phenomenological approach to resurrecting this paradigm. We consider a primordial power spectrum which incorporates a bump, arbitrarily placed at k_b, and characterized by a Gaussian in log k of standard deviation σ_b and amplitude A_b, that is superimposed on to a scale-invariant power spectrum. We generate a range of theoretical models that include a bump at scales consistent with cosmic microwave background and large-scale structure observations, and perform a simple χ² test to compare our models with the COBE DMR data and the recently published BOOMERanG-98 and MAXIMA-1 data [2]. Unlike models that include a high baryon content, our models predict a low third acoustic peak. We find that low ℓ observations (20 < ℓ < 200) are a critical discriminant of the bumps because the transfer function has a sharp cutoff on the high ℓ side of the first acoustic peak. We show that the concordance cosmology can be resurrected using our phenomenological approach and our best-fitting model is in agreement with the PSCz observations. A more detailed account of this work has been submitted to Monthly Notices of the Royal Astronomical Society for publication [3].