The impact of non-Planckian effects on cosmological radio background

Abstract

Non-Planckian (NP) spectral modifications of the CMB radiation spectrum can be produced due to the existence of a non-zero value of the plasma frequency at the recombination epoch. We present here an analysis of NP effects on the cosmological radio background and we derive, for the first time, predictions of their amplitude on three different observables: the CMB spectrum, the Sunyaev-Zel'dovich (SZ) effect in cosmic structures, and the 21-cm background temperature brightness change. We find that NP effect can manifest in the CMB spectrum at ν ≲ 400 MHz as a drastic cut-off in the CMB intensity. Using the available CMB data in the relevant ν range (i.e., mainly at ≲ 1 GHz and in the COBE-FIRAS data frequency range), we derive upper limits on the plasma frequency νp = 206, 346 and 418 MHz at 1, 2 and 3 σ confidence level, respectively. We find that the difference between the pure Planck spectrum and the one modified by NP effects is of the order of mJy/arcmin2 at ν ≲ 0.5 GHz and it becomes smaller at higher frequencies where it is ∼ 0.1 mJy/arcmin2 at ν ≳ 150 GHz, thus indicating that the experimental route to probe NP effects in the early universe is to observe the cosmological radio background at very low frequencies. We have calculated for the first time the NP SZ effect (SZNP) using the upper limits on νp allowed by the CMB data. We found that the SZNP effect shows a unique spectral feature, i.e. a peak located exactly at the plasma frequency νp and this is independent of the cluster parameters (such as its temperature or optical depth). This offers a way, therefore, to measure directly and unambiguously the plasma frequency in the early universe at the epoch of recombination by using galaxy clusters in the local universe, thus opening a unique window for the experimental exploration of plasma effects in the early universe. We have shown that the SKA-LOW has the potential to observe such a signal integrating over the central regions of high-temperature clusters. The studies of NP effects through the SZNP can be done by intensive observations of only one galaxy cluster, or with a stacked spectrum of a few well known clusters, thus avoiding the need of large statistical studies of source populations or wide area surveys. Finally, we also show that future low-ν observations of the cosmological 21-cm background brightness temperature spectral changes have the possibility to set global constraints on NP effects by constraining the spectral variations of the temperature brightness change δ Tb induced by the plasma frequency value at the epoch of recombination.