The impact of non-Planckian effects on cosmological radio backgrounds

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 radio cosmological 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 \(\nu \simlt 400\) MHz as a drastic cut-off in the CMB intensity. Using the available CMB data in the relevant \(\nu\) range (i.e., mainly at \(\simlt 1\) GHz and in the COBE-FIRAS data frequency range), we derive upper limits on the plasma frequency \(\nu_p\) = 206, 346 and 418 MHz at 1, 2 and 3 \(\sigma\) 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/arcmin\(^2\) at \(\nu \simlt 0.5\) GHz and it becomes smaller at higher frequencies where it is \(\sim 0.1\) mJy/arcmin\(^2\) at \(\nu \simgt 150\) GHz, thus indicating that the experimental route to probe NP effects in the early universe is to observe the radio cosmological background at very low frequencies.(abridged)