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 \nu \raise - 2.0 pt \hbox { \hbox to 0.0 pt { \hbox { $ \sim$ } } \raise 5.0 pt \hbox { $ < $ } } 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 \raise - 2.0 pt \hbox { \hbox to 0.0 pt { \hbox { $ \sim$ } } \raise 5.0 pt \hbox { $ < $ } } 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 \raise - 2.0 pt \hbox { \hbox to 0.0 pt { \hbox { $ \sim$ } } \raise 5.0 pt \hbox { $ < $ } } 0.5 GHz and it becomes smaller at higher frequencies where it is \sim 0.1 mJy/arcmin ^ { 2 } at \nu \raise - 2.0 pt \hbox { \hbox to 0.0 pt { \hbox { $ \sim$ } } \raise 5.0 pt \hbox { $ > $ } } 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 ( SZ _ { NP } ) using the upper limits on \nu _ { p } allowed by the CMB data .
We found that the SZ _ { NP } effect shows a unique spectral feature , i.e .
a peak located exactly at the plasma frequency \nu _ { p } and this is independent of the cluster parameters ( such as its temperature or optical depth ) .
This offers therefore a way to measure directly and unambiguously the plasma frequency 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 SZ _ { NP } 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- \nu 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 \delta T _ { b } induced by the plasma frequency value at the epoch of recombination .