The cosmological ultra-low frequency radio background: a solution to the Hubble tension and the 21-cm excess trough

Abstract

Recent observations of the Cosmic Microwave Background and the Hubble Space Telescope obtain two different values of the Hubble constant with a discrepancy of at least 4.4 sigma , which is now known as the Hubble tension. On the other hand, observations of 21-cm signal from the EDGES experiment reveal an unexpected excess absorption trough. These results give a big challenge to the standard cosmological model – the Lambda CDM model. In this article, we propose that the existence of a cosmological ultra-low frequency radio background (ULFRB) can simultaneously reconcile both discrepancies. The best-fit amounts of the energy density of this additional radio component based on the two problems give an excellent agreement with each other. The extra radio component could be arisen from the simulated decay of axions in the early universe.