Understanding the early Universe, when the first stars and galaxies formed (the “Cosmic Dawn”), is one of the major science goals of a number of new observatories. The James Webb Space Telescope (JWST), for example, will directly image these early galaxies in deep near-infrared surveys. Observations of the redshifted 21-cm line of atomic hydrogen have provided several upper limits on the 21-cm power spectrum. A tentative detection of the sky-averaged signal at redshift z≈17, made with the Experiment to Detect the Global EoR Signature (EDGES) low-band antenna, was reeported in 2018. However, this is yet to be confirmed.

SARAS 3 is a radiometer based on a monocone antenna that has made observations of the sky from a location in Southern India. The experiment is the first global 21-cm experiment of its kind to take observations while floating on a body of water, which is expected to improve the total efficiency of the antenna. Fifteen hours of observations were integrated in the frequency range 55–85 MHz (z≈15–25), reduced after radio frequency interference filtering, with corrections made for emission from the water beneath the antenna and receiver noise temperature. No evidence for an EDGES-like signal was seen, and the data were used to constrain a population of radio-luminous galaxies ~200 million years after the Big Bang. The results are presented in this week’s issue of Nature Astronomy . (Image credit: https://arxiv.org/pdf/2104.03522.pdf)