Has light from the first stars after the big bang been detected?

“Astronomers detect light from the Universe’s first stars” is the headline of a Nature news article, which appeared February 28, 2018.1  It relates to observations made by a team of astronomers led by Judd Bowman of Arizona State University in Tempe. The team published their results in Nature the same week.2 According to Bowman,

“This is the first time we’ve seen any signal from this early in the Universe, aside from the afterglow of the Big Bang.”

They used a small radio-telescope situated in the Western Australian desert, far away from human settlement to minimise interference from radio signals generated by human technology. (See Fig. 1.) The antenna was tuned to a waveband of about 78 MHz, which is at the low end of FM radio, so isolation from human generated radio signals was essential.

Figure 1: The small radio telescope in Western Australia used to detect evidence of light allegedly from the Universe’s first stars. Credit: CSIRO

To understand what the astronomers interpret from this research I quote an editorial summary from Nature:3

“As the first stars heated hydrogen in the early Universe, the 21-cm hyperfine line—an astronomical standard that represents the spin-flip transition in the ground state of atomic hydrogen—was altered, causing the hydrogen gas to absorb photons from the microwave background. This should produce an observable absorption signal at frequencies of less than 200 megahertz (MHz). Judd Bowman and colleagues report the observation of an absorption profile centred at a frequency of 78 MHz that is about 19 MHz wide and 0.5 kelvin deep. The profile is generally in line with expectations, although it is deeper than predicted. An accompanying paper by Rennan Barkana suggests that baryons were interacting with cold dark-matter particles in the early Universe, cooling the gas more than had been expected.”

Let’s look at this in two stages. What was observed and what is the interpretation of the recorded data. Continue reading