Has Light from the First Stars After the Big Bang been Detected?

Cosmic Dawn

What was observed and what is the interpretation of the recorded data

by Dr John Hartnett, Physicist and Cosmologist

“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. 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.

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.

What was observed

Astronomers recorded the spatially averaged, all-sky radio-frequency emissions received at their antenna for all frequencies between 50 MHz and 100 MHz. This was filtered and averaged over hundreds of hours. It was then represented as a brightness temperature.

The astronomers used an antenna tuned to a region less than 200 MHz because of cosmological theory. The theory dictates/predicts that the so-called “cosmic dawn” occurred several hundred million years after the alleged big bang. This is the period in the big bang story that when sufficient neutral hydrogen clumped together the first stars turned on. According to standard ΛCDM cosmology the “cosmic dawn” ended at a redshift of about 20 and started some 100 million years before that.

The neutral atomic hydrogen of those first stars allegedly absorbed radiation from the big bang fireball (now called the cosmic microwave background (CMB) radiation). This resulted in spin-flips of the atomic hydrogen atom when the atoms absorbed radiation at a wavelength of 21 cm, or about 1420 MHz. As a result the researchers looked for a 21-cm absorption dip in the sky-averaged radio-spectrum. But if you want to see this absorption dip today after the universe has allegedly expanded by a factor of about 20 you have to look down in the FM radio frequency band. Roughly said: divide 1420 MHz by 20 equals approximately 70 MHz. That means the 21-cm absorption feature has been redshifted down to about 70 MHz for a period in the big bang history characterised by a redshift z =19.4

The astronomers had to be sure the source of this radiation they were observing, together with its absorption feature, was not from Earth or the Galaxy itself. Both have radio emissions at these frequencies at much higher intensities than what was expected to be seen from the ‘cosmic dawn’. So they spent several years with a second antenna making sure it was not interference from Earth or the Galaxy…