With a new analysis of data collected by the Murchison Widefield Array (MWA) radio telescope, scientists are now closer than ever to detecting the ultra-faint signature of this turning point in cosmic history.
The first analysis of data from a new configuration of the MWA designed to look for the signal of neutral hydrogen, the gas that dominated the universe during the cosmic dark age.
The analysis sets a new limit to the lowest limit set for the strength of the neutral hydrogen signal.
Despite its importance in cosmic history, little known about the period when the first stars formed, known as the Epoch of Reionization (EoR).
The first atoms that formed after the Big Bang charged hydrogen ions atoms whose electrons stripped away by the energy of the infant universe.
As the universe cooled and expanded, hydrogen atoms reunited with their electrons to form neutral hydrogen.
And that’s about all there was in the universe until about 12 billion years ago, when atoms started clumping together to form stars and galaxies.
Light from those objects re-ionized the neutral hydrogen, causing it to disappear from interstellar space.
The goal of projects like the one happening at MWA is to locate the signal of neutral hydrogen from the dark ages and measure how it changed as the EoR unfolded.
Doing so could reveal new and critical information about the first stars the building blocks of the universe we see today. But catching any glimpse of that 12-billion-year-old signal is a difficult task that requires instruments with exquisite sensitivity.
When it began operating in 2013, the MWA was an array of 2,048 radio antennas arranged across the remote countryside of Western Australia.
The antennas bundled together into 128 tiles, whose signals combined by a supercomputer called the Correlator.
In 2016, the number of tiles doubled to 256, and their configuration across the landscape altered to improve their sensitivity to the neutral hydrogen signal. This new paper is the first analysis of data from the expanded array.
Neutral hydrogen emits radiation at a wavelength of 21 centimeters. As the universe has expanded over the past 12 billion years, the signal from the EoR is now stretched to about 2 meters, and that’s what MWA astronomers are looking for.
The problem is there are myriad other sources that emit at the same wavelength — human-made sources like digital television as well as natural sources from within the Milky Way and from millions of other galaxies.
Even an FM radio signal that’s reflected off an airplane that happens to be passing above the telescope is enough to contaminate the data.”
To home in on the signal, the researchers use a myriad of processing techniques to weed out those contaminants. At the same time, they account for the unique frequency responses of the telescope itself.
Correcting for the telescope response is critical for then doing the separation of astrophysical contaminants and the signal of interest.
Those data analysis techniques combined with the expanded capacity of the telescope itself resulted in a new upper bound of the EoR signal strength.
It’s the second consecutive best-limit-to-date analysis to released by MWA and raises hope that the experiment will one day detect the elusive EoR signal.
The fact that MWA has now published back-to-back the two best limits on the signal gives momentum to the idea that this experiment and its approach has a lot of promise.”
The research supported in part by the U.S. National Science Foundation (grant #1613040). The MWA receives support from the Australian government and acknowledges Wajarri Yamatji people as the traditional owners of the observatory site.