Searching for a signal from the Cosmic Dark Ages

Using cutting-edge telescope technology astronomers look back 12 billion years into the universe’s history to detect signals from the building blocks of the first stars.

Approximately 2 billion years into the universe’s 13.8 billion year history the first stars ignited and illuminated what had previously been darkness. Now, a new analysis of data collected by the Murchison Widefield Array (MWA) radio telescopes has brought researchers closer than ever to the detection of an ultra-faint signal from the end of this cosmic dark age.

The mission of the MWA project and similar endeavours is to spot the signal of neutral hydrogen from this dark era of the universe and to trace how it changes in step with the evolution of the universe.

In their paper published in the Astrophysical Journal, Wenyang Li and Jonathan Prober — both from Brown University — detail their analysis of the data. An analysis specifically designed to search for the telltale signs of neutral hydrogen — the gas that dominated the early universe during this cosmic dark age.

The study set a new lower limit for the strength of such a signal from neutral hydrogen.

“We can say with confidence that if the neutral hydrogen signal was any stronger than the limit we set in the paper, then the telescope would have detected it,” says Prober, an assistant professor of physics at Brown University and corresponding author on the new paper. “These findings can help us to further constrain the timing of when the cosmic dark ages ended and the first stars emerged.”

In doing this, it is possible to gather new information about the very first stars — which provided the building blocks and raw materials for the current generation of stars and planets.

The Epoch of Reionisation. A Dark Age in more ways than one

A visualisation of the Epoch of Reionisation showing the recoupling of hydrogen ions and electrons leading to the formation of the first stars and galaxies. (https://www.kicc.cam.ac.uk/images/reionization)

The period in which the stars first ignited — known as the Epoch of Reionization (EoR) — is one of vital importance in the history of the cosmos. But, despite this importance, relativity little is known by scientists about this period.

One thing that we do know is that the universe was extremely sparse, to say the least, 12 billion years ago. In fact, it was likely populated by little more than neutral hydrogen atoms — formed after expansion and cooling resulted in positively charged hydrogen ions reuniting with electrons stripped by the violent, intensely hot conditions of the infant universe.

At this point, these atoms could then begin to ‘clump’ together, thus forming stars and galaxies. In turn, the light from these stellar objects then caused these early hydrogen atoms to re-ionise and disappear from interstellar space.

As you may imagine, catching a signal from neutral hydrogen 12 billion years in the universe’s history is no easy task and thus requires instruments of extraordinary sensitivity and complexity.

An instrument like the MWA.

The Murchison Widefield Array — hunting for a signal from the dawn of time

The Murchison Widefield Array radio telescope, a portion of which is pictured here, is searching for a signal emitted during the formation of the first stars in the universe. (Goldsmith/MWA Collaboration/Curtin University)

The Murchison Widefield Array (MWA) began operations in 2013 consisting of an array of over 2 thousand radio antennas scattered across the Western Australian countryside. Initially, these antennas were arranged into 128 groups or ‘tiles’ this population was upped to 256 in 2016. Also at this time, the configuration of these tiles was adjusted with the specific goal of being more sensitive to the signal emitted by neutral hydrogen.

Li and Prober’s paper represents the first findings delivered from data collected from this upgraded and expanded array.

Whilst neutral hydrogen in lab conditions emits at a wavelength of roughly 21cm, the expansion of the universe over 12 billion years has stretched this wavelength to approximately 2 metres. What makes the task of the astronomers at the MWA even tougher is the fact that there are numerous other sources that emit electromagnetic radiation at the same wavelength. This includes natural sources within the Milky Way and man-made sources as mundane as digital television, making contamination of their data extremely difficult to avoid.

“All of these other sources are many orders of magnitude stronger than the signal we’re trying to detect,” explains Prober. “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.”

Thus, researchers must use careful processing techniques to filter their data and remove such interference.

“If we look at different radio frequencies or wavelengths, the telescope behaves a little differently,” adds Prober. “Correcting for the telescope response is absolutely 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 be released by MWA and raises hope that the experiment will one day detect the elusive signal of neutral hydrogen.

“This analysis demonstrates that the phase two upgrade had a lot of its desired effects and that the new analysis techniques will improve future analyses,” concludes Prober.

“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.”