Probing the Cosmic Dawn
At a glance
- The first stars and galaxies are difficult to study because they are exceptionally faint
- Hydrogen, the most abundant element in the Universe, is the clue that astronomers use to unlock when the first galaxies began to shine
- The SKA-Low telescope has been specifically designed to help date when hydrogen’s emissions changed from being electrically neutral to ionised by light photons
One of the final frontiers in cosmology is exploring the time after the Big Bang when the very first stars and galaxies started lighting up the darkness. The SKA telescopes will be the most sensitive radio telescopes available to study these periods, known as the Cosmic Dawn and the Epoch of Reionisation.
Revealing the conditions that existed when the Universe was just a few hundred million years old has proven to be challenging. The signal emitted by early celestial objects, dating back over 13 billion years, is indeed exceptionally faint. Much of this light is also absorbed by intervening gas as it travels toward us.
Surveys by large optical and infrared telescopes have shown that galaxies from the early Universe may be unlike anything we can observe closer by. One reason is that the first stars likely formed from pristine material: mostly hydrogen, a bit of helium, and some heavier elements. The properties of stars that formed from gas enriched by the supernova explosions of an older generation of stars look very different.
How do radio astronomers go about studying the early Universe then?
The key to mapping what happened lies with the Universe’s most abundant element: hydrogen. Indeed, the SKA concept arose from the desire to answer a simple question: How can we fill in the gaps in our understanding of the Universe by reading its history as written in the language of hydrogen?
During the period right after the Big Bang called the Dark Ages, hydrogen was fairly evenly distributed and electrically neutral. Every now and again, the hydrogen atom’s lone electron makes a “spin-flip” transition that produces radio emission that radio astronomers can detect.
In theory, astronomers should be able to observe neutral hydrogen’s weak radiation at a wavelength of 1420MHz, or 21cm. Due to the expansion of the Universe, the wavelength of the emitted radiation is lengthened (this phenomenon is called redshift), meaning that for observers on Earth, we have to look at much lower frequencies to be able to detect such radiation. In practice, the signal is very faint and difficult to detect through the Earth’s ionosphere. To measure this signal requires a low-frequency telescope with a large collecting area and an inside-out understanding of the instrument's properties.
Searching for first light
In an attempt to detect a signal from the Cosmic Dawn, the US-funded EDGES experiment set up an antenna at the SKA-Low telescope site in Western Australia. Its receiver looks deceptively simple, resembling a table tennis table made of metal.
In 2018, the team announced the possible detection of hydrogen from 180 million years after the Big Bang. Confirming the signal is not an easy task. Part of the challenge is disentangling it from everything else the telescope can see.
Early 2022, another team working with a receiver operating on a deep lake in India has tried to replicate the result (unsuccessfully). They theorise that the EDGES signal might have been due to instrument interference.
Both teams have new instruments planned for follow-up studies, and so the hunt continues, ahead of the SKA-Low telescope coming online.
Another method: dating hydrogen reionisation
While radio astronomers continue chasing a direct detection of neutral hydrogen, they also use another method to study the Cosmic Dawn. This involves observing the brightness of light from the early stars and galaxies directly. Doing so will tell them how many photons travelled into the surrounding pockets of gas, ionising the hydrogen.
Labelled reionisation, this important event effectively switches off neutral hydrogen’s emission. Studying the imprint of this process in full can allow astronomers to more accurately date the transition from the Cosmic Dawn to the Epoch of Reionisation.
Models predict that the end of the Epoch of Reionisation corresponds to a hydrogen frequency of 200MHz. This forms one of the motivations for the range of frequencies that the SKA-Low telescope is designed to collect. The telescope will contain more than 130,000 antennas acting as one telescope, registering emissions between 50MHz and 350MHz.
Once up and running, the SKA-Low telescope will be able to take the best possible measurements of the Universe’s first light sources. It should also be able to take snapshots of hydrogen emissions before, during, and after reionisation.
So far, studies of this kind have already led to huge shifts in what we think happened during the early Universe. Since the first galaxies may have formed in different ways than our own Milky Way, their stars could be orders of magnitude larger than our own Sun and have had much shorter lifetimes. Pinpointing when the Epoch of Reionisation started will be key to unlocking these mysteries.