The Cosmic Dawn
The Big Bang: the dawn of time; the dawn of space; the dawn of everything we know today. However, it wasn’t until much much later that the first light mustered its way out of a star to hail what is known as the Cosmic Dawn.
A recent study by researchers at UCL and the University of Cambridge has determined the date of this event to be between 250 million and 350 million years after the Big Bang. So how did this Cosmic Dawn come about, and why is this information so important? [1]
Artist's concept of the first population III stars
Credit: NASA/WMAP Science Team
The Prologue
The run up to the Cosmic Dawn is the story of how the first atoms of hydrogen came together into a population III star: the ‘purest’ of stars, made almost exclusively out of hydrogen and helium. After the Big Bang, when all the primordial ions and atoms eventually came into being, the passage of time led to them being pulled into clumps which eventually collapsed to form the hot fusion furnaces that then led to what we see today in the form of our Sun (a population I star, containing heavier elements than population III stars [11]) [2].
The ultraviolet (UV) radiation emitted from these population III stars was enough to excite the surrounding hydrogen gas, leading to that gas absorbing energy. This frequency of radiation absorbed was precisely 1.4GHz (1.4 billion Hertz), leading to a characteristic absorption line in spectra of light from around that time [3].
Then, due to the way in which the Universe is expanding, this absorption line will be observed at a lower frequency (redshifted light). To detect this low frequency signal, a small horizontal antenna was utilised, at just around 2 metres long, called EDGES (‘Experiment to Detect the Global Epoch of Reionisation Signature’: a fancy way of saying “a telescope to find the absorption line left by the first stars”).
Even though the signal they were looking for was so far away and so old, theories stated that the amount of UV light emitted by these population III stars was so great that the absorption signal would still be prominent even now, even though noise can be 10,000 times brighter than this signal, which is “like being in the middle of a hurricane and trying to hear the flap of a hummingbird’s wing” according to Peter Kurczynski who oversaw NSF funding of the EDGES programme [4][5][6].
And this turned out to be true when, sure enough, a signal was found at the much lower frequency of 78MHz (78 million Hertz), and a year of doing repeat observations with different variables such as the orientation of the antenna being changed, the lack of a change from that frequency of absorption line was strong evidence for these population III stars and the Cosmic Dawn [7].
EDGES ground based spectrometer
Credit: © Copyright CSIRO Australia, (2018)
Why is the Cosmic Dawn so Important?
The way in which the figure for the Cosmic Dawn (250-350 million years) was worked out was by analysing images not of individual stars but galaxies, taken by the Hubble Space Telescope and Spitzer Space Telescope, due to how bright collections of stars are.
The galaxies analysed were 6 of the most distant (and therefore oldest) galaxies known, of redshifts at z ≥ 9 (including: MACS0416-JD, MACS1149-JD1, GN-z10-3, GN-z9-1, GS-z9-1, UVISTA-1212; great names!), calculating how far away these galaxies are from Earth. Then, observing for 70 hours on three Chilean telescopes (the Atacama Large Millimetre Array, the Very Large Telescope, and Gemini South) along with the twin Keck telescopes in Hawaii, they confirmed their age, 550 million years, and from this they could work out when Cosmic Dawn happened [8][9].
It then predicts that the first galaxies could have been formed at a time which will be visible to the new James Webb Space Telescope to be launched in October of this year: something that could unlock many more answers to mysteries about how the early Universe worked, revealing the inner workings of stellar evolution [8][12].
James Webb Space Telescope
Credit: NASA
The First Galaxies
Telescopes have found that the frequency of galaxies rapidly declines after a redshift of z = 6 (around 1 billion years after the Big Bang), coming to 0 at redshift z = 10 (when the Universe was 500 million years old).
However, using simulations of the evolution of the 6 galaxies in the study, the researchers discovered that 69% of the stellar mass of the galaxies observed was formed by redshift z = 10, meaning that the time when those galaxies will have finished forming will be visible to the James Webb Space Telescope (hence the prediction in the previous section).
Only with these observations will we be able to get a clear picture of how the first galaxies actually formed, and know what actually happened in a galaxy far, far away [8][9][10].
Image of galaxy MACS1149-JD1.
Credit: ALMA (ESO/NAOJ/NRAO), NASA/ESA Hubble Space Telescope, W. Zheng (JHU), M. Postman (STScI), the CLASH Team, Hashimoto et al., CC BY 4.0
by George Abraham, ADAS member.
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Click here to watch the video on the discovery of the absorption line at 78MHz
Click here to see the paper on the discovery of the date of the Cosmic Dawn
Click here to see where MACS0416-JD is; here for MACS1149-JD1, here for GN-z10-3, here for GN-z9-1, here for GS-z9-1, and here for UVISTA-1212
References
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"The First Stars and Galaxies". Uppsala Universitet. Archived from the original on 27th June 2021.
"Fingerprinting the very first stars". Astronomy.com. Archived from the original on 27th June 2021.
"The birth of the first stars". YouTube, National Science Foundation. Archived from the original on 27th June 2021.
"Astronomers detect ancient signal from the first stars in the universe". National Science Foundation. Archived from the original on 27th June 2021.
"Experiment to Detect the Global EoR Signatures (EDGES)". LoCo Lab. Archived from the original on 27th June 2021.
"EDGES: Experiment to Detect the Global EoR Signature". MIT Haystack Observatory. Archived from the original on 27th June 2021.
"Astronomers work out when the first stars shone". BBC News. Archived from the original on 27th June 2021.
"Probing cosmic dawn: Ages and star formation histories of candidate z ≥ 9 galaxies". Monthly Notices of the Royal Astronomical Society. Archived from the original on 27th June 2021.
"The First Galaxies". UCL. Archived from the original on 27th June 2021.
"What Kind of Star is the Sun?" Universe Today. Archived from the original on 27th June 2021.