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The EoR, in the middle of this image between the Big Bang on the right, and present day on the left, was characterised by bubbles of ionised gas forming and gradually joining over time. CREDIT: NASA/STScI
Let’s talk about... the dawn of the Universe BY CASSANDRA CAVALLARO (SKAO)
If we cast our minds back to the early 1990s, astronomers involved in the early days of the SKA project were beginning to think about an ambitious goal: to look back further into the history of the Universe than ever before, to the time when the very first stars and galaxies began to light up the darkness. This was the Cosmic Dawn. That goal would require the detection of signals so faint that many considered it impossible, and indeed it would require major advances in technology to achieve. “Detection of the faint signal of hydrogen from the infant Universe will be transformational to our understanding of the foundations on which the entire currently visible Universe is built.” It doesn’t get much more emphatic than that statement from Prof. Leon Koopmans of Kapteyn Astronomical Institute in the Netherlands, an expert
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C O N TA C T
in this field. In the period following the Big Bang, 13.8 billion years ago, the Universe entered an era that we refer to as the Dark Ages. With no starlight, and full of neutral hydrogen, it was an extremely dark, bleak place. That began to change with the Cosmic Dawn, as gravity caused small pockets of gas to collapse and form the first stars and galaxies. Let’s quickly remind ourselves of some high school physics. Hydrogen is the most abundant element in the Universe. In its neutral state it comprises one electron and one proton; they are in
balance, so there is no electrical charge. If that balance is upset, the atom becomes charged, or ionised. During the Cosmic Dawn the ionisation process began. The first stars and stellar remnants emitted ultraviolet, Lyman-alpha and X-ray radiation, which together caused the neutral hydrogen in and between galaxies to warm up and start absorbing background radiation from the Big Bang, known as the Cosmic Microwave Background. This process created bubbles of ionised gas which gradually grew until they joined up. This period is called the Epoch of Reionisation (EoR). Astronomers want to directly observe the full imprint from these processes. Using highly sensitive radio telescopes, they aim to look back in time and detect the signal from neutral hydrogen in the intergalactic medium during the early Universe and map its distribution over hundreds of millions of years. “Light travels at a constant velocity, and
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