Carina Cheng, a graduate student working on HERA, explains that they are interested in a time called the The Epoch of Reionization (EOR). It is an era in cosmic history which occurred about a million to a billion years after the Big Bang.
“Here’s what happened: After the universe began with an explosion of energy, it expanded and cooled and neutral hydrogen atoms were eventually able to form.” (The universe is mostly made of hydrogen.)
For a long time the universe was dark and neutral, explains Cheng. But there were fluctuations (slight over-densities of matter), left over from the Big Bang. These slowly grew. Eventually the denses regions collapsed to form the first luminous structures. These were the first stars and galaxies.
“These stars and galaxies emit light, and their energies ionized the neutral hydrogen. This transition — from neutral hydrogen to ionized hydrogen — because of the first stars and galaxies, is called the epoch of reionization,” explains Cheng. “It is thought to have begun a few million years after the Big Bang and end about a billion years later. This era is particularly exciting because it is relatively unexplored.”
“As telescopes have become more and more sensitive, we’ve been able to look at galaxies further and further away [further and further back in time[, but we’re only just beginning to be able to see these ‘first’ galaxies, now,” Cheng says.
“Instead, mapping the hydrogen itself — as it alters phase from neutral to ionized — using radio telescopes is a promising and complementary way to probe this era. If we can map out how the gas changes over time, we will learn about the properties of the first stars and galaxies that are responsible for the ionizing. For example, whether the hydrogen gas was ionized quickly or slowly — or over large scales or small scales — will tell us about the characteristics of the first stars and galaxies,” Cheng says.
But there is currently a problem. Telescopes can receive signals from times very far back — and then they hit a “curtain”. It is called the cosmic microwave background radiation (CMB) – at about 380,000 years after the Big Bang.
Prof. David DeBoer, astronomer from the University of California at Berkeley, the university leading the scientific team, explains that HERA has been optimised to remove this “curtain” or signals which are very bright. We will be able to see through it.
How do astronomers distinguish between the very first stars and galaxies and the “noise and signals” in the foreground?
“This is the hardest part of it all, since the foreground material is about a million times brighter, so we need to get rid of it. They do have a different spectrum from what we are trying to find. So the Epoch of Reionisation would sound like the static you heard when you dial between two radio stations, while the foreground would sound like someone whistling. We can filter that out and measure the static,” DeBoer says.