To look through space is to look through time, so getting a glimpse of the early days of the universe is a matter of peering deeper and deeper into space. Now, astronomers from MIT and Arizona State University have peered right back to the "Cosmic Dawn" – the time when the first stars were beginning to fire up – by picking up an extremely faint radio signal that marks the earliest evidence of hydrogen, just 180 million years after the Big Bang.
In the early years, the Universe was a very dark, very cold place. Light basically didn't exist, and the hydrogen gas that made up the majority of the interstellar medium was virtually indistinguishable from the cosmic background radiation, left over from the Big Bang.
But over time pockets of matter clumped together, grew larger and eventually exerted such high pressure that nuclear fusion kicked into gear. This resulted in the first stars flickering on across the Universe, and the UV radiation they emitted interacted with the surrounding hydrogen gas. The hydrogen atoms absorbed the background radiation, and it's this change that the new study was able to detect as radio waves.
"This is the first real signal that stars are starting to form, and starting to affect the medium around them," says Alan Rogers, co-author of the new study. "What's happening in this period is that some of the radiation from the very first stars is starting to allow hydrogen to be seen. It's causing hydrogen to start absorbing the background radiation, so you start seeing it in silhouette, at particular radio frequencies."
The team managed to trace the signal back 13.6 billion years – a mere 180 million years after the explosive beginning of the Universe. That makes it the most ancient signal ever detected, by a pretty wide margin. ALMA has observed clouds of star-forming gas from 800 million years after the Big Bang, a Johns Hopkins team managed to peer back to 500 million years after, and Hubble holds the previous record at 400 million years after the Big Bang.
This new detection was made using the Experiment to Detect Global Epoch-of-Reionization Signature (EDGES), a small radio antenna that's about the size of a tabletop. To make sure this incredibly sensitive piece of equipment was able to pick up the faintest of signals from the early universe, the researchers set it up in a remote region of outback Australia, hundreds of kilometers from any human sources of radio interference.
Neutral hydrogen emits radiation at a frequency of 1,420 MHz, but thanks to the ongoing expansion of the Universe and the Doppler Effect, that radiation shifts to a lower frequency. It was estimated that by the time the signal reaches Earth here and now, it would be somewhere around 100 MHz. So, the team initially set the antenna up to receive frequencies between 100 and 200 MHz.
At first, that didn't work. The team went back to the drawing board and eventually realized that the 100-MHz estimate was based on the assumption that the hydrogen gas was hotter than everything else around it. When they adjusted the calculations to account for the gas being much colder, the model returned a frequency range of 50 to 100 MHz.
"As soon as we switched our system to this lower range, we started seeing things that we felt might be a real signature," says Rogers. "We see this dip most strongly at about 78 MHz, and that frequency corresponds to roughly 180 million years after the Big Bang. In terms of a direct detection of a signal from the hydrogen gas itself, this has got to be the earliest."
That course correction raises some interesting questions of its own. The results suggest that the primordial Universe must have been twice as cold as previously thought – a very brisk -454° F (-270° C). The researchers aren't sure what would cause this considerable chill, but suggest that interactions with the eternally-mysterious dark matter could be at play.
The research was published in the journal Nature. The team describes the work in the video below.
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