In the beginning, it was very bright. Suddenly it was dark. The lights went out 380,000 years after the Big Bang, a blink of the eye in cosmic terms. It was the start of what astronomers call the Dark Ages.
What started the Dark Ages was hydrogen. The universe had cooled enough for massive clouds of hydrogen to form. It was uniform, dark and pretty boring. And being dark, it’s hard for astronomers to see what was going on.
While the timing of the start of the Dark Ages is certain, the end is less so. That’s when sparks began to fly as newborn stars began to chase away the darkness.
Astronomers have found a way to peer into the darkness by exploring traces that hydrogen left behind. Though it was dark in the visible spectrum, faint radio signals offer a clue.
It turns out that the hydrogen was not all that dull. Nor were all the hydrogen atoms the same. Some had electrons and protons that spun in the same direction (see diagram) and some the opposite.
The same-spin orientation contains more energy so when the parts of the atom flip, a little radio wave is given off. Those waves still wander through the universe and can be detected today. They carry clues from the Dark Ages.
Where the start of the Dark Ages is well-defined by the formation of hydrogen, the emergence of light is less certain. In general, hydrogen clumped into spheres and formed stars, galaxies and black holes. But there are still a number details to work out.
Virtually all of that old hydrogen from the Dark Ages is gone now, blown away by the explosive structures born from early hydrogen. The universe has been slowly swept clear of hydrogen.
Radio astronomers can detect amount and age of hydrogen from those ancient signals. When the hydrogen signals disappear, that defines the end of the Dark Ages.
The age of the signals can be detected by their wavelength. Wavelength can be thought of as the distance between coils of a spring. Young waves start at a wavelength of 21 cm and as they age, they stretch out. They stretch out because the universe is expanding. Imagine a spring being stretched out and the distance between the coils increasing.
Our universe is 10 times bigger than the era of the Dark Ages 13 billion years ago so the hydrogen signals from that time are now 210 cm in length. The wavelength reveals the age of the hydrogen and the strength reveals how much hydrogen existed.
The advantage of radio astronomy is that the sky is relatively transparent at those frequencies. The disadvantage is that large antennas are required because of the relatively low frequency and weak signal strength.
As reported in Discover science magazine, new ways of detecting these faint messages from the era of hydrogen in the Dark Ages will reveal the beginning of the reign of starlight.