The Cosmological Dark Ages: The Deepest Black
Black everywhere, forever it extends;
it was hard to imagine we’d see light again.
This period of the Universes life is pretty simple. Everything was very dark. The heat and fury of the Big Bang was fading by turning into a thin mist of gas spread out across the Universe. However, this gas was almost completely uniform so there weren’t really any ‘clumps’. There were no planets, no stars, no nebula, just an even soup of atoms. Over time gravity pulled these particles together to create clumps, that turned into galaxies and planets and the Universe we know. But we are still years hundreds of millions of years away from that. For now, it was just dark, deep, and slowly getting colder and darker.
The Cosmological Dark Ages
After the Big Bang, the Universe started cooling and fading to black. Around 370,000 years after the Universe was created, the temperature of the Universe had cooled to about 4,000 degrees celsius. That’s still pretty hot, just slightly colder than the temperature at the center of the earth. However, it is plenty cool enough for atoms and matter to start forming a thin cloud across the entire Universe – instead of the billion-degree-crazy -energy-fields-world that existed right after the Big Bang.
When this soup of atoms started to form, it caused a big flash of warm light. When electrons and protons come together to form hydrogen atoms, they produce photons, a little packet of light. The Universe at this time was full of trillions and trillions of these protons and electrons, all combining together at the same time to form hydrogen atoms, so large numbers of these packets of light were created. The packets of light were all the same color, a very specific wavelength (known as the 21cm wavelength), which we can recreate in labs today. This all combined together to create one large flash of light. This light is still visible today and is the main component of the cosmic microwave background (see our last post)
This is exciting for two reasons. Firstly, it is the earliest moment that the Universe becomes transparent. Previously, because there were large numbers of free electrons, light would have run into them and scattered in all sorts of directions. You wouldn’t have been able to see through it, just in the same way you can’t see through a rock, so there is no sense in which the Universe had ‘a color’. Secondly, the hydrogen line being emitted is actually visible to the naked human eye as a faint orange glow. This means, this moment is the first time you can really say that the Universe “looked” like something – the entire Universe was a uniform solid orange.
However, pretty quickly after this, the Universe faded to black. As all the free electrons and protons were used up, and clicked together to form the hydrogen atoms, there was less and less light and less and less heat. Over the next billion years, the Universe faded from 4,000 degrees celsius to roughly -200 degrees celsius. We have finally got to the familiar black of space that dominates the night sky today.
There are still no ‘structures’ in the Universe at this point. There were no stars, no galaxies, no nebula or pretty pictures of space. Instead, the Universe was an almost completely uniform soup of atoms, and nothing was producing light. Hence the obvious name of the “Cosmological Dark Ages”.
The Habitable Epoch: A random fun sidenote
The most exciting period of this time is what’s referred to as “the habital epoch”. As the temperature of the Universe cooled to that of modern space, there was a period of roughly 6.6 million years where it was between 0 and 20 degrees celsius – that is, the normal temperature on Earth today. This means that an ordinary human would have been able to walk around and survive in this environment.
One particular researcher has actually even argued that there was sufficient density for a single star to emerge and that there is a theoretical possibility of there being life in this period, more than 13 billion years ago (see Loeb).
This is very unlikely. If there were dense pockets in the Universe, they would have been extremely rare. And it took more than 3 billion years for us to evolve on earth, so the idea that complex life popped into existence in this brief period as the earth cooled seems almost impossible. However, what it does demonstrate is the weird and wonderful variety of states of the early Universe. We here at The Universal Story think, at the very least, that it is very amusing and fun to think about.
