The hot Big Bang was an energetic, brilliantly luminous event. Today’s Universe is alight with stars. But in between, the dark ages ruled.
Today, in all directions, no matter where we look, there are luminous sources of energy to behold. Stars, galaxies, nebulae, and even energy-emitting black holes populate the Universe wherever matter has clumped and clustered together sufficiently. Even though there are great cosmic voids that span up to around a billion light-years in diameter, they’re merely holes in the cosmic “Swiss cheese” of structure. From all directions, the light still gets in, and illuminates even the darkest corners of the Universe.
But that’s what things are like now, 13.8 billion years after the Big Bang. As we look deeper and deeper into the Universe, we see that the story gradually begins to change. Past a certain threshold, galaxies appear redder and fainter than expected: as though something were in the way, blocking that light. That effect gets more severe with distance, where only the brightest of galaxies can be perceived at all. At last, we run out of light to see, suggesting that there were “dark ages” beyond a certain point. What were those dark ages like? That’s what Predrag Branković wants to know, asking:
“How was the dark age of the universe really dark?”
The darkness was real, but there are actually three things at play, all together, that caused them. Here’s how to understand the dark ages, and why they finally came to an end.
Initial light fades away
Back at the start of the Universe as we know it — during the earliest stages of the hot Big Bang — everything was brilliantly hot and dense. Not only was the Universe filled with quanta of light, photons, of terrifyingly high energies, but all the other particles (and antiparticles) that the laws of physics allowed to come into existence. Given that:
- energies were tremendous, possibly as high as trillions of times what the Large Hadron Collider at…