Why Was The Universe Dark For So Long?

November 6, 2017

The expanding Universe, full of galaxies and the complex structure we observe today, arose from a smaller, hotter, denser, more uniform state. Once neutral atoms form, however, it takes roughly 550 million years for the ‘dark ages’ to end.

At the moment of the Big Bang, the Universe was full of matter and radiation, but there were no stars. As it expanded and cooled, you formed protons and neutrons in the first fraction of a second, atomic nuclei in the first 3-4 minutes, and neutral atoms after about 380,000 years. After another 50-100 million years, you form the very first stars. But the Universe remains dark, and observers within it are unable to see that starlight, until 550 million years after the Big Bang. Why so long? Iustin Pop wants to know:

One thing I wonder though is why did the dark ages last hundreds of millions of years? I would have expected an order of magnitude smaller, or more.

Forming stars and galaxies is a huge step in the creation of light, but it isn’t enough to end the “dark ages” on its own. Here’s the story.

The early Universe was full of matter and radiation, and was so hot and dense that it prevented protons and neutrons from stably forming for the first fraction-of-a-second. Once they do, however, and the antimatter annihilates away, we wind up with a sea of matter and radiation particles, zipping around close to the speed of light.

Try and imagine the Universe as it was when it was only a few minutes old: before the formation of neutral atoms. Space is full of protons, light nuclei, electrons, neutrinos, and radiation. Three important things happen at this early stage:

The Universe is very uniform in terms of how much matter there is in any location, with the densest regions only a few parts in 100,000 more dense than the least dense regions.

Gravitation works hard to pull matter in, with overdense regions exerting an extra, attractive force to make that happen.

And radiation, mostly in the form of photons, pushes outwards, resisting the gravitating effects of the matter.

As long as we have radiation that’s energetic enough, it prevents neutral atoms from stably forming. It’s only when the expansion of the Universe cools the radiation enough that neutral atoms won’t immediately get reionized.

After this occurs, 380,000 years into the history of the Universe, that radiation (mostly photons) simply free-streams in whatever direction it was traveling last, through the now-neutral matter. 13.8 billion years later, we can view this leftover glow from the Big Bang: the Cosmic Microwave Background. It’s in the “microwave” part of the spectrum today because of the stretching-of-wavelengths due to the Universe’s expansion. But more importantly, there’s a pattern of fluctuations in there of hot-and-cold spots, corresponding to overdense and underdense regions of the Universe.

Once you form neutral atoms, it becomes much easier for gravitational collapse to ensue, since photons interact very easily with free electrons, but much less so with neutral atoms. As the photons cool to lower and lower energies, the matter becomes more important to the Universe, and so gravitational growth begins to occur. It takes roughly 50-100 million years for gravity to pull enough matter together, and for the gas to cool enough to allow collapse, so that the very first stars form. When they do, nuclear fusion ignites, and the first heavy elements in the Universe come into existence.

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