It all started very hot. Pure radiation, at first: drawn from some primordial pulse now lost in the overstretched obscurity of space-time, hidden behind a wall of fire that burned in every femtometer of the primordial universe. There was no source of light, nor a ignition point to propagate from; It was everything, everywhere, and that was growing everywhere. The universe was inflated, and space was fleeing itself, spreading light over the face of creation until droplets formed: matter was born hot and screaming.
The first particles rip the scorching plasma in waves, and sound vibrations scatter and collide. The universe was a sea of ions – unpaired protons and electrons, with a sprinkling of helium and other light nuclei – born hot, nuclear from the mass furnace. The universe shot out fire that slowly turned into atomic ash. Positively charged protons and negative electrons spiraled together to form neutral atoms – the first in the universe – mostly hydrogen, and no longer plasma, not ions, but gas. Gas cools. I became calm. The universe rested for a hundred million years.
We call the next stage the Dark Ages. (We astronomers are literal beings.) The light from the Big Bang was fading, stretching toward the radio spectrum. The first stars have not yet lit. Over eons, the universe filled with a dark hydrogen fog, pervasive as the universe expanded, and the residual heat waned.
The heaviness of the fog could be felt. As the raging plasma waves cooled, they left behind wave crests, imprinted in the gas as tiny imperfections. Few atoms here; A little thinner set there. The atom clump is small, but give it time and it will find its neighbors.
Form thick mist clouds. The densest knot clouds formed. The knots became heavier, pulling the gas into orbit around them, spinning and colliding with this force that drove the gas to ignite. The same gas that had been dormant for countless ages, in the midst of the narrowest cloud, was transformed again into a heat-burning nuclear furnace. The first star was born: the cosmic dawn.
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Amidst the dense mist of the universe, stars sprang to life: tiny dots of bright light, shining in the dark. Clustered together where the biggest clumps gathered: the age of galaxies began.
Each galaxy was born in a shallow pool of its light, shrouded in the thick, dark clouds that formed it, like a city light smothered in mist. The vagaries of atomic physics make hydrogen an effective astral shield: give a hydrogen atom a photon of visible light and it will consume it all, pushing its electron into a higher energy state, only to later eject the light in a random direction.
But this shield is something that limits itself. The light emitted by the first galaxies carried even stronger radiation: ultraviolet light, so intense that an unsuspecting hydrogen atom could make its electron unexcited, but fly away completely. Bubbles of hydrogen-scarred galaxy light began to grow, punching holes in the cool, calm bulk of intergalactic gas. Over a billion years, bubbles filled the universe and ripped nearly every hydrogen atom in two, leaving protons and electrons roaming the universe separately again—not fire this time, but a pervasive, dispersing mist of ionized gas again.
We are still learning the story of how the first stars divided the universe. We call it “reionization,” and our knowledge of it comes primarily from its end. The ionized universe is the transparent universe of visible light. When we look across the universe, at galaxies whose ancient light comes to us from farther back in time, we can begin to see the quenching of that light, as if we were watching the cosmic movie in reverse – thick neutral gas spreads out and covers the galaxies until they completely disappear from view.
We’re fortunate, though, that we have ways to cut through the fog. Visible light may be consumed, but the longer wavelengths of radiation—infrared, microwave, and radio—can travel unimpeded, and starlight is full-spectrum. Using new telescopes such as the James Webb Space Telescope, we can study the epoch of reionization by capturing the infrared portion of the galaxies’ light. With new radio telescopes, we may do a better job, tuning into the lower frequencies of radio light emitted or absorbed by the neutral hydrogen itself.
To understand reionization is to know the first stars and galaxies. Maybe one day we’ll watch them sculpt their birth caps, little sparks stretching across a sea of darkness to change it, essentially, forever.
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