People are like stained-glass windows. They sparkle and shine when the sun is out, but when the darkness sets in their true beauty is revealed only if there is light from within. -Elisabeth Kübler-Ross
Welcome back to The Greatest Story Ever Told, where we’re covering the natural history of the Universe from before the Big Bang to the present day. You can catch up on the first eight parts here, going forward from Inflation in part 1 to parts 2, 3, 4, 5, 6, 7, and reaching the formation of the first neutral atoms in part 8. But during all of this time, gravity has been working its magic.
In other words, the Universe started off almost perfectly smooth, with roughly the same matter and energy densities at all points in space. But it wasn’t perfectly smooth, there were teeny tiny differences from place to place, just like if you went out to the middle of the ocean and measured how level is the water’s surface.
Sure, you might glance at it and say the ocean is flat. But if you looked closely, you’d realize there were tiny crests, where the water’s a little higher than average, and tiny troughs, where the water level’s a little lower.
Thanks to inflation and quantum physics, we can state that the Universe started out almost exactly the same way. Here’s how. The Universe has the same average density everywhere, but every once in a while, you get a quantum fluctuation.
Well, during Inflation, space is expanding exponentially fast! This is so fast that you make these quantum fluctuations, and by time these particles are ready to find each other and disappear again, they’re already stretched out across the Universe! And this is, roughly, how tiny quantum fluctuations create regions with slight overdensities and slight underdensities in our Universe!
And gravity works on these over time, attracting more and more matter towards the overdense regions and attracting less and less matter towards the underdense ones. By time you get to the cosmic microwave background and the formation of neutral atoms, some spots appear significantly cooler (shown in blue) due to increased density and others appear significantly hotter (shown in red) due to a deficiency in density.
And then the fun begins. You see, gravitation is a runaway process. If I take a completely uniform Universe and make one region just a tiny bit denser than everywhere else, then everything in the Universe gets preferentially attracted to that region!
So what does this mean for space? It means that the regions where the overdensities live are going to become the places where stars, galaxies, and clusters of galaxies form! What do we owe this to? Gravitational attraction, and eventually, gravitational collapse.
From inflation to the formation of neutral atoms took about 380,000 years. It takes about another 50 million years until you get enough gravitational collapse to make a dense enough gas cloud that the atoms at the center start fusing together. What happens then?
Congratulations! We’ve just covered how the first star in the Universe formed! And this happens all over the Universe over the next few million years, as the Universe goes from a dark, cool place to one suddenly illuminated by the glow of billions of stars. Want to see a video (courtesy of New Scientist)?