A Universal Chronicle E2: The Dark Ages

Why the Dark Age?

There is certainly plenty of light to go around but there is no new light being made. We won't see any new light until the stars start to form. Hence the dark ages.Where we left off, about 300,000 years after the big bang the universe is a balmy 3,500 degrees celsius. More or less the entire universe looks like the area just above the surface of the sun. At this point something subtle begins to happen. During the early stages of the universe (inflation) tiny quantum fluctuations slowly grew until they were very large.The regions of slight hot/cold we discussed in the previous episode which make up the cosmic microwave background now form regions of slightly high/low density. High density areas have slightly more gravity and pull more material towards them while the low density regions lose material.This continued on for hundreds of millions of years. Slowly material piled together into the first galaxies and compressed further to become the first stars.At this point the universe would be 200-400 million years old and about -200 degrees celsius. We’ve come a long way from the over a thousand trillion degrees we had early in the last episode!

The First Stars:

You may recall that at this point the universe is filled with hydrogen, helium, and a bit of lithium. This is the material which would make the first stars.This has two major effects. First, it means the gas would cool slowly. For gas to form stars it needs to cool down and collapse to become very compact. Second, the stars themselves would be very different. It’s quite possible that the first generation of stars with only hydrogen and helium to power their fusion would have grown to enormous sizes of around 1,000 times the mass of our sun.The first stars would have been incredible candles in the darkness. They would shine incredibly brightly, each one thousands of times brighter than our sun.So bright in fact that they would be able to split off electrons from hydrogen molecules. The first population of stars would have been created in the first galaxies.

The First Galaxies:

The first galaxies would be quite different from the Milky Way Galaxy that we call home today. These galaxies would have been much smaller.A galaxy is basically a region with above average dark matter and gas and some stars.These galaxies would immediately begin merging with each other. The universe would be noticeably smaller and more cramped than it is today.This means that galaxies would be constantly colliding with each other. Quickly, we end up with big galaxies that swallow up smaller ones in their path, astronomers call the process “Hierarchical Merging”.

The Universe Today:

So about 3 billion years after the big bang, star formation had really picked up. From this point, and for a couple billion more years, the universe would be in its most active star formation period. During the next 10 billion years, the galaxies would settle to what we know today. Some ancient galaxies that look like giant balls of stars whizzing around in all directions, they dont really make new stars for themselves, they just absorb other galaxies. The latest generation of stars also has the kinds of material around them to form rocky planets.At this point we have essentially made it up to today as far as astronomy is concerned.

Links to Science Outreach Material:

McDonald InstituteRoyal Astronomical SocietyAstronomy on Tap

Special thanks to Colin Vendromin for the music, also thanks to Zac Kenny for the logo!