In a fascinating new paper in the Monthly Notices of the Royal Astronomical Society, astronomers led by Nicolas Laporte have done what I can only describe as an archeological dig through the light of the most distant known galaxies to try and understand the ages of the stars in this old light. 

Let’s break this down. Light takes time to travel. This means that when we look at galaxies that are thirteen billion light-years away, we are seeing light that was released from stars and hot gas when the universe was only 550 million years old. We know how to roughly judge the age of a galaxy or a star cluster by looking at what distribution of stars is required to create the light we observe. 

To figure out when stars started to form, we can look at light from the oldest galaxies and then figure out how old their oldest stars are. When the universe was just 500 million years old, there were already stars that were 200-300 million years old. This means that there were already stars forming when the universe was 250-350 million years old. This gives a new and improved timeline for our universe. 

At time equals zero, there was a universe. People can argue that it was already there but in a different form, and I’m not here to argue this detail. It is enough to say, at T=0, there was a universe. By the end of the first three minutes, the energy had started to form matter, and that matter (by which I mean protons and electrons and such) underwent simple nuclear reactions and created the initial mix of hydrogen, helium, and traces of lithium and beryllium nuclei that were the ingredients available for the first stars. 

For roughly 400,000 more years, the universe was way too hot and dense for any structures to form, not even neutral atoms, and things expanded with reverberating sound waves shaping the mix of energy and nuclei. Everything was opaque, and not even light could travel any real distance without being absorbed and reemitted. 

Then one day, about 400,000 years after it all started, everything was finally cool enough and diffuse enough that the electrons and the nuclei could finally form atoms, and with this simple change, light that we now see as the Cosmic Microwave Background was released to shine everywhere. And then, for a few million years, that not entirely smooth mix of material continued to expand and cool, but fighting against that expansion, areas with a bit more matter gravitationally pulled on their surroundings, pulling in material in that would collapse to form stars and galaxies. To go from a smooth distribution of material with place-to-place density differences of a fraction of a percent to a distribution having material dense enough to form stars? That slow collapse against the expansion of the universe only took 250-350 million years.

That sounds like a lot of time, but even here on the planet Earth, it really isn’t. 300 million years ago, the oceans were teeming with the ammonite snails so many of us either have as jewelry or collect in Animal Crossing. Mammals, reptiles, amphibians all existed and lived in the shadow of the apex predator, the Dimetrodon. In the time it took life to get from Dimetrodons ruling the world to humans being humans… that is the same amount of time it took our universe to go from zero to stars.

And that is your moment of cosmic “wow”.

More Information

University College London press release

“Probing cosmic dawn: Ages and star formation histories of candidate z ≥ 9 galaxies,” N Laporte et al., 2021 June 24, Monthly Notices of the Royal Astronomical Society