First light

After the Big Bang the universe was dark for hundreds of million years, until the formation of the earliest stars and galaxies. And one of those galaxies was GN-z11, the oldest we have ever observed.

NASA, ESA, P. Oesch (Yale University), G. Brammer (STScI), P. van Dokkum (Yale University), and G. Illingworth (University of California, Santa Cruz), Public domain, via Wikimedia Commons

The Big Bang was very hot and very bright. The light from those earliest times is still kicking around in the form of cosmic background radiation. But once things cooled off a bit, nothing produced new light for a long time.

There is a period in the cosmic timeline known as the Dark Ages. From around 370,000 years after the Big Bang until the formation of the first stars and galaxies, almost nothing was producing new light. (I say “almost” nothing because there were a few errant photons produced by neutral hydrogen’s spin-flips, for example.) But, otherwise, the universe was dark.

When the first stars showed up, they were the first things to produce light in a long long time – perhaps hundreds of millions of years. We don’t have a good picture of those stars because a lot has happened in the intervening aeons. However, with a sufficiently powerful telescope we can spot some very old ones.

Currently, the record-holder for oldest observed galaxy is GN-z11. It was around and shining brightly just 400 million years after the Big Bang. It’s also a long way away from us – it’s the most distant galaxy we’ve ever seen. These two records (age and distance) are of course related because of the speed of light and the expansion of the universe. The further away the galaxy observed, the older it must be.

GN-z11 is now 32 billion light years away from us. How is this possible, you may ask, given that the universe has only been around for less than half of that time? Well, the short answer is that when the light we’re seeing from GN-z11 was emitted, it was a lot closer to us. Since then, the universe has been expanding and that galaxy has kept moving. I’ll get into the complexities of measuring distances in the universe another time, because it is more complicated than you might think.

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