Look at the ingredients on a tube of toothpaste and you will probably read something like “contains sodium fluoride”. Fluoride, as you probably know, is important for healthy teeth. It strengthens enamel, the hard, protective layer around a tooth and so helps prevent cavities.

You may not think too deeply about toothpaste. But like all things on Earth, from the majestic to the mundane, fluoride – and the story of a smile – has a cosmic origin. Now, my colleagues and I have published a paper in Nature Astronomy that sheds some light on it.

Virtually all natural elements were formed long ago in the history of the universe. Hydrogen is the oldest element: it formed very shortly after the big bang, about 14 billion years ago. Within a few minutes of the big bang, the light elements helium, deuterium and lithium were also formed in a process called big bang nucleosynthesis. Since then, nearly every other element has been forged in processes associated with the life and death of stars. But those stars were not always around.

We still do not know exactly when the first stars ignited in the universe, but it probably did not happen for about 100 million years or so after the big bang. Before this, the universe was filled with a fog…

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