Imagine that you are an intergalactic space traveller, voyaging through the ancient cosmos. You initially seem to be flying through a star, surrounded in all directions by a uniform glowing plasma. Gradually, the glow fades as the ionized gas around you cools and recombines into neutral atoms. All is dark. All is cold.
Eventually, after 100 million years, a few stars begin to appear. At first, these are little more than distant points of light that you can only just pick out with a telescope. Stars then begin to appear more frequently, and soon you can see entire galaxies. Suddenly, the whole region around you has reionized and you are once again surrounded by a hot plasma, albeit tenuous and faint.
In recent years, the first and last parts of this story of the earliest billion years have been revealed to us in glorious detail. The first chapter has been uncovered using the fading remnants of the early glow. So too has the last chapter, by observing gas and galaxies in the recent universe. But the heart of the book - the entire story of how the glowing, uniform gas transformed into a universe of stars and planets - has been missing.
In the February issue of Physics World, Rennan Barkana discusses the recent advances that have been made to aid our understanding of the early universe.