14 billion years ago, just after the Big Bang, the Universe was very different from today. There were no stars or galaxies, planets, like those in our solar system, had yet to form, and most of the elements required for life – e.g. carbon, nitrogen, oxygen – had not been synthesized. 14 billion years ago, the Universe was a hot mixture of hydrogen and helium. 

However, as the Universe expanded, it began to cool. The cooling reduced the gas pressure, which allowed tiny imperfections in the density field to grow under the influence of gravity. ~100 million years after the Big Bang, gravity became so strong and enough gas had collapsed on itself that the first generation of stars began to form (Cosmic Dawn) and shed light on an otherwise dark Universe, ending the period of time known as the Cosmic Dark Ages. Powered by nuclear fusion, chemical reactions in these first stars converted hydrogen and helium into the first heavy elements (e.g. carbon, nitrogen, oxygen). When all of the hydrogen had been fused, the stars reached the end of their lifetimes and erupted in violent supernova explosions, releasing their elements back into the Universe, and finally creating the conditions necessary for all future generations of stars, galaxies, and planets that we see today. 

While the first generation of stars undoubtedly set the stage for the subsequent evolution of the Universe, they have never been directly observed. Hence the properties of the first stars and how the Universe transitioned out of the Cosmic Dark Ages remains one of the fundamental open questions in modern astrophysics.  

Much of the information about the first stars is encoded in their spectral signatures and the surrounding gas that they illuminate. Interpreting these spectral signatures represents a key theoretical challenge which currently limits our ability to understand Cosmic Dawn. One of the primary goals of the DiRAC dp265 is to self-consistently model Cosmic Dawn and the transition from a Universe free from heavy elements to one that is highly enriched. Our simulations predict the spectral signatures of this transition that can be directly compared with existing and upcoming JWST data to elucidate the physics of the early Universe. We show in Figure 1 a rendering of what our own Milky Way galaxy may have looked like 13.5 billion years ago, highlighting the cosmic web of gas density (purple), ionized hydrogen (white), and emission from gas recently enriched with oxygen (yellow).  

PI: Harley Katz