Our Cosmic Roots

Snapshot from the Lumina computer simulation (650 million light-years across), showing massive galaxy clusters, intertwined with filaments that feed them with dark matter (blue) and gas (orange) like the roots of tree trunks. (Image credit: O. Zier, Center for Astrophysics| Harvard & Smithsonian)Today, I responded with answers (A) to the following questions (Q):Q: What do you see as the most consequential uncertainty in our understanding of the first stars and galaxies, and how can observations of the early universe help distinguish between competing accounts of how the first sources of light transformed the cosmos?A: The most exciting question is how early did the first stars form after the Big Bang. Their formation marks the transition of the Universe into complexity from the simple initial conditions that can be summarized on a single sheet of paper (in terms of the matter constituents [dark matter and ordinary matter] and the power-spectrum of primordial density fluctuations on smaller scales than were probed so far). The first stars also mark the beginning of the productions of Carbon, Oxygen, Nitrogen, Phosphorus and Sulphur, elements that are essential for the chemistry of life-as-we-know-it. They represent our cosmic roots and the scientific version of the biblical phrase “Let there be light!”About 15 million years after the Big Bang (or equivalently, a redshift of about z=110), the cosmic background radiation had room temperature. If Earth-like planets had existed at that time, they would have not needed to be close to stars in order for their surface to be warmed to habitable conditions. The popular cosmological model predicts that the first stars form 50 million years later (redshift of z~50). But we need to check if that is indeed the case. I discussed this in 2014 in a paper titled “The Habitable Epoch of the Universe” here, and discussed the broader context in two follow-up papers here and here.There are several observational signatures that can reveal