The first stars and galaxies fundamentally transformed the universe. They formed in the smallest dark matter halos, produced large amounts of ionizing photons, and polluted the universe with the first heavy elements. Near-field cosmology probes this early era through detailed study of nearby relic galaxies that have survived from ancient times. Most of the remaining information is encoded in the elemental abundances of old stars, but decoding these stellar abundances requires connecting physics spanning 37 orders of magnitude in scale: from nuclear and stellar astrophysics to galaxy formation and hierarchical assembly. I will use elements synthesized in the rapid neutron-capture process (r-process) to illustrate how stellar abundances open a window to the first stars and galaxies. My work has shaped our current understanding of the origin of r-process elements, informed multi-messenger observations of neutron star mergers, produced unique constraints on first galaxy formation, and now enables reconstruction of the hierarchical assembly of our Milky Way's stellar halo. I will conclude with a blueprint for how stellar abundances across the Local Group will make near-field cosmology an observational pillar for accessing the high-redshift universe.
Speaker: Alexander Ji, Carnegie Institute
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