Jump-starting Cosmic Dawn with Enhanced Small-scale Structure

This award funds the research activities of Professor Adrienne Erickcek at the University of North Carolina at Chapel Hill. The first stars formed when gas fell into gravitationally bound clumps of matter and cooled enough to undergo gravitational collapse. The structures that are thought to host the first stars are much smaller than those that contain galaxies and are therefore difficult to observe. Consequently, the onset of star formation during an epoch known as Cosmic Dawn provides a unique and valuable probe of structure formation on sub-galactic scales. The density fluctuations on these scales encode information about the evolution of the early universe and the nature of dark matter (an invisible and poorly understood substance whose gravity holds galaxies together). Research in this area advances the national interest by promoting the progress of science in its quest to understand our cosmic origins and the composition of the universe. Professor Erickcek will determine how the enhanced inhomogeneity on sub-galactic scales that could result from new physics in the early universe would accelerate star formation and lead to an earlier Cosmic Dawn. She will also investigate how the reduction in density fluctuations associated with warm dark matter would instead delay the formation of the first stars. To connect these possibilities to observations, she will predict their imprints on the radio signal emitted by hydrogen gas in the early universe and on the population of small galaxies orbiting the Milky Way. To strengthen our understanding of the first stage of structure formation, Professor Erickcek will use machine learning to better predict the properties of the dark matter clumps that form when regions with higher-than-average dark matter densities collapse. The broader impacts of this award include training the next generation of scientists and increasing general awareness of cosmological research. Professor Erickcek will involve graduate and undergraduate students in this investigation; these students will be trained in computational physics and will gain the research experience needed to advance their careers. She will also continue to teach the "How Do You Weigh a Galaxy?" curriculum, which she designed in partnership with the Morehead Planetarium and Science Center and has already taught to over 275 high school students. Finally, she will share the quest to understand dark matter with general audiences by giving public lectures. More technically, Professor Erickcek will develop a comprehensive model that connects the matter power spectrum to the minimum halo mass required for star formation. She will first determine how enhancements to the small-scale power spectrum, such as those generated by very early dark energy, axion kination, and axion miniclusters, alter the halo mass function and the concentration-mass relation, and she will validate these predictions against cosmological simulations. She will then establish how the streaming velocity of baryons relative to dark matter sets the minimum halo mass at high redshift, and she will use the resulting model to predict how enhanced small-scale power alters the global 21-cm absorption signal and the velocity-induced acoustic oscillations in the 21-cm power spectrum. Professor Erickcek will also use machine learning to improve the peak-to-halo algorithm her group developed to predict halo properties directly from the initial density field; a neural network trained on wavelet-compressed density fields will identify which peaks in that field collapse into halos. This advance will enable her to determine how the central density cusps of the first halos affect their ability to capture baryons, which has direct implications for the signatures of warm dark matter in the 21-cm background. Finally, Professor Erickcek will determine how earlier structure formation affects the galaxy occupation fraction, which will provide the missing link needed to use the abundance of satellite galaxies to set an upper bound on the matter power spectrum on sub-galactic scales. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.