The Dawn of Hydrogen Cosmology: First Stars, Dark Matter, and the Thermal History of the Early Universe

Arizona State University, Scottsdale AZ

Investigators

Abstract

The goal of cosmology is to understand the history of the Universe from the Big Bang through the present day. Initially, the universe was filled with a gas of hydrogen and helium atoms. Eventually gravity caused stars to form from this gas. Astronomers have seen distant galaxies--collections of stars--as they looked when the universe was less than 500 million years old. But the stars in these galaxies are probably not the first stars that formed in the universe. It is important to determine the properties of the earliest stars in the universe. These stars created new elements like carbon, nitrogen, and oxygen for the first time. This changed how future stars would form and evolve. Detecting light from early stars is very challenging. This research will instead analyze radio observations. Radio signals from the early universe can record the effects of the first stars indirectly. By studying these signals, this research will help astronomers understand the role of early stars in shaping the modern universe. Early-career researchers at the undergraduate, graduate, and postdoctoral levels will be trained as part of this project. Results from the research will be shared with children and adults at local and regional science events. The formation of the first stars, galaxies, and black holes during Cosmic Dawn is the next frontier in observational cosmology. During this epoch, the intergalactic medium (IGM) is dominated by neutral hydrogen gas, accessible to observations through its redshifted 21 cm line. The thermal and ionization history of the early IGM encodes unique information about the properties of the first luminous objects. Once the first stars form at z ~ 30, they produce a background of Lyman-alpha photons that couple the neutral hydrogen spin temperature to the physical gas temperature in the IGM. This causes the 21 cm line to become visible in absorption against the warmer cosmic microwave background (CMB). Later, X-rays traveling long distances deposit their energy as heat and raise the IGM temperature. This heating drives the average 21 cm signal to switch into emission above the CMB, until the neutral gas is eventually ionized during the epoch of reionization, leaving no detectable signal. The evolution of these absorption and emission features in the early IGM will be imprinted in the all-sky radio spectrum since redshift maps to frequency for the 21 cm line. This research will use observations from the EDGES dual-band all-sky radio spectrometer. EDGES operates in the frequency range between 50 and 200 MHz, spanning redshifts 30 > z > 6. The goal of the research is to constrain plausible astrophysical model parameters for star formation, heating, and ionization in the early Universe. Theoretical models from 21CMFAST and other simulation codes will be fit to the observations. Collaborators will incorporate additional physics into their codes to investigate any contributions from non-standard physics to the data. This project will provide support for one postdoctoral research and the final year of Ph.D. thesis research for one graduate student. It will provide opportunities for undergraduate students to engage in astrophysical research, with special efforts to make sure that all interested students at the host institution are encouraged to pursue these opportunities, including students from underrepresented populations and non-traditional backgrounds. 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.