Collaborative Research: Probing Dark Lights with Big Bang Nucleosynthesis and the Cosmic Microwave Background

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This award funds the research activities of Professor Brian Fields at the University of Illinois and Professor Lloyd Knox at the University of California Davis.The theory of big bang nucleosynthesis (BBN) describes the production of the lightest elements--hydrogen, helium, and lithium--in the first seconds of the big bang. BBN predictions for cosmic light-element abundances are in broad agreement with observations; this concordance is a major success of the hot big bang model. Moreover, BBN provides a measure of the ordinary matter ("baryon") content of the universe, as well as total content in relativistic particles ("radiation") in both visible and invisible form (e.g., neutrinos and any other forms of "dark light"). In the past decade, the cosmic microwave background (CMB) has emerged as a precise and independent measure of cosmic baryons. Comparing BBN and CMB baryon measures thus has provided a new fundamental test of the hot big bang cosmology, and reveal a "lithium problem." This BBN/CMB connection is now poised to open qualitatively new possibilities. Recent and upcoming measurements of the CMB for the first time will simultaneously probe not only cosmic baryons, but also cosmic radiation as well as the helium abundance itself. This will enable powerful new tests of cosmology: for the first time, the predictions of BBN can be tested entirely with very precise CMB data. Agreement would dramatically demonstrate the consistency of the standard cosmology, while any difference probes new physics. Professors Fields and Knox will both evaluate and interpret the BBN-CMB comparison, develop tools to efficiently and accurately compare BBN and CMB data. For the first time, a Markov-chain Monte Carlo approach will be used to evaluate BBN uncertainties; this will allow a direct and consistent means of combining BBN predictions and CMB data and statistically characterizing the results. As part of this analysis, a new open-source BBN code will be produced, tested, and publicly released.It is expected that this work will have substantial broader impacts. A publicly available open source BBN code will be released. Moreover, new and strong links will be forged among the CMB and early-universe particle astrophysics communities, and more widely among the particle astrophysics, cosmology, and astronomy communities. Graduate students, both in formal courses in research mentoring, will be trained to synthesize these approaches and to work in an interdisciplinary way. In addition, Profs. Fields will maintain vigorous, sustained outreach programs which include school visits and public lectures bringing this forefront science to a wider audience.