Precision frontiers of big bang nucleosynthesis

Yeh, Tsung-Han

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https://hdl.handle.net/2142/116055 Copy

Description

Title

Precision frontiers of big bang nucleosynthesis

Author(s)

Yeh, Tsung-Han

Issue Date

2022-07-08

Director of Research (if dissertation) or Advisor (if thesis)

Fields, Brian D

Doctoral Committee Chair(s)

Shelton, Jessie

Committee Member(s)

• Peng, Jen-Chieh
• Filippini, Jeffrey P

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Big Bang Nucleosynthesis
• Cosmology
• Early Universe
• Effective Cosmological Neutrinos
• Particle Physics-Cosmology Connection
• Primordial Light Element Abundance
• Primordial Nucleosynthesis

Abstract

We present in this dissertation the current precision frontiers of Big Bang nucleosynthesis (BBN) in response to the latest developments in key nuclear reaction experiments, astronomical observations for primordial abundances and the cosmic microwave background (CMB) measurements. BBN delineates the very first nucleosynthesis in the universe, explaining the cosmic origin of the lightest elements, such as D, 4He, and 7Li. Based on the comparison of light element abundance predictions and observations, BBN probes the physics of the early universe and provides constraints on important cosmological parameters, in particular, the baryon-to-photon ratio η and the effective neutrino flavors Nν. These cosmological parameters can also be determined independently from CMB measurements. We have performed likelihood analyses of η and Nν using the latest Planck CMB data and our state-of-the-art BBN calculation that convolves with precision D and 4He observations. Our results display excellent concordance between separate BBN and CMB determinations of these parameters, representing a remarkable success of the hot big bang cosmology. This agreement allows us to combine BBN and CMB constraints, yielding a new limit Nν = 2.884 ± 0.140 (i.e., Nν < 3.164 at 2σ) on new physics. In addition, the improved precision of the independent BBN and CMB constraints now provides a new window to new physics: we can investigate potential changes in η and/or Nν between these two cosmic epochs. We find that the present data still agrees with the Standard Model (i.e., no changes), but some modest departures are allowed. Furthermore, we forecast the future BBN+CMB-S4 joint precision on Nν, and discuss its impact on probing the early universe physics with better resolution. To compete with the projected CMB-S4 Nν precision, we point out that future 4He measurements with a factor of ∼3 reduction in the current error are needed on the BBN side. Compared with BBN, Planck CMB data now provides better precision on η while comparable uncertainty on Nν. Accordingly, we use CMB η (and Nν) as necessary inputs to make accurate primordial abundance predictions against precision observations. The predicted D/H uncertainty from BBN+CMB is dominated by the thermonuclear rate errors from deuterium destruction reactions d(p,γ)3He, d(d,n)3He, and d(d,p)t. We have included a new set of precision d(p,γ)3He cross section data by the LUNA Collaboration in our rate evaluation, and show that the uncertainty of this rate has been reduced by a factor of 2. We also have re-evaluated the existing d(d,n)3He and d(d,p)t data using the same evaluation procedure, mildly improving their uncertainties. However, the predicted D/H error (∼3%) is still large relative to the observed counterpart (∼1%). We point out that new precision measurements of d + d rates at BBN energies are critical to sharpen D/H prediction. Moreover, we report a new 7Be(n,p)7Li rate using new neutron capture measurements from the n_TOF Collaboration, but we find this new rate has marginal impact on resolving the famous discrepancy (> 4σ) between the predicted and observed 7Li/H. Because this “Lithium Problem” remains outstanding, we do not utilize 7Li/H as a BBN probe throughout this dissertation study.

Graduation Semester

2022-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Tsung-Han Yeh

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