Big Bang Nucleosynthesis: Physical Science

• Physical science explores the fundamental laws and principles governing the universe's physical aspects
• It encompasses fields like physics, chemistry, astronomy, and earth science
• Physical science seeks to understand the nature of matter, energy, space, and time

Big Bang Nucleosynthesis

• Big Bang Nucleosynthesis (BBN) is the theory explaining the formation of light elements in the early universe
• BBN occurred within the first few minutes after the Big Bang
• The universe was extremely hot and dense during this period
• As the universe expanded and cooled, conditions became favorable for forming light atomic nuclei
• BBN primarily produced hydrogen (¹H), helium (⁴He), and trace amounts of deuterium (²H), tritium (³H), helium-3 (³He), and lithium (⁷Li)

Timeline of BBN

• t=0 seconds: Big Bang occurs; universe is an extremely hot, dense plasma of fundamental particles
• t=1 second: Temperature drops to around 10 billion Kelvin; neutrinos decouple; ratio of neutrons to protons is established through weak interactions
• t=1-3 minutes: Temperature drops to around 1 billion Kelvin; nuclear reactions begin; deuterium forms, followed by helium-4
• t=3-20 minutes: Nucleosynthesis continues until the temperature and density become too low for further fusion
• t>20 minutes: Nucleosynthesis effectively stops; the elemental abundances are fixed

Key Nuclear Reactions

• The primary nuclear reactions in BBN involve isotopes of hydrogen and helium
• Deuterium formation: ¹H + n → ²H + γ (neutron capture by a proton forms deuterium)
• Deuterium is fragile, easily broken apart by high-energy photons
• Helium-3 formation: ²H + ¹H → ³He + γ (deuterium and hydrogen fuse to form helium-3)
• Helium-4 formation: ³He + n → ⁴He + γ, ²H + ²H → ⁴He + γ, ³H + ¹H → ⁴He + γ (various pathways to form helium-4)
• Trace amounts of lithium-7 are produced through reactions like ⁴He + ³H → ⁷Li + γ

Neutron-Proton Ratio

• The neutron-to-proton ratio (n/p) is critical for determining the final elemental abundances
• Initially, at very high temperatures, neutrons and protons were in thermal equilibrium through weak interactions
• n + νe ↔ p + e⁻ (neutron interacts with electron neutrino to produce a proton and electron)
• p + νe ↔ n + e⁺ (proton interacts with electron antineutrino to produce a neutron and positron)
• As the universe cooled, these interactions became less frequent, and the n/p ratio began to freeze out
• The neutron is slightly heavier than the proton, so the equilibrium shifted towards fewer neutrons at lower temperatures
• Free neutron decay (n → p + e⁻ + νe) further reduced the number of neutrons
• By the time nucleosynthesis began, the n/p ratio was approximately 1/7

Abundance of Helium-4

• Helium-4 is the second most abundant element in the universe after hydrogen
• BBN predicts a helium-4 mass fraction of approximately 25%
• Almost all available neutrons at the time of nucleosynthesis end up bound in helium-4 nuclei due to its high binding energy
• The abundance of helium-4 is relatively insensitive to the exact conditions of the early universe, making it a robust prediction of BBN

Deuterium as a Baryometer

• Deuterium is a sensitive probe of the baryon density of the universe
• The final abundance of deuterium is inversely proportional to the baryon density
• A higher baryon density leads to more efficient deuterium burning into heavier elements, resulting in a lower deuterium abundance
• Measurements of deuterium abundance in pristine, high-redshift systems provide a precise estimate of the baryon density

Lithium Problem

• The predicted abundance of lithium-7 from BBN is higher than observed in old, metal-poor stars in the galactic halo
• This discrepancy is known as the "lithium problem."
• Possible solutions include:
  • Astrophysical solutions: Destruction of lithium in stars through mixing or nuclear reactions
  • Nuclear physics solutions: Uncertainties in the nuclear reaction rates affecting lithium production
  • New physics solutions: Exotic particles or modified cosmological models altering BBN predictions

Importance of BBN

• BBN is a cornerstone of the Big Bang theory
• It provides strong evidence for the hot, dense early universe
• The predictions of BBN are consistent with observed elemental abundances
• BBN provides constraints on fundamental physics, such as the number of neutrino species and the variation of fundamental constants
• BBN complements other cosmological probes, such as the cosmic microwave background (CMB), in establishing the standard cosmological model

BBN and the Cosmic Microwave Background (CMB)

• The CMB provides an independent measurement of the baryon density
• The baryon density inferred from the CMB is consistent with the value inferred from deuterium abundance measurements
• This concordance between BBN and CMB strengthens the Big Bang theory

Limitations of BBN

• BBN only explains the formation of light elements up to lithium
• It cannot account for the observed abundances of heavier elements such as carbon, oxygen, and iron
• These heavier elements are produced in stars through stellar nucleosynthesis and in explosive events like supernovae
• BBN assumes a homogeneous and isotropic universe
• Inhomogeneities in the early universe could affect the predictions of BBN, but such effects are generally small

Explore Big Bang Nucleosynthesis, the theory of light element formation in the early universe. Learn about BBN's timeline and the creation of elements like hydrogen and helium. Understand the conditions during the first few minutes after the Big Bang.