Big Bang Nucleosynthesis

Big Bang Nucleosynthesis

Big Bang Nucleosynthesis (BBN) refers to the brief period—roughly between 1 second and 20 minutes after the Big Bang—when the universe was hot and dense enough for nuclear fusion to synthesize the first stable nuclei of hydrogen, helium, and a small amount of lithium. By the end of this epoch, the basic chemical composition of the early universe was set until stars began forging heavier elements billions of years later.

1. Why BBN Matters

  1. Testing the Big Bang Model
    The predicted abundances of light elements (hydrogen, helium, deuterium, and lithium) can be compared to observations in ancient, nearly pristine gas clouds. A strong match provides a direct test of our cosmological models.
  2. Establishing Baryon Density
    Measurements of primordial deuterium help us determine how many baryons (i.e., protons and neutrons) there are in the universe, a key input for broader cosmological theories.
  3. Early-Universe Physics
    BBN probes extreme temperatures and densities, offering a glimpse of particle physics beyond what can be replicated in modern laboratories.

2. Setting the Stage: The Universe Before Nucleosynthesis

  • End of Inflation
    After cosmic inflation ended, the universe was a hot, dense plasma of particles (photons, quarks, neutrinos, electrons, etc.).
  • Cooling Down
    As space expanded, the temperature dropped below about 1012 K (100 MeV of energy), allowing quarks to combine into protons and neutrons.
  • Neutron-Proton Ratio
    Free neutrons and protons were interconverting via weak interactions. As the universe cooled below a certain energy threshold, these interactions froze out, setting a neutron-to-proton (n/p) ratio around 1 neutron for every 6–7 protons. This ratio strongly influenced how much helium could ultimately form.

3. The Timeline of Big Bang Nucleosynthesis

  1. Around 1 Second to 1 Minute
    Temperatures remained extremely high (1010 K to 109 K). Neutrinos decoupled from the plasma, and the n/p ratio became nearly fixed.
  2. From 1 Minute Onward
    As the universe cooled to around 109 K (roughly 0.1 MeV), protons and neutrons started fusing to form deuterium (a nucleus with one proton and one neutron). However, photons at these energies could still break deuterium apart. Only when the universe cooled a bit more did deuterium become stable enough for further fusion processes.
  3. Peak Nucleosynthesis (About 3–20 Minutes)
    • Deuterium Fusion
      Once stable deuterium nuclei formed, they quickly fused into helium-3 and tritium (hydrogen-3).
    • Helium-4 Formation
      Helium-3 and tritium could combine with other protons or neutrons (or each other) to form helium-4 (two protons + two neutrons).
    • Trace Lithium
      Small amounts of lithium-7 were also created through various fusion and decay processes.
  4. End of BBN
    After about 20 minutes, the universe’s density and temperature dropped too low for sustained fusion. The abundances of light elements were effectively “locked in” at this point.

4. The Key Nuclear Reactions

Let’s represent isotopes in simpler forms:

  • H (hydrogen-1): 1 proton
  • D (deuterium, or hydrogen-2): 1 proton + 1 neutron
  • T (tritium, or hydrogen-3): 1 proton + 2 neutrons
  • He-3 (helium-3): 2 protons + 1 neutron
  • He-4 (helium-4): 2 protons + 2 neutrons
  • Li-7 (lithium-7): 3 protons + 4 neutrons

4.1. Deuterium (D) Formation

  • Proton (p) + Neutron (n) → Deuterium (D) + Photon (γ)
    This step was initially hindered by high-energy photons that broke deuterium apart. Only after further cooling could deuterium survive.

4.2. Building Helium

  • D + D → He-3 + n (or T + p)
  • He-3 + n → He-4 (via intermediate reactions)
  • T + p → He-4

As soon as deuterium became stable, it rapidly fused into helium-4, which is the most stable light nucleus (besides hydrogen) and contains two protons and two neutrons.

4.3. Lithium Synthesis

Some helium-4 nuclei combined with tritium or helium-3 to form beryllium-7 (Be-7), which then decayed into lithium-7 (Li-7). The overall amount of Li-7 produced was very small compared to hydrogen and helium.

5. Final Abundances

By the close of BBN, the universe’s light-element makeup was roughly:

  • Hydrogen-1: About 75% (by mass)
  • Helium-4: About 25% (by mass)
  • Deuterium: A few parts in 105 relative to hydrogen
  • Helium-3: Even less
  • Lithium-7: Around a few parts in 109 or 1010 relative to hydrogen

These proportions have been slightly modified over billions of years by stellar processes, but in regions with minimal stellar nucleosynthesis (e.g., certain ancient gas clouds), the primordial ratios are largely preserved.

6. Observational Evidence

  1. Helium-4 Measurements
    Astronomers look at helium abundances in metal-poor dwarf galaxies and find values close to 24–25% by mass, matching BBN predictions.
  2. Deuterium as a “Baryometer”
    Deuterium abundance is highly sensitive to the number of protons and neutrons. Observations of deuterium in distant gas clouds (using quasar absorption lines) help determine the baryon density of the universe. These measurements agree closely with cosmic microwave background (CMB) data, reinforcing the standard cosmological model.
  3. The Lithium Problem
    Although helium and deuterium measurements fit predictions well, there is a discrepancy for lithium-7. The observed amounts in old stars are lower than predicted, known as the “lithium problem.” Possible explanations include lithium destruction in stars, inaccuracies in nuclear reaction rates, or undiscovered physics.

7. Why BBN Is Central to Cosmology

  • Cross-Checking the Big Bang
    BBN provides a clear test of the standard model since it predicts specific abundances of light elements. Observations match these predictions for helium and deuterium extremely well.
  • Consistency with CMB
    The baryon density inferred from BBN matches that from detailed studies of the CMB’s temperature fluctuations, offering a compelling, independent confirmation of the Big Bang framework.
  • Constraints on New Physics
    BBN’s sensitivity to particle physics at high temperatures means it can reveal or rule out exotic particles, extra neutrino species, or subtle shifts in fundamental constants that would have altered primordial element production.

8. The Bigger Picture: Cosmic Evolution

After the BBN epoch ended, the universe continued to expand and cool:

  • Formation of Neutral Atoms
    Around 380,000 years later, electrons and nuclei combined, giving rise to the cosmic microwave background.
  • Star and Galaxy Formation
    Over hundreds of millions of years, regions with slightly higher density collapsed under gravity to form stars and galaxies. In stellar cores, heavier elements (carbon, oxygen, iron, etc.) would be forged, further enriching the universe.

Thus, Big Bang Nucleosynthesis set the initial chemical blueprint. All subsequent cosmic evolution—from the first stars to life on Earth—built upon those primordial abundances.

Big Bang Nucleosynthesis is a cornerstone of cosmology, connecting the earliest high-energy phases of the universe to the chemical composition we observe in ancient gas clouds and modern stellar populations. Its success in predicting the relative abundances of hydrogen, helium, deuterium, and trace lithium provides one of the most compelling lines of evidence for the Big Bang theory. While some puzzles remain—like the precise level of primordial lithium—the broad agreement between BBN calculations and observations underscores our deep understanding of how the cosmos took shape in its very first minutes.

Sources:

Steigman, G. (2007). “Primordial Nucleosynthesis in the Precision Cosmology Era.” Annual Review of Nuclear and Particle Science, 57, 463–491.
– A comprehensive review of BBN, discussing both the theoretical framework and how observational data (e.g., light-element abundances) test our cosmological models.

Olive, K. A., Steigman, G., & Walker, T. P. (2000). “Primordial Nucleosynthesis: Theory and Observations.” Physics Reports, 333–334, 389–407.
– This paper reviews the predictions of light-element abundances and compares them with observations, providing insights into baryon density and early-universe physics.

Cyburt, R. H., Fields, B. D., & Olive, K. A. (2008). “An Update on the Big Bang Nucleosynthesis Prediction for 7Li: The Problem Worsens.” Journal of Cosmology and Astroparticle Physics, 11, 012.
– Focuses on the lithium problem in BBN and discusses discrepancies between predicted and observed lithium-7 abundances.

Fields, B. D. (2011). “The Primordial Lithium Problem.” Annual Review of Nuclear and Particle Science, 61, 47–68.
– Reviews the current status and challenges associated with lithium-7 predictions, offering a detailed discussion of one of BBN’s outstanding puzzles.

Kolb, E. W. & Turner, M. S. (1990). The Early Universe. Addison-Wesley.
– A classic textbook that provides a solid foundation in early-universe physics, including detailed treatments of BBN, its nuclear reactions, and its role in cosmology.

Sarkar, S. (1996). “Big Bang Nucleosynthesis and Physics Beyond the Standard Model.” Reports on Progress in Physics, 59(12), 1493–1610.
– Discusses how BBN constrains new physics (e.g., extra neutrino species, exotic particles) and outlines the sensitivity of nucleosynthesis to the conditions of the early universe.

返回博客