Baryon Density and BBN Constraints

Because the light-element yields of the Big Bang depend on how many baryons there were, nucleosynthesis measures the cosmic baryon density and constrains physics in the first seconds of the universe.

Definition

The baryon density is the mean density of ordinary matter in the universe, conventionally expressed through the baryon-to-photon ratio; Big Bang nucleosynthesis constrains it because the predicted light-element abundances vary with this ratio, and the abundances also limit any new physics that would change the early expansion rate.

Scope

This topic covers the dependence of primordial abundances on the baryon-to-photon ratio, the resulting determination of the cosmic baryon density, the remarkable agreement with the independent value from the cosmic microwave background, and the additional constraints that nucleosynthesis places on the expansion rate, the number of neutrino species, and other early-universe physics.

Core questions

• How does nucleosynthesis measure the cosmic baryon density?
• Why is the agreement with the cosmic microwave background significant?
• What other physics does nucleosynthesis constrain?

Key concepts

• Baryon-to-photon ratio
• Baryon density parameter
• Deuterium baryometer
• Effective number of neutrinos
• Expansion-rate constraint
• Concordance with the CMB

Key theories

Baryometer from abundances

The primordial deuterium abundance varies steeply with the baryon-to-photon ratio, so measuring it pins down the baryon density precisely, a determination independent of the cosmic microwave background.

Constraints on relativistic species

A faster early expansion, as caused by extra relativistic species, would leave more neutrons and raise the helium abundance, so the observed helium limits the effective number of neutrino species in the early universe.

Mechanisms

Running nucleosynthesis codes over a range of baryon-to-photon ratios produces predicted abundance curves; matching the measured deuterium and helium to these curves yields the baryon density and bounds any nonstandard expansion, since changes in the early expansion rate alter the neutron-to-proton freeze-out and hence the yields.

Clinical relevance

The baryon density from nucleosynthesis agrees with the value derived from the cosmic microwave background to within uncertainties, a concordance that strongly validates the standard cosmological model and shows that ordinary matter accounts for only a few percent of the cosmic energy budget, with the rest dark matter and dark energy.

History

Schramm, Steigman, and others developed nucleosynthesis as a baryometer and as a constraint on neutrino species in the 1970s and 1980s, famously bounding the number of light neutrino families before collider experiments; later precision deuterium measurements and the Planck cosmic-microwave-background results brought the two baryon-density determinations into close agreement.

Debates

Tension and the lithium anomaly

While deuterium and helium agree well with the cosmic-microwave-background baryon density, lithium does not, raising debate over whether the residual tension signals new physics in the early universe or unresolved astrophysical and nuclear systematics.

Key figures

• Gary Steigman
• David Schramm
• Keith Olive
• Brian Fields

Seminal works

• cyburt2016

Frequently asked questions

How can the first minutes of the universe tell us how much ordinary matter exists?

The amount of deuterium that survives depends sharply on how densely packed baryons were, so measuring primordial deuterium effectively counts the baryons, yielding the cosmic baryon density.

Why does agreement with the cosmic microwave background matter?

Nucleosynthesis probes the universe at one second while the cosmic microwave background probes it at 380,000 years; their two completely independent measurements of the baryon density agree, a powerful consistency check on the entire Big Bang framework.