Baryonic Matter
Baryonic matter refers to ordinary matter composed of baryons — protons, neutrons, and the electrons that accompany them — making up all atoms, molecules, stars, planets, and gas clouds that can be observed through electromagnetic radiation. Despite being the familiar constituent of everything we can directly see and touch, baryonic matter constitutes only approximately 4.9% of the total mass-energy content of the universe, with the remainder being dark matter (~27%) and dark energy (~68%). The cosmic density of baryons, parameterised by Ω_b ≈ 0.049, is precisely measured through Big Bang nucleosynthesis predictions, CMB acoustic peaks, and the Lyman-alpha forest.
Key Formula
Ω_b = ρ_b / ρ_crit; ρ_crit = 3H₀² / (8πG)
LaTeX: \Omega_b = \frac{\rho_b}{\rho_{\rm crit}}, \quad \rho_{\rm crit} = \frac{3H_0^2}{8\pi G}
| Symbol | Meaning | Unit |
|---|---|---|
| Ω_b | Baryonic density parameter (dimensionless) | dimensionless |
| ρ_b | Observed baryon density | kg/m³ |
| ρ_crit | Critical density of the universe | kg/m³ |
| H₀ | Hubble constant | km/s/Mpc |
| G | Gravitational constant | N·m²/kg² |
Worked Example
Problem
Calculate the critical density of the universe ρ_crit using H₀ = 70 km/s/Mpc and G = 6.674 × 10⁻¹¹ N·m²/kg², then find the baryonic matter density ρ_b given Ω_b = 0.049.
Solution
Step 1 – Convert H₀ to SI: H₀ = 70 km/s/Mpc = 70 × 10³ / (3.086 × 10²²) = 2.269 × 10⁻¹⁸ s⁻¹ Step 2 – Critical density: ρ_crit = 3H₀² / (8πG) = 3 × (2.269 × 10⁻¹⁸)² / (8π × 6.674 × 10⁻¹¹) = 3 × 5.149 × 10⁻³⁶ / (1.676 × 10⁻⁹) = 1.545 × 10⁻³⁵ / 1.676 × 10⁻⁹ = 9.22 × 10⁻²⁷ kg/m³ Step 3 – Baryonic density: ρ_b = Ω_b × ρ_crit = 0.049 × 9.22 × 10⁻²⁷ = 4.52 × 10⁻²⁸ kg/m³
Answer
Critical density ρ_crit ≈ 9.22 × 10⁻²⁷ kg/m³; baryon density ρ_b ≈ 4.52 × 10⁻²⁸ kg/m³.
Distribution of Baryonic Matter in the Universe
| Form of Baryonic Matter | Fraction of Ω_b (%) | Temperature (K) | Location |
|---|---|---|---|
| Stars | ~7 | 10³–10⁷ | Galaxies |
| Cold gas (HI, H₂) | ~10 | 10–100 | Galaxy disks, ISM |
| Warm-Hot IGM (WHIM) | ~30–40 | 10⁵–10⁷ | Cosmic filaments |
| Hot intracluster gas (ICM) | ~3–5 | 10⁷–10⁸ | Galaxy clusters |
| Compact objects (WDs, NSs) | ~1–2 | Variable | Galaxies |
| Undetected (missing baryons) | ~30 | Unknown | Filaments/voids |
Interactive Tools
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Related Terms
Dark Matter
Dark matter is a hypothetical form of matter that does not interact with the electromagnetic force, making it invisible to the entire electromagnetic spectrum, yet its existence is inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe. It is estimated to constitute approximately 27% of the total mass-energy content of the universe, compared to only 5% for ordinary baryonic matter. Leading candidates include Weakly Interacting Massive Particles (WIMPs), axions, and sterile neutrinos, though no direct detection has been confirmed as of 2025.
Dark Energy
Dark energy is a hypothetical form of energy that permeates all of space and is postulated to drive the accelerated expansion of the universe, constituting approximately 68% of the total energy content of the cosmos. It was first inferred from Type Ia supernova observations in 1998, which showed that distant supernovae were fainter than expected, implying the expansion of the universe is speeding up rather than slowing down. The simplest model for dark energy is the cosmological constant (Λ), representing a constant energy density of free space, though alternative models such as quintessence propose a dynamic scalar field.
Big Bang Theory
The Big Bang Theory is the prevailing cosmological model describing the origin and evolution of the universe, proposing that all matter, energy, space, and time originated from an extremely hot, dense singularity approximately 13.8 billion years ago. The rapid expansion from this primordial state led to cooling, the formation of fundamental particles, and eventually atoms, stars, and galaxies. Evidence supporting this model includes the observed expansion of the universe, the cosmic microwave background radiation, and the abundance of light elements like hydrogen and helium.
"Baryonic" derives from Greek "barys" (heavy) + suffix "-on" (particle), as baryons (protons and neutrons) are among the heaviest subatomic particles. The term was coined in particle physics in the 1950s. "Matter" from Latin "materia" (substance, wood, stuff). The distinction between baryonic and non-baryonic matter became cosmologically significant with the development of Big Bang nucleosynthesis theory in the 1960s–70s.