Jeans instability of a local gas model
Linear perturbations in a uniform Newtonian gas model obey ω²=c_s²k²−4πGρ₀.
Key knowledge
Linear perturbations in a uniform Newtonian gas model obey ω²=c_s²k²−4πGρ₀. Modes with k<k_J grow or decay exponentially; k>k_J supports acoustic oscillation. The marginal scale is k_J²=4πGρ₀/c_s² and λ_J=2π/k_J. This conditional instability illustrates competition between pressure response and self-gravity.
Values and equations
ω²=c_s²k²−4πGρ₀; k_J²=4πGρ₀/c_s²; λ_J=c_s√[π/(Gρ₀)]
- ω
- angular mode frequency, possibly imaginary s⁻¹
- k,k_J
- angular wavenumber and marginal Jeans wavenumber m⁻¹
- c_s
- sound speed for the chosen closure m s⁻¹
- ρ₀
- background mass density kg m⁻³
- G
- gravitational constant m³ kg⁻¹ s⁻²
- λ_J
- marginal wavelength m
Conditions and scope
- Small perturbations, positive uniform mass density and sound speed, an inviscid nonrotating unmagnetized fluid and a specified pressure closure. Here c_s²=(∂P/∂ρ)_s for an adiabatic closure.
- Subtract the uniform gravitational background in the local perturbation model: an infinite static homogeneous self-gravitating gas is not an exact global equilibrium. Expansion, cooling, turbulence, boundaries and magnetic fields require adapted models; this is not a universal star-formation threshold or exact finite-sphere collapse time.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Read the actual linear dispersion and explicit inconsistent-background warning. Defined SI mass density ρ₀=m n̄, avoiding the notes' later energy-density convention; derived λ_J by 2π/k_J. Earlier Cambridge-host fetch failed; this author-hosted body was actually inspected.
2026-10-10 · Independent AI editorial review: All44 exact root-authored compact records, equations/variable units/conditions and140 origin edges accepted after independently reading final6/18 with bounded primary bodies and matching prior38/122 scientific fingerprints unchanged. Exact claim/edge review is distinct from combined inventory scope and separate CLE26 approval; no entire cited books or old-corpus experimental revalidation is asserted. Receipt: cosmos-completion-independent-content-and-inventory-review.1.
Entry details
Permanent link to this entry ↗- Record identity
- uni-cosmos-jeans-cloud-instability
- Revision
- core.1
- Structured reference record
- University core expansion