Lambda-CDM model: Difference between revisions

Revision as of 23:30, 21 January 2025

Revision as of 23:32, 21 January 2025

Line 78:

If the cosmological constant were actually zero, the critical density would also mark the dividing line between eventual recollapse of the universe to a [[Big Crunch]], or unlimited expansion. For the Lambda-CDM model with a positive cosmological constant (as observed), the universe is predicted to expand forever regardless of whether the total density is slightly above or below the critical density; though other outcomes are possible in extended models where the [[dark energy]] is not constant but actually time-dependent.{{citation needed|date=February 2024}}

It is standard to define theThe present-day '''density parameter''' <math>\Omega_x</math> for various species is defined as the dimensionless ratio<ref name=Peacock-1998/>{{rp|74}}

: <math>\Omega_x \equiv \frac{\rho_x(t=t_0)}{\rho_\mathrm{crit} } = \frac{8 \pi G\rho_x(t=t_0)}{3 H_0^2}</math>

where the subscript <math>x</math> is one of <math>\mathrm b</math> for [[baryon]]s, <math>\mathrm c</math> for [[cold dark matter]], <math>\mathrm{rad}</math> for [[radiation]] ([[photon]]s plus relativistic [[neutrino]]s), and <math>\Lambda</math> for [[dark energy]].{{citation needed|date=February 2024}}