Cosmology · undergrad

Cosmological Constant

The constant energy density term associated with accelerated expansion.

Published Apr 26, 2026 · 1 min

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In Einstein’s Equation

The cosmological constant appears as

G_{\mu\nu}+\Lambda g_{\mu\nu}=\frac{8\pi G}{c^4}T_{\mu\nu}.

It can be moved to the matter side and interpreted as a constant vacuum energy density.

Vacuum energy has

p_\Lambda = -\rho_\Lambda c^2.

The negative pressure makes it contribute positively to $\ddot a$ in the Friedmann acceleration equation.

A standard introductory path through expansion, Friedmann dynamics, distance scales, and the thermal history of the universe.

The density that makes the spatial curvature term vanish for a given Hubble rate.

The equations governing the expansion of a homogeneous and isotropic universe.