Astronomers use the Greek capital letter lambda (Λ) to represent the concentration, or density, of dark energy in the universe. Sometimes, because dark energy also affects the geometry of the universe, the dark energy density is represented by a subscripted component of omega (Ω_Λ).

If dark energy does indeed exist, then the combined effect of the matter and energy density in the universe determines the geometry of the universe. Thus, if (Ω Λ) or, equivalently, (Ω_A Ω_M) is smaller than one, then the universe is open; if it is greater than one, then the universe is closed; and if it is equal to one, then the universe has a flat geometry.