While Einstein introduced his cosmological constant to get a static universe, he was never satisfied with it. This wasn’t merely due to the aesthetics of imposing such a solution. A significant problem with his cosmological constant was that while it made the universe static, it also made it unstable. To illustrate this, consider Einstein’s static universe to be comprised only of non-relativistic matter with mass density ρ, and a cosmological constant, Λ = 4πGρ. Suppose, now, that some of the non-relativistic matter is converted into radiation (e.g., by stars). Will the universe start to expand or contract ?

While Einstein introduced his cosmological constant to get a static universe, he was never satisfied with it. This wasn’t merely due to the aesthetics of imposing such a solution. A significant problem with his cosmological constant was that while it made the universe static, it also made it unstable. To illustrate this, consider Einstein’s static universe to be comprised only of non-relativistic matter with mass density ρ, and a cosmological constant, Λ = 4πGρ. Suppose, now, that some of the non-relativistic matter is converted into radiation (e.g., by stars). Will the universe start to expand or contract ?