crookedindifference:

The ultimate fate of an expanding universe

Top: Diagrams of three possible geometries of the universe: closed, open and flat from top to bottom, corresponding to a density parameter Ω₀ which is greater than, less than or equal to 1. The closed universe is of finite size and, due to its curvature, traveling far enough in one direction will lead back to one’s starting point. The open and flat universes are infinite and traveling in a constant direction will never lead to the same point.

Bottom: The age and ultimate fate of the universe can be determined by measuring the Hubble constant today and extrapolating with the observed value of the deceleration parameter, uniquely characterized by values of density parameters (Ω_M for matter and Ω_Λ for dark energy). A “closed universe” with Ω_M > 1 and Ω_Λ = 0 comes to an end in a Big Crunch and is considerably younger than its Hubble age. An “open universe” with Ω_M ≤ 1 and Ω_Λ = 0 expands forever and has an age that is closer to its Hubble age. For the accelerating universe with nonzero Ω_Λ that we inhabit, the age of the universe is coincidentally very close to the Hubble age.