Ultimate fate of the universe
The long-term evolution and possible end state of the universe, shaped by cosmic expansion, matter, and dark energy.
Big Freeze: A hypothesized future in which continued expansion leaves the universe increasingly cold, dilute, and unable to sustain useful energy flows. This is the leading forecast if dark energy remains nearly constant and expansion continues accelerating.
Cosmic expansion: The increase over time of distances between large-scale, gravitationally unbound regions of the universe. Its rate and acceleration determine whether cosmic distances keep growing or eventually shrink.
Friedmann equations: Equations from general relativity that govern the expansion of homogeneous and isotropic cosmological models. They connect the universe’s contents and spatial curvature to its changing expansion rate.
Dark energy equation of state: The pressure-to-density relation of dark energy, often summarized by the parameter w. A changing value could overturn the constant-dark-energy forecast and alter the universe’s fate.
Big Crunch: A hypothetical ending in which cosmic expansion reverses and the universe collapses into a hot, dense state. It is the classic alternative in which gravity eventually halts and reverses expansion.
Dark energy: The name for the component inferred to drive the observed acceleration of cosmic expansion. Its density and evolution largely decide which long-term expansion scenario is possible.
ΛCDM model: The standard cosmological model combining a cosmological constant, cold dark matter, ordinary matter, and initial hot Big Bang conditions. Its measured parameters currently favor continued accelerated expansion and a Big Freeze.
Cosmological constant problem: The discrepancy between the observed vacuum energy density and estimates from quantum field theory. Its unresolved origin leaves the physical basis of the universe’s late-time acceleration unexplained.
Big Rip: A hypothetical ending in which accelerating expansion grows strong enough to unbind galaxies, stars, and ultimately atoms. It requires dark energy to become more repulsive with time than in the standard forecast.
Equation of state (cosmology): A relation between pressure and energy density that characterizes the behavior of a cosmic component. Its value for dark energy distinguishes steady acceleration from scenarios such as the Big Rip.