The shape and fate of the entire universe
General relativity also describes the universe as a whole. It predicts a universe that expands, and ours turns out to be speeding up.
Click Our Universe, then add Einstein-de Sitter and Big Crunch to overlay their expansion histories. Notice which keep growing, which slow down, and which collapse.
Click presets to overlay cosmological models. The amber dot marks the present epoch (a=1, t=0).
General relativity doesn't just describe black holes and orbits — it governs the evolution of the entire universe. The Friedmann equations, derived directly from Einstein's field equations, predict an expanding (or contracting) universe. The discovery that our universe is not only expanding but accelerating is one of the great mysteries of modern physics.
The scale factor a(t) describes how the universe expands over time. Its behavior depends on the universe's composition: matter, radiation, dark energy, and spatial curvature. Adjust the density parameters to see expanding, decelerating, accelerating, and even collapsing universes.
Try it: Our universe has Ω_m ≈ 0.3, Ω_Λ ≈ 0.7. Set these values to see the expansion history. Then try Ω_m = 1 (Einstein-de Sitter) or Ω_m > 1 (Big Crunch) to see the dramatic alternatives.
Change the expansion rate and watch photon wavelengths stretch as space grows. A higher redshift z means more expansion since the light set out.
As space expands, photon wavelengths stretch (redshift). Higher z means more expansion since emission.
As the universe expands, the wavelength of light stretches with it. A photon emitted by a distant galaxy arrives at Earth with a longer wavelength — shifted toward the red end of the spectrum. The redshift z tells us how much the universe has expanded since the light was emitted: 1 + z = a(now)/a(then).
Switch between flat, spherical, and hyperbolic space and watch what happens to parallel lines and the angles of a triangle.
Parallel lines stay parallel forever. Euclidean geometry.
General relativity allows three possible spatial geometries for the universe: flat (k = 0, parallel lines stay parallel), spherical (k = +1, parallel lines converge), and hyperbolic (k = -1, parallel lines diverge). Observations indicate our universe is very nearly flat.
Trace the particle horizon, the edge of what we can see, and the event horizon, the edge of what we can ever reach.
Conformal spacetime diagram. The particle horizon bounds what we can see; the event horizon bounds what we can ever reach.
The observable universe has a finite size — about 46 billion light-years in radius — even though the universe may be infinite. The particle horizon is the farthest distance from which light has had time to reach us. The event horizon is the distance beyond which light emitted now will never reach us.
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