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Expanding Universe

A lattice of galaxies whose separations grow with the scale factor a(t) of the Friedmann equation, integrated live from the density you choose. Every galaxy recedes from every other in proportion to distance (Hubble law) with no special centre. Matter alone decelerates the expansion; enough matter closes the universe so it expands, halts, and recollapses to a Big Crunch; dark energy makes the expansion accelerate and the lattice thin out forever. Click a galaxy to send a light pulse to you and read its redshift, which is exactly the ratio of scale factors between emission and now.

Figure 1. Comoving galaxy lattice observed from the centre, proper separations scaled by the live Friedmann solution a(t), galaxies coloured by recession velocity with the Hubble radius ringed. Click a galaxy to read the cosmological redshift of its light. Lower panel: a(t) history and the radiation/matter/dark-energy density eras. Source: Ryden, Introduction to Cosmology, 2nd ed., Ch. 5-6.

WHAT TO TRY

  • Switch to the dark-energy preset and watch the lattice thin out as the expansion accelerates; switch to closed (Big Crunch) and watch it expand, halt, and recollapse.
  • Click a galaxy: a photon flies in to you, reddened by its cosmological redshift z = a(now)/a(emit) - 1, which the readout reports.
  • Watch the Hubble-radius ring: galaxies outside it recede faster than light (deep red), yet their light still reaches you as space evolves.
  • Drag the cosmic-time slider to scrub the expansion by hand, or drag the scene to orbit the lattice.