CHRONOSCOPE · time, at every scale Home
◌ Workbench
ConceptsThe big ideas, with the evidence — and every lab that shows them The mathsEvery model's equations, constants and worked examples — each checked by the test suite For teachersClass links, completion codes and printable worksheets Your passportTour stamps, missions, your prediction record — and save it to a file Quizzes and reviewTour quizzes, and questions that come back to check what stuck About ChronoscopeWho made it, how it was built, and why you can trust it
Mode
← All concepts
Concept

◎ Expansion and redshift

Established Contested

The universe isn't exploding into empty space. The space between galaxies is growing — everywhere at once.

Hubble's law

The farther a galaxy, the faster it recedes: speed = H₀ × distance. Found in the 1920s (Lemaître 1927, Hubble 1929). Every galaxy sees the same law, so no galaxy is the centre. Established

Picture dots drawn on a balloon being blown up: every dot sees every other dot move away. Analogy — the balloon's surface is the universe; there's no inside or outside to it.

Redshift is stretching

Light's wavelength stretches by exactly as much as the universe has grown while it travelled: a redshift of z = 1 means the universe has doubled in size since the light set out. Everything the light carries is stretched with it — which is why distant supernovae appear to unfold in slow motion. Established

Faster than light? Yes — and that's fine

Galaxies beyond about 14.5 billion light-years (the Hubble distance) recede faster than light. Relativity forbids anything moving through space faster than light nearby; it doesn't limit how fast space between distant galaxies grows. We can even see some galaxies that are receding faster than light today. Established

What doesn't expand

Atoms, people, the solar system and galaxies are held together by their own forces and gravity; they don't grow with the universe. Expansion shows up only between groups of galaxies. Established

Speeding up — and the open questions

In 1998 two teams found distant supernovae fainter than expected: the expansion is accelerating (Nobel Prize 2011). Whatever drives it — 'dark energy', about 68% of the universe's energy — is unknown. Established (the acceleration). Recent maps hint that dark energy may change over time, and two ways of measuring today's expansion rate disagree (67 vs 73 km/s per megaparsec). Contested Holes H9 and H10.

How we know

  • Redshift grows with distance for millions of galaxies.
  • The microwave background: the cooled glow of the hot early universe, with a perfect thermal spectrum (COBE, 1990).
  • The amounts of hydrogen, helium and lithium made in the first minutes.
  • Supernova light curves stretched by (1 + z) (DES, 2024).
  • Patterns frozen into the galaxy distribution (baryon acoustic oscillations).

See it in the labs

Expanding universeCosmic horizonsHow sure are we?

Further exploring

Other free sites that explore this well — not part of Chronoscope.

Sources: Lemaître (1927); Hubble, PNAS 15 (1929); Riess et al., AJ 116 (1998); Perlmutter et al., ApJ 517 (1999); Mather et al. (1990); DES (2024); Planck (2020); DESI (2024–25); Davis & Lineweaver (2004).

New here?