Worked example: is the universe really expanding?
Scientists don't settle arguments by vote. They find a test where two ideas predict different things — then look.
Distant galaxies look redder (redshift). Two explanations were on the table from 1929: space is expanding, or light simply gets tired on a long journey (Fritz Zwicky's idea — a natural one). How do we decide? Find tests where the two ideas predict different things, then look.
| Test | If space expands | If light gets tired | What we see | Winner |
|---|---|---|---|---|
| Supernovae unfold in slow motion | Stretched by exactly (1 + z) | No stretching | Stretched by (1 + z) — Dark Energy Survey, 2024, ~1,500 supernovae | Expansion |
| The microwave background's spectrum | A perfect thermal curve | A distorted curve | Perfect thermal curve — COBE, 1990 | Expansion |
| The background was hotter in the past | Temperature × (1 + z) | No change | Temperature × (1 + z) — measured in distant gas clouds | Expansion |
| Distant galaxies stay sharp | Sharp | Blurred by scattering | Sharp | Expansion |
| Surface brightness dims | By (1 + z)⁴ | By (1 + z) | Consistent with (1 + z)⁴ once galaxy ageing is included | Expansion (with caveats) |
Verdict: expansion is Established. Independent tests agree, and tired light fails several of them.
What is still genuinely uncertain
- How fast? Two methods disagree (67 vs 73 km/s per megaparsec) — the Hubble tension. Established that the disagreement exists.
- What drives the acceleration? "Dark energy" is a name, not an explanation. DESI hints it may be weakening. Contested
- Is the acceleration even real? Timescape cosmology argues it's an effect of clocks ticking differently in voids. Contested
The ladder every claim climbs
- Idea — anyone can have one.
- Proposal — worked out mathematically and published. Speculative
- Tested, disputed — evidence exists, experts disagree. Contested
- Confirmed — independent tests agree; alternatives fail. Established
- Ruled out — failed its tests. Still worth learning from: tired light is here. Ruled out