The Hubble Plot
Almost every galaxy is receding, and the farther one is, the faster it goes — which is not what an explosion in space would look like.
Phase 0 · The phenomenon
Everything is moving away
A scatter chart with distance in megaparsecs from 0 to 200 on the horizontal axis and recession velocity in kilometres per second from −1,000 to 14,000 on the vertical axis. Eight survey galaxies lie close to a straight rising line through the origin; Andromeda sits just below zero velocity at almost zero distance.
Measure the spectra of galaxies and nearly all their lines are shifted toward red: they are moving away from us. Only a few close neighbours, like the Andromeda galaxy, are approaching.
Measure their distances too, and a pattern appears. A galaxy twice as far away is receding about twice as fast.
It is tempting to picture a giant explosion with us near the middle. In this lab you will build the Hubble plot yourself — and then test whether an explosion could really explain it.
Why are almost all galaxies moving away from us, faster the farther they are — and what does that pattern tell us about the history of the universe?
What this lab works toward
HS-ESS1-2Explain the Big Bang using evidence from starlight spectra, the motion of distant galaxies and what the universe is made of.
Plain-English summaries. The course maps these performance expectations to this lab; the official wording is published at nextgenscience.org.
The stations
- Redshift ReaderHow fast is each galaxy moving toward or away from us?
- Standard CandlesHow does the faintness of a standard candle reveal its distance?
- Hubble FitWhat single number links distance and velocity — and what does it say about time?
- Explosion TestCould an explosion from a centre produce what we see — or does the evidence point to expanding space?
Each station runs the same loop: predict before you see any data, investigate the model and record runs, reveal what the measurements show, then explain the gap between your prediction and the result. A claim-evidence-reasoning checkpoint and an exit ticket close the lab.
- Galaxies G1–G8 are a simulated survey built from a Hubble constant of 70 km/s/Mpc plus realistic random motions, so the answers can be checked. Andromeda is real.
- Velocities use the simple Doppler formula v = cz, which is accurate only for redshifts much smaller than 1 — true for every galaxy here.
- Distances from standard candles assume every Type Ia supernova peaks at the same brightness and that no dust dims the light.
- The explosion test is a flat, two-dimensional picture. The real argument uses three-dimensional galaxy surveys and the cosmic microwave background.