Red Shift: Evidence for an Expanding Universe
Simulation Description
This simulation demonstrates cosmological redshift — the observation that light from distant galaxies is shifted toward the red end of the spectrum — which is core AQA GCSE Physics evidence for the Big Bang and the expanding universe (Paper 2, Space Physics).
The scene is viewed from the Milky Way at the centre. Six objects are distributed in 3D space: Andromeda (the exception — approaching us and blueshifted), the Virgo Cluster, the Coma Cluster, and three progressively more distant galaxies stretching toward cosmological distances. Each galaxy pulses light wave rings outward toward the observer — near galaxies emit near-white rings, and rings from progressively more distant galaxies become visibly deeper amber and red, showing the wavelength stretching visually before any number is read.
As the expansion slider runs, all galaxies recede simultaneously from the central Milky Way following Hubble’s Law. Critically, more distant galaxies move outward faster than nearby ones — students can see this directly. This is not galaxies flying through space; it is space itself expanding, carrying the galaxies with it. Andromeda — governed by local gravitational pull rather than cosmological expansion — moves toward us, its rings remaining blue throughout.
The dual spectrum strip (bottom-left, always visible) is the simulation’s most important teaching feature. The top strip shows the laboratory rest spectrum — what light from a stationary hydrogen source looks like. The bottom strip shows what an observer on Earth actually detects from the selected galaxy. Five absorption lines are marked: Ca K (393 nm), Hδ (410 nm), Hγ (434 nm), Hβ (486 nm), and Hα (656 nm). Each line shifts by its own rest wavelength multiplied by z — so the whole pattern of lines shifts rightward toward red for receding galaxies, leftward for Andromeda. The relative spacing between lines is preserved, which is exactly how astronomers identify galaxies and measure their recession speeds in reality.
Clicking any galaxy updates the spectrum, the info card (top-right), and the focus panel immediately. The info card shows distance in light-years, recession speed in km/s, the redshift parameter z, and the H-alpha wavelength shift in nm — all calculated from Hubble’s Law (v = H₀d, H₀ = 70 km/s/Mpc) and the non-relativistic approximation z = v/c as specified by AQA. Toggling 🏷 Labels adds galaxy names, recession direction arrows, and the five-step GCSE causal chain.
Suggested Class Activity
“Reading the Universe” — Spectrum Analysis and Hubble’s Law Suitable for: GCSE Physics Year 10/11 — Space Physics, redshift and expanding universe. 25–30 minutes.
Setup (2 min) Open the simulation with expansion paused (slider at 0). Select the Coma Cluster by clicking it. Labels off. Draw students’ attention to the two spectrum strips.
Stage 1 — What are these lines? (5 min) Point to the rest spectrum. Ask: “These dark lines appear in the spectrum of hydrogen gas in a laboratory on Earth. What do you think they represent?” Target: absorption lines — specific wavelengths of light absorbed by hydrogen atoms. Each element has a unique fingerprint pattern of lines.
Now ask students to compare the two strips. Ask: “What has happened to the pattern of lines in the observed spectrum? Has the pattern changed, or has something else happened?” Target: the pattern is the same — same lines in the same relative positions — but the entire pattern has shifted to the right. Toward the red end.
Stage 2 — Measuring the shift (6 min) Click through each galaxy in turn from nearest to furthest (Virgo → Coma → Distant A → Distant B → Very Distant C). Students record the redshift z and recession speed from the info card.
| Galaxy | Distance | z | Recession speed |
|---|---|---|---|
| Virgo Cluster | 54 Mly | 0.004 | 1,155 km/s |
| Coma Cluster | 326 Mly | 0.023 | 7,000 km/s |
| Distant A | 1.63 Gly | 0.117 | 35,000 km/s |
| Distant B | 4.89 Gly | 0.350 | 105,000 km/s |
| Very Distant C | 13.0 Gly | 0.933 | 280,000 km/s |
Ask: “What pattern do you notice between distance and redshift?” Target: the further the galaxy, the greater the redshift and the higher the recession speed. This is Hubble’s Law.
Stage 3 — Andromeda: the exception (4 min) Click Andromeda. The spectrum lines shift left. The info card shows a negative recession speed. Ask: “Andromeda is the only galaxy here with a blueshift. Does this disprove the idea of an expanding universe?” Target: no — Andromeda is so close that the local gravitational attraction between it and the Milky Way dominates over the expansion of space at that scale. The expansion is only detectable at very large distances. Galaxies in the same local group can still be gravitationally bound and approaching each other.
Stage 4 — Run the expansion (5 min) Set the slider to ×2 and play. Ask students to watch carefully and answer: “All galaxies are moving away from us. Does this mean we are at the centre of the universe?” This is a common misconception. Target: no — every galaxy sees every other galaxy moving away from it, because space is expanding everywhere. There is no special centre. An analogy: dots on a balloon all move away from each other as the balloon is inflated; no dot is the centre.
Then ask: “What does the fact that almost all galaxies are redshifted tell us about the history of the universe?” Target: if everything is currently moving apart, then in the past it must have been closer together — all matter once occupied the same point. This is the evidence for the Big Bang.
Stage 5 — Written explanation (5 min) Students answer: “Explain how observations of redshift provide evidence for the Big Bang theory.”
Use the 🏷 Labels chip sequence as a writing frame. A strong answer will include: light from distant galaxies is redshifted → wavelengths are longer than expected → galaxies are moving away from us → the further the galaxy, the greater the redshift (Hubble’s Law) → the universe is expanding → if we reverse time, all matter was once concentrated at one point → this supports the Big Bang model.
Adaptation notes
- For lower-confidence learners: focus only on the spectrum comparison (rest vs observed) and the simple rule “further = more redshifted”; skip the Andromeda discussion
- For higher ability: ask why z = v/c breaks down for Very Distant C where v approaches c; introduce the concept that space itself can expand faster than light even though nothing moves through space faster than light
- For SEND learners: the reading ruler supports work with the spectrum strips; the colour progression of the wave rings (white → red) gives a visual analogy that does not require reading any numbers; extra-slow expansion lets students watch a single galaxy’s recession at their own pace