--- title: "Red-shift and the expanding universe" description: "Red-shift means that observed spectral lines have longer wavelengths than the same lines measured in a laboratory. Across many distant galaxies, larger distances go with larger red-shifts on average, " canonical: https://lightmysky.com/learn/science/red-shift-and-the-expanding-universe-mt_JeAfpxw49W source: https://lightmysky.com/learn/science/red-shift-and-the-expanding-universe-mt_JeAfpxw49W.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Red-shift and the expanding universe Red-shift means that observed spectral lines have longer wavelengths than the same lines measured in a laboratory. Across many distant galaxies, larger distances go with larger red-shifts on average, supporting expansion of space. The cosmic microwave background provides separate evidence for an early hot, dense universe. Subject: Science · Area: Space Systems & Earth's History · Ages 15 to 16 Page: https://lightmysky.com/learn/science/red-shift-and-the-expanding-universe-mt_JeAfpxw49W ## Ready when they can - Defines red-shift as an increase in observed wavelength and explains how recession or expansion of space can produce it - States the average distance-red-shift pattern for distant galaxies and explains how it supports expansion - Identifies the cosmic microwave background as evidence for an early hot, dense universe and explains why different observations support a model without ending further testing ## Lesson: Red-shift and cosmic expansion A galaxy contains stars whose light can have dark or bright spectral lines. These lines mark wavelengths absorbed or emitted by atoms. Wavelength is the distance between wave crests; nm means nanometres, a length unit. Compare the same line with its laboratory wavelength. A longer observed wavelength is a red-shift. The light need not look red. *(drawing: Example: a line at 450 nm in a laboratory appears at 600 nm in a galaxy's light. Its wavelength has increased.)* Across many distant galaxies, larger distances go with larger red-shifts on average. Expansion of space explains this pattern: gaps between galaxies not held together by gravity grow, and light stretches during its journey. Some nearby galaxies approach us because their own motions can outweigh expansion. A departing siren sounds lower in pitch because its sound waves reach you farther apart. A source moving away through space can also red-shift light. Cosmological red-shift instead comes from space expanding during the light's journey. The siren analogy compares wave spacing, but it does not show the cause of cosmic expansion. In the 1920s, distance measurements established that some fuzzy patches were galaxies beyond the Milky Way. Slipher had already measured many red-shifts. Hubble compared galaxy distances with recession inferred from spectra. The pattern also supported Lemaître's expanding-universe model. Both distances and shifts were needed. Expansion means the universe was denser in the past. The hot Big Bang model describes an early hot, dense universe that expanded and cooled. It predicts radiation left from that hot state. Expansion stretches this radiation to microwave wavelengths. Astronomers detect this cosmic microwave background in all directions. This supports an early hot universe; expansion alone does not establish that it was hot. *(drawing: Follow the radiation through time. Its wavelength grows as space expands.)* One galaxy's red-shift could result from its own motion through space. The distance-red-shift pattern across many galaxies supports expansion. The microwave background supports an early hot state. These different observations support the hot Big Bang model by testing different predictions. Agreement strengthens support, but scientists still check other explanations and test more predictions. **Recap.** The distance-red-shift trend supports expanding space. The cosmic microwave background also supports an early hot, dense universe. *Lesson, questions and flashcards written for this dataset. OpenStax astronomy texts were consulted for factual background; their prose was not adapted.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Galaxies and the universe](https://lightmysky.com/learn/science/galaxies-and-the-universe-mt_v9uYnIY5-B) ## Optional connections - [Life Cycle of a Star](https://lightmysky.com/learn/science/life-cycle-of-a-star-mt_lHasFmgvnT) ## Opens up - [Galaxies, Redshift and the Expanding Universe](https://lightmysky.com/learn/science/galaxies-redshift-and-the-expanding-universe-mt_-POhqrng-Q)