--- title: "Galaxies, Redshift and the Expanding Universe" description: "Calibrated standard candles give distances, and small redshifts estimate recession speeds. The velocity-distance slope gives the present expansion rate H0. Its inverse is an age scale; cosmic age also" canonical: https://lightmysky.com/learn/science/galaxies-redshift-and-the-expanding-universe-mt_-POhqrng-Q source: https://lightmysky.com/learn/science/galaxies-redshift-and-the-expanding-universe-mt_-POhqrng-Q.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`. # Galaxies, Redshift and the Expanding Universe Calibrated standard candles give distances, and small redshifts estimate recession speeds. The velocity-distance slope gives the present expansion rate H0. Its inverse is an age scale; cosmic age also depends on expansion history. Uniform expansion has no preferred centre. Subject: Science · Area: Astronomy & Astrophysics · Ages 21 to 22 Page: https://lightmysky.com/learn/science/galaxies-redshift-and-the-expanding-universe-mt_-POhqrng-Q ## Ready when they can - Calculates a standard-candle distance from luminosity and flux, and estimates recession speed from a small redshift - Calculates the Hubble time 1/H0 and explains why it need not equal the universe's age - Explains why a homogeneous uniform-expansion model gives comoving observers the same large-scale relation without a preferred centre ## Lesson: Measuring an expanding universe A standard candle has a calibrated luminosity L, its emitted power. Flux F is received power per unit area. For a nearby source emitting equally in all directions, with dust dimming corrected, F = L/(4 pi d^2), so d = sqrt(L/(4 pi F)). At fixed luminosity, doubling distance reduces flux to one quarter. Faintness alone cannot distinguish distance from low luminosity. For Cepheids of the same calibrated class and observing band, a longer pulsation period means greater mean luminosity. Measure the period, apply the calibrated relation, then compare luminosity with flux. Independent nearby distances set the calibration. Type Ia supernovae extend the ladder after brightness corrections. These white-dwarf explosions differ from the core-collapse supernovae discussed previously. *(drawing: Both luminosity and flux are needed. A flux measurement alone does not determine distance.)* Redshift is z = (observed wavelength - rest wavelength)/rest wavelength. Expansion stretches light during its journey. For small z, v is approximately cz, where c is light's speed. This estimates recession speed when local motion is negligible. Large cosmological redshifts require an expansion model for interpretation. Nearby galaxies have local motions and can approach us. For low-redshift galaxies whose local motions are small, v = H0 d. Plot speed v vertically and distance d horizontally: the slope is H0, today's fractional expansion rate. Thus d/v = 1/H0, the Hubble time. A megaparsec (Mpc) is 3.09 x 10^19 km. For H0 in km/s/Mpc, divide that kilometre value by H0 to get seconds. Divide seconds by 3.15 x 10^16 to get billion years. Use many galaxies to reduce local-motion effects. *(drawing: For the example H0 = 75 km/s/Mpc, divide 3.09 x 10^19 by 75, then convert seconds to billion years.)* Extrapolating today's distance and recession speed back to zero separation at constant speed gives the Hubble time. Expansion has changed over time, so cosmic age requires a model of that history. A reliably dated star gives another constraint: the universe must be at least as old as the star. This lower bound does not determine H0 or the exact cosmic age. In a homogeneous model with uniform expansion, large-scale separations grow by the same factor. Comoving observers follow this expansion; each finds the same distance-speed relation at a given cosmic time. The model selects no expansion centre. Our recession pattern alone does not prove the model. Rising dough illustrates separation, but its edge and centre are not features of this cosmological model. Gravity-bound galaxies need not expand. **Recap.** Calibrated candles give distance. Small redshifts estimate speed. H0 gives a Hubble time; cosmic age needs expansion history. Uniform expansion has no preferred centre. *Lesson, questions and flashcards written for this dataset. Supplied OpenStax Astronomy records were consulted for scientific facts and teaching structure.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Red-shift and the expanding universe](https://lightmysky.com/learn/science/red-shift-and-the-expanding-universe-mt_JeAfpxw49W) - [Stellar Death: White Dwarfs, Neutron Stars and Black Holes](https://lightmysky.com/learn/science/stellar-death-white-dwarfs-neutron-stars-and-black-holes-mt_k1oUF3mJlu) - [Flux, Magnitudes and the Distance Ladder](https://lightmysky.com/learn/science/flux-magnitudes-and-the-distance-ladder-mt_q1vEUas3AY) ## Optional connections - [Simple Linear Regression and Inference on the Slope](https://lightmysky.com/learn/mathematics/simple-linear-regression-and-inference-on-the-slope-mt_W2huNTKsMj) ## Opens up - [Galaxy Types, the Milky Way's Structure and Active Nuclei](https://lightmysky.com/learn/science/galaxy-types-the-milky-ways-structure-and-active-nuclei-mt_Eps5CXEB5C) - [The Cosmic Microwave Background and the Light Elements](https://lightmysky.com/learn/science/the-cosmic-microwave-background-and-the-light-elements-mt_oz0MJXiuVt) - [The Scale Factor, the Friedmann Equation and the Age of the Universe](https://lightmysky.com/learn/science/the-scale-factor-the-friedmann-equation-and-the-age-of-the-universe-mt_v3VEzinvJe) - [Dark Matter, Dark Energy and What the Universe Is Made Of](https://lightmysky.com/learn/science/dark-matter-dark-energy-and-what-the-universe-is-made-of-mt_GKKPGcHqJH)