---
title: "Galaxies, Redshift and the Expanding Universe"
description: "Galaxy distances measured by standard candles and speeds measured by redshift give a linear relation whose slope sets the expansion rate. The expansion is of space itself, so there is no centre to poi"
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: 2026-09-12
---

> **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

Galaxy distances measured by standard candles and speeds measured by redshift give a linear relation whose slope sets the expansion rate. The expansion is of space itself, so there is no centre to point at.

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

- Builds a distance from a standard candle and a speed from a measured redshift
- Estimates the age of the universe from the expansion rate
- Explains why every observer sees the same recession and why that means no centre

## Lesson: Measuring an expanding universe

Distances need standard candles: objects of known true brightness. Comparing apparent and true brightness yields the distance, like judging a streetlamp range from its glare. Cepheid variable stars anchor the nearby steps, and type Ia supernovae extend the ladder to billions of light years. Slow pulsing Cepheids are truly brighter: the pulsation period reveals the true luminosity. And light dims with the square of the distance, so a galaxy ten times farther looks one hundred times dimmer.

Speeds come from redshift: distant galaxy light is stretched to longer, redder wavelengths because space itself expands while the light travels. The rule linking speed and distance is Hubble law, and its slope gives the present expansion rate. Dividing distance by speed gives a rough cosmic age near 14 billion years.

**Example.** Picture raisins in rising dough: every raisin sees its neighbors recede, wherever it sits. Galaxies work the same way, so every observer sees the same recession. There is no center to point at, because the expansion is of space itself.

**Tip.** To estimate the cosmic age yourself, take one far galaxy, read its speed from redshift, read its distance from a standard candle, and divide distance by speed. One division carries you back near the beginning.

**Recap.** Candles give distance, redshift gives speed, and their ratio gives an age with no center anywhere.

## 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)
- [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)