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.
What a learner can do afterwards
- 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
1 · Read
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.
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.
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.
Candles give distance, redshift gives speed, and their ratio gives an age with no center anywhere.
2 · Watch
Take it off screen
Where it sits
Learn first
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.