The Expanding Universe
Lesson notes
Overview
Space itself expands: distant galaxies recede faster in proportion to distance. This topic is rated advanced and is number 42 of 98 in Earth & Space. Deeper models, formal results and harder reasoning.
What you are seeing
The panel above shows a model of an Earth or space system that you can adjust. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.
In depth
Edwin Hubble found in 1929 that distant galaxies are moving away from us, and the farther they are, the faster they recede: v = H₀d, with H₀ about 70 km/s per megaparsec. This does not mean we are at the centre: space itself is expanding, so every galaxy sees the others receding, like dots on an inflating balloon. Running the expansion back leads to a hot, dense start, the Big Bang, about 13.8 billion years ago.
Reading the relationship
The expression "v = H₀d" is a compact summary of the idea. It uses the symbols v, H₀d; name what each one stands for before you start. Then use the sliders to see how changing one quantity changes the others.
A guided exploration
1. Run the animation once with the default settings and describe in words what happens. 2. Change one slider at a time and note which quantity responds most. 3. Push each slider to its extreme and decide whether the result still makes sense.
Try this
Try to explain why the pattern holds, not just what it is. Look for limiting cases and what happens at the edges.
Common pitfalls
Earth and space systems work on very different scales of time and distance. Check which scale a number refers to before comparing it with another.
How it connects
It builds on “Galaxy Rotation Curves and Dark Matter”, the topic before it in Earth & Space. Once you are comfortable, continue with “Cosmological Redshift”.
Why it matters
Earth and space science helps us understand our planet, its climate and the wider universe.
Where you might use it
- Revising “The Expanding Universe” before a class, exam or certification
- Forecasting weather and understanding climate records and their uncertainty
- Planning for earthquakes, floods and other natural hazards in cities
- Interpreting satellite images, space missions and the age of rocks and stars
Frequently asked questions
What is The Expanding Universe?
Space itself expands: distant galaxies recede faster in proportion to distance. It is an advanced topic in Earth & Space.
How do I use the The Expanding Universe simulation?
The panel at the top shows a model of an Earth or space system that you can adjust. Move the sliders one at a time, then press Space to pause the animation and look closer.
What does the formula mean?
The expression v = H₀d packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.
Who is this topic for?
Deeper models, formal results and harder reasoning. If the quiz feels hard, reread the lesson notes and try the previous topic first.
What should I learn before this?
Start with “Galaxy Rotation Curves and Dark Matter”, the previous topic in Earth & Space. Each topic in a subject builds on the ones before it.
How do I track my progress?
Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.
Quick quiz
More in Earth & Space
See all →Cosmological Redshift
Light from receding galaxies is stretched to longer wavelengths.
Timeline of the Universe
From the Big Bang to the present: expansion, cooling and the formation of structure.
How Gravity Assists Work
A spacecraft flies past a moving planet and steals a little of its orbital energy.