Slow Cosmic Expansion
Lesson notes
Overview
A smaller expansion rate means galaxies recede more slowly. This topic is rated beginner and is number 52 of 98 in Earth & Space. Core ideas and first intuitions. No prior knowledge needed.
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
If the Hubble constant H₀ were smaller, galaxies at a given distance would recede more slowly, v = H₀d, and the expansion age of the universe, about 1/H₀, would be longer. A lower expansion rate means that the universe would have taken more time to reach its present size, leaving more time for stars and galaxies to form. Different measurements of H₀ currently give values between about 67 and 73 km/s/Mpc, a puzzle called the Hubble tension.
Reading the relationship
The expression "low H₀" is a compact summary of the idea. It uses the symbols l, o, w, H₀; 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
Start by only moving one slider at a time. Say aloud what you expect, then check.
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 “A Sun-Like Star”, the topic before it in Earth & Space. Once you are comfortable, continue with “Fast Cosmic Expansion”.
Why it matters
Earth and space science helps us understand our planet, its climate and the wider universe.
Where you might use it
- Revising “Slow Cosmic Expansion” 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 Slow Cosmic Expansion?
A smaller expansion rate means galaxies recede more slowly. It is a beginner topic in Earth & Space.
How do I use the Slow Cosmic Expansion 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 low H₀ 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?
Core ideas and first intuitions. No prior knowledge needed. If the quiz feels hard, reread the lesson notes and try the previous topic first.
What should I learn before this?
Start with “A Sun-Like Star”, 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.