The Expanding Universe
The light from distant galaxies carries a secret: they are all rushing away from us. Follow the red-shift evidence out to the edge of the sky and back to the Big Bang.
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The Expanding Universe 🔭
When we split the light from a distant galaxy, we see the same patterns of dark lines as from our own Sun — but shifted. That tiny shift is one of the most important discoveries in all of science. It tells us the universe is not sitting still. It is growing.
Red-shift 🔴
When a galaxy moves **away** from us, the light waves it sends out get **stretched** to **longer wavelengths** — shifted towards the **red** end of the spectrum. We call this **red-shift**. The faster the galaxy recedes, the more its light is stretched, and the bigger the red-shift.
Reading the shift
The light from a distant galaxy is red-shifted. What does this tell us about the galaxy?
- It is moving away from us
- It is moving towards us
- It is completely stationary
- It is getting hotter
Further means faster 🌌
Astronomers measured red-shift for galaxy after galaxy and found a striking pattern: **the more distant a galaxy is, the greater its red-shift** — so the faster it is moving away from us. Nearby galaxies drift away slowly; the most distant ones race away fastest.
Plot the galaxy
An interactive activity.
Everything is spreading out 💥
If every distant galaxy is moving away, and the far ones fastest, there is one simple explanation: the **whole universe is expanding**. Space itself is stretching, carrying the galaxies apart. Think of raisins in a rising loaf — as the dough expands, every raisin moves away from every other one, and the furthest apart separate fastest.
Back to the beginning
An interactive activity.
Be the astronomer
An interactive activity.
Match each term to its meaning
- Red-shift
- Recession
- Expanding universe
- Big Bang theory
- Light stretched to longer wavelengths as a source moves away
- A galaxy moving away from us
- All distant galaxies moving further apart over time
- The universe began from a tiny, hot, dense region
Tour summary
Light from distant galaxies is _____-shifted, meaning its wavelength is stretched to become _____. This shows the galaxies are moving away, and the more distant ones recede _____. The whole universe is _____, which is the key evidence supporting the Big Bang theory.