Redshift, Hubble's Law and the Expanding Universe
Light from distant galaxies arrives stretched and reddened. Learn what redshift reveals, calculate it, use Hubble's law to link a galaxy's speed to its distance, and see why the whole Universe must be expanding.
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Light stretched by distance 🔭
When we split the light from a distant galaxy, its features are shifted toward the **red** end of the spectrum. This **redshift** is the key clue that the Universe is not standing still. This module measures redshift, uses **Hubble's law** to link a galaxy's speed to its distance, and shows why the whole Universe must be **expanding**.
The language of cosmology 🔑
Four ideas run through this module:
Why is it redshifted? 🏃
Why is the light from distant galaxies redshifted?
- Because the galaxies are moving away from us
- Because the galaxies are moving toward us
- Because the galaxies are extremely hot
- Because the galaxies are made only of red stars
Work out the redshift 📏
An interactive activity.
Two ways to explain expansion 🌌
An expanding Universe once fitted two rival theories. Later observations came to favour the Big Bang.
Hubble's law 📐
Hubble's law says a galaxy's recession speed is **proportional to its distance**: **v = H0 x d**, where H0 is the **Hubble constant**. So the further away a galaxy is, the faster it is moving away from us. Dividing 1 by the Hubble constant gives a rough estimate of the **age of the Universe**.
Work out the recession speed 🚀
An interactive activity.
Match each symbol to its meaning 🔗
- z
- v
- d
- H0
- The redshift of a galaxy
- How fast a galaxy is receding
- The distance to a galaxy
- The Hubble constant
Cosmology in words 🧩
Light from a distant galaxy is shifted to longer, _____ wavelengths because the galaxy is _____ from us. The greater a galaxy's distance, the greater its _____. Hubble's law links recession speed to distance through the Hubble _____. That the galaxies are all receding is evidence that the Universe is _____.
Which two are true? ✅
Select the TWO statements that are true about redshift and Hubble's law.
- A larger redshift means a galaxy is moving away faster
- The further away a galaxy, the greater its recession speed
- Redshift shows that galaxies are moving toward us
- Hubble's law says all galaxies are the same distance away
From redshift to the Big Bang 🪜
An interactive activity.
Spot the true statements 🖍️
An interactive activity.
Measure the Universe 🧭
An interactive activity.
Evidence for an expanding Universe ✍️
An interactive activity.