8 · IGCSE Physics
Red-shift and the expanding universe
Set how fast a distant galaxy is receding and watch its hydrogen absorption lines shift towards the red end of the spectrum, then see where it sits on the speed–distance graph.
z = Δλ / λ₀ ≈ v / c
v = H₀ d
c = 3.00 × 10⁸ m s⁻¹
1 Mpc = 3.09 × 10²² m
v = H₀ d
c = 3.00 × 10⁸ m s⁻¹
1 Mpc = 3.09 × 10²² m
Try this
- Set v = 15 000 km s⁻¹ and note z. Double the speed to 30 000 km s⁻¹: what happens to z and to Δλ?
- Keep v fixed and switch between Hα and Hδ. Δλ changes but z does not. Explain why.
- Keep v fixed and raise H₀ from 50 to 90. How does the galaxy's distance change, and what happens to the age estimate 1/H₀?
Exam tip: red-shift is a fractional change, so divide Δλ by the original (laboratory) wavelength λ₀, not by the observed wavelength.