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LearnAQA7408First teaching 2015Paper 3B · Option: AstrophysicsCosmology

Paper 3B · Module

Cosmology

3 subtopics≈ 95 min in total

Start the module: Standard candles Open graphing calculator

Standard candles
A star's luminosity $L$ is the total power it radiates. At distance $d$ , that power spreads over a sphere of area $4\pi d^2$ , so the radiant flux intensity received is $F=\dfrac L{4\pi d^2}$ . A standard candle is an object whose luminosity is known, such as a Cepheid variable star or a type Ia supernova. Measuring its flux then gives its distance: $d=\sqrt{\dfrac L{4\pi F}}$ .
≈ 30 min
The Doppler effect
When a sound source moves, its wavefronts bunch up in front and spread out behind. A stationary observer in front receives a shorter wavelength and a higher frequency; behind, a longer wavelength and lower frequency. For a source moving at $v_s$ towards or away from you, $f_o=\dfrac{f_sv}{v\mp v_s}$ .
≈ 30 min
Hubble's law and the Big Bang
Spectral lines from distant galaxies are shifted to longer wavelengths: redshift. For speeds much less than $c$ , $\dfrac{\Delta\lambda}\lambda\approx\dfrac{\Delta f}f\approx\dfrac vc$ , giving the recession speed. Hubble's law: a galaxy's recession speed is proportional to its distance, $v=H_0d$ , with $H_0\approx2.3\times10^{-18}$ s⁻¹. Every galaxy moving away from every other means the universe is expanding; running it backwards points to a hot, dense beginning: the Big Bang, about $\dfrac1{H_0}\approx14$ billion years ago.
≈ 35 min