A level Physics · Edexcel 9PH0 · Topic 10D

Cosmology

Key points

Light from distant galaxies is redshifted: $z = \frac{\Delta\lambda}{\lambda} \approx \frac{\Delta f}{f} \approx \frac{v}{c}$ (for $v \ll c$ ), so they are moving away.
Hubble's law: recession velocity is proportional to distance, $v = H_0d$ . Further galaxies recede faster.
This supports an expanding universe that began in a hot, dense state (the Big Bang). The age of the universe is roughly $\frac{1}{H_0}$ .
Uncertainty in $H_0$ comes from difficulties measuring large distances accurately.
The ultimate fate (expand forever, slow to a stop or collapse) depends on how the universe's density compares with a critical density.
Dark matter: galaxy rotation curves show outer stars moving too fast for the visible mass, so there must be unseen mass. Its nature is still unknown, which makes the fate of the universe uncertain.

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The light from distant galaxies is stretched to longer wavelengths, and the further away a galaxy is, the more it is stretched. This tells us that the universe is expanding, and lets us estimate its age.

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