PhysicsChapter 9: Cosmology
Redshift and Hubble's Law
Redshift
Definition
Redshift is the observed shift of spectral features toward longer wavelengths. For distant galaxies, cosmological redshift is a key indicator of cosmic expansion. Hubble's law relates a galaxy's recession rate to its distance.
Meaning
Redshift can be written: z = λobs − λemitλemit where: `λ_obs` = observed wavelength `λ_emit` = emitted/reference wavelength. For small redshifts, an approximate velocity relation is: v ≈ cz Hubble's law for nearby cosmological distances is: v = H₀d where: `v` = recession velocity/rate in the low-redshift approximation `d` = distance `H₀` = Hubble constant. If distance increases, recession rate generally increases. The relation provides strong observational evidence that cosmic distances between widely separated, gravitationally unbound galaxies increase over time.
Formula / Key Relation
**z = Δλ/λemit v ≈ cz for sufficiently small `z` v = H₀d**
Exam preparation
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Review the key facts, formulas and checks before you move on.Redshift and Hubble's Law
Sticky Notes · Quick Revision
Redshift:
- `z = (λobs−λemit)/λemit`
- Small z:
- `v≈cz`
- Hubble:
- `v=H₀d`
60-second revision
- Galaxy redshift and Hubble's law provide key evidence for cosmic expansion.
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