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6.2.5Space Physics

Galactic Redshift — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 10 parts Lesson 22 of 24 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

6.2.5 Galactic Redshift — IGCSE Physics Notes — IGCSE PHYSICS notes: Explain redshift of light from distant galaxies and what it shows about the motion of galaxies and the expansion of the Universe.Waves from a stationary source — they spread out symmetrically.Moving source — the…

  • Applies to
  • Source moving towards the observer
  • Source moving away
  • Comparison
  • Observation
  • All distant galaxies

Section 1 of 10Doppler effect

1Waves from a stationary source
  • Waves from a stationary source they spread out symmetrically.
2Moving source
  • Moving source the waves are squashed together or spread out, and this change in wavelength is the Doppler effect.
3Applies to
  • Applies to both sound waves and electromagnetic waves such as light.
4Sound
  • Sound an ambulance siren rises in pitch as it approaches and drops as it moves away.

Section 2 of 10Redshift and blueshift

1Source moving towards the observer
  • Source moving towards the observer the observed wavelength decreases and the light shifts towards the blue end (blueshift).
2Source moving away
  • Source moving away the observed wavelength increases and the light shifts towards the red end (redshift).
3Red light
  • Red light it has the longest wavelength and the smallest frequency in the visible spectrum.
4Frequency
  • Frequency it is lower for a receding source.

Section 3 of 10Redshift in distant galaxies

Comparison
  • Comparison astronomers compare light from glowing hydrogen in distant galaxies with light from hydrogen on the Earth.
Observation
  • Observation the light from the galaxies is redshifted, with the pattern of dark absorption lines at longer wavelengths.
Meaning
  • Meaning the wavelength increased as the light travelled, so distant galaxies are moving away (receding) from the Earth.

Section 4 of 10Pattern in the observations

All distant galaxies
  • All distant galaxies nearly all show redshift, so almost everything is moving away from us.
Further away
  • Further away the greater the observed redshift, the greater the distance to the galaxy.
Speed
  • Speed a larger redshift also means the galaxy is moving away faster.

Section 5 of 10Expansion of the Universe

Conclusion
  • Conclusion if galaxies are moving apart, the Universe must be expanding.
No centre
  • No centre an observer in any other galaxy would see the others receding in the same way.
Stretching of space
  • Stretching of space the wavelength of the light is stretched by the expansion of space itself, not just the motion of the galaxies.
Balloon model
  • Balloon model the balloon is space and points on it are galaxies, so as it inflates all points move further apart by the same amount.
Limits
  • Limits the expansion does not stretch objects held together by gravity or electromagnetic forces.

Section 6 of 10Hubble's law (Extended)

Law
  • Law the speed of recession is proportional to the distance of the galaxy from the Earth.
Graph
  • Graph recession speed against distance is a straight line through the origin, and the gradient is H₀.
Hubble constant
  • H₀ = v / d, the ratio of the speed of recession to the distance.
Quantities
  • H₀ is the Hubble constant, in per second (s-1).
  • v is the recessional velocity, in km/s.
  • d is the distance to the object, in km.
Value
  • H₀ is about 2.2 × 10-18 per second.

Section 7 of 10Measuring recession speed and distance (Extended)

Recession speed
  • Recession speed found from the change in wavelength of a galaxy's starlight caused by redshift.
Distance
  • Distance found from the brightness of a supernova in that galaxy.
Standard candles
  • Standard candles certain supernovae have the same peak brightness, so they can be used to find distances to the most distant galaxies.

Section 8 of 10More from the specification

1
Cosmic expansion over billions of years
  • Cosmic expansion over billions of years photons losing radiant energy across billions of light years shift towards redder wavelengths.
2
Stellar Nebula and Protostar Formation
  • Stars begin their life cycle within a cold stellar nebula of gas and dust, collapsing under mutual gravitational attraction until core temperature and pressure initiate nuclear fusion reactions.
3
Main Sequence Stars and Fusion Equilibrium
  • In a main sequence star, inward gravitational collapse is counteracted by outward thermal and radiation pressure generated by the continuous fusion of hydrogen into helium.
4
Red Giant Phase and Heavy Element Synthesis
  • When core hydrogen runs out, the star expands into a red giant, igniting helium fusion reactions that forge carbon, oxygen,
  • and heavier elements before shedding planetary nebulae.
5
Galactic Redshift and Expanding Universe
  • Light from distant galaxies observed throughout the Milky Way exhibits redshift, showing that distant galaxies are receding and providing primary evidence for cosmological cosmic expansion.
6
Stellar Nebula and Protostar Formation
  • Stars begin their life cycle within a cold stellar nebula of gas and dust, collapsing under mutual gravitational attraction until core temperature and pressure initiate nuclear fusion reactions.
7
Main Sequence Stars and Fusion Equilibrium
  • In a main sequence star, inward gravitational collapse is counteracted by outward thermal and radiation pressure generated by the continuous fusion of hydrogen into helium.
8
Red Giant Phase and Heavy Element Synthesis
  • When core hydrogen runs out, the star expands into a red giant, igniting helium fusion reactions that forge carbon, oxygen,
  • and heavier elements before shedding planetary nebulae.
9
Galactic Redshift and Expanding Universe
  • Light from distant galaxies observed throughout the Milky Way exhibits redshift, showing that distant galaxies are receding and providing primary evidence for cosmological cosmic expansion.
10
Background radiation
  • Radiation that is always present in the environment.
11
Planetary nebula
  • The shell of gas ejected by a red giant.
12
Fusion
  • Joining of light nuclei into a heavier one.
13
Radiation
  • Transfer by electromagnetic waves, requiring no medium
14
Reaction
  • The partner force acting on the other object.
15
Nebula
  • Cloud of gas and dust in which stars form.
16
Helium
  • Product of the fusion of hydrogen in stars.
17
Light-year
  • The distance light travels in one year, 9.5 × 10¹⁵ m.
18
Stable
  • Returns to its original position when tilted slightly.
19
White dwarf
  • Small, hot, dense core left by a low-mass star.
20
v = H0d
What the symbols mean: v = recession speed of the galaxy (km s−1) · H0 = Hubble constant (km s−1 Mpc−1) · d = distance (Mpc)

Section 9 of 10Key terms

TermMeaning
Doppler effectChange in observed wavelength and frequency due to the motion of a source.
RedshiftIncrease in the wavelength of light from a source moving away from the observer.
BlueshiftDecrease in the wavelength of light from a source moving towards the observer.
Hubble constantH₀ = v / d, about 2.2 × 10^{-18} per second.
RecessionMovement away from the observer.
Standard candleA supernova of known peak brightness used to measure distance.

Section 10 of 10Exam technique

1
Saying galaxies move away because the Earth is at the centre — all galaxies move away from each other.
2
Saying redshift means the light has turned red — it means the wavelength has increased.
3
Mixing up redshift and blueshift — redshift means moving away and blueshift means moving towards.
4
Thinking speed is independent of distance — speed of recession is proportional to distance.

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