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6.2.6Space Physics

The Big Bang Theory — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 23 of 24 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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6.2.6 The Big Bang Theory — IGCSE Physics Notes — IGCSE PHYSICS notes: Describe the Big Bang theory and explain the two main pieces of evidence for it: redshift and the cosmic microwave background.Beginning — around 13.8 billion years ago the Universe began from a single point th…

  • Not in space
  • First instant
  • Observation
  • Distant galaxies
  • Further away
  • Conclusion

Part 1 of 7The Big Bang theory

Beginning
  • Beginning around 13.8 billion years ago the Universe began from a single point that was extremely hot and dense.
Expansion
  • Expansion a giant explosion, known as the Big Bang, made the Universe expand outwards.
Cooling
  • Cooling as each point moved away from the others, the Universe cooled.
Present
  • Present the Universe is still expanding as a result of the initial explosion.
Not in space
  • Not in space space itself began to expand everywhere at once, so there is no centre.

Part 2 of 7Early history

1First instant
  • First instant it was so hot that matter as we know it could not exist.
2Particles
  • Particles protons and neutrons formed as it cooled.
3Nuclei
  • Nuclei simple nuclei of hydrogen and helium formed in the first few minutes.
4Atoms
  • Atoms after about 380 000 years it was cool enough for atoms to form.
5Structure
  • Structure over hundreds of millions of years gravity pulled gas into the first stars and galaxies.

Part 3 of 7Evidence 1: galactic redshift

Observation
  • Observation light from glowing hydrogen in distant galaxies is redshifted compared with hydrogen on the Earth.
Distant galaxies
  • Distant galaxies all are moving away from us.
Further away
  • Further away the further a galaxy, the faster it moves away and the more redshifted its light.
Conclusion
  • Conclusion the Universe is expanding, so tracing it back leads to a Big Bang.
Reason
  • Reason the stretching is due to the expansion of space itself.

Part 4 of 7Evidence 2: cosmic microwave background radiation (Extended)

CMBR
  • CMBR a form of electromagnetic radiation emitted shortly after the beginning of the Universe.
Detection
  • Detection it is detected everywhere, from every direction, at similar intensity.
Temperature
  • Temperature it corresponds to about 2.7 K.
Map
  • Map it shows the temperature of the Universe is almost uniform, with tiny variations that became galaxies.

Part 5 of 7Why the CMBR supports the theory (Extended)

Prediction
  • Prediction theory says the early Universe was extremely hot and dense.
Original radiation
  • Original radiation the Big Bang released extremely high-energy gamma radiation of short wavelength.
Stretching
  • Stretching as the Universe expanded the wavelength increased, which is redshift.
Now
  • Now over billions of years the radiation has been redshifted into the microwave region.
Uniformity
  • Uniformity the very uniform CMBR indicates the Universe was initially much smaller than it is now.
Leftover
  • Leftover the CMBR can be interpreted as radiation left over from the Big Bang.

Part 6 of 7Other support

Light elements
  • Light elements the theory predicts about 75% hydrogen and 25% helium by mass, as observed in old gas clouds.
Steady state theory
  • Steady state theory it cannot explain the CMBR, so it was abandoned.
Open questions
  • Open questions scientists still investigate the earliest moments, dark matter and dark energy.

Part 7 of 7More from the specification

1
Cosmic microwave background analysis
  • Cosmic microwave background analysis measuring isotropic 2.7 Kelvin blackbody radiation provides an empirical means to validate the expanding universe model.
2
Hubble's Law and Cosmic Expansion
  • Hubble's law states that the recession speed v of a distant galactic cluster is directly proportional to its distance d from Earth, expressed as v = H₀·d,
  • where H₀ represents the Hubble constant, supporting the Big Bang theory of cosmic expansion.
3
Stellar Life Cycle and Thermonuclear Fusion
  • Stars condense out of a giant interstellar cloud of gas and dust (nebula);
    • gravitational contraction raises core temperature until hydrogen nuclei begin nuclear fusion reactions, sustaining the stable main sequence phase of the stellar life cycle.
4
Galactic Structures and the Milky Way
  • The Milky Way is our local barred spiral galactic system;
    • spectral observations of light from galaxies outside our Milky Way show systematic cosmological redshift, demonstrating uniform universal expansion.
5
Hubble's Law and Cosmic Expansion
  • Hubble's law states that the recession speed v of a distant galactic cluster is directly proportional to its distance d from Earth, expressed as v = H₀·d,
  • where H₀ represents the Hubble constant, supporting the Big Bang theory of cosmic expansion.
6
Stellar Life Cycle and Thermonuclear Fusion
  • Stars condense out of a giant interstellar cloud of gas and dust (nebula);
    • gravitational contraction raises core temperature until hydrogen nuclei begin nuclear fusion reactions, sustaining the stable main sequence phase of the stellar life cycle.
7
Galactic Structures and the Milky Way
  • The Milky Way is our local barred spiral galactic system;
    • spectral observations of light from galaxies outside our Milky Way show systematic cosmological redshift, demonstrating uniform universal expansion.
8
Galactic redshift
  • Shift of galaxy spectral lines to longer wavelength due to recession.
9
White dwarf
  • Small, hot, dense core left by a low-mass star.
10
Planetary nebula
  • The shell of gas ejected by a red giant.
11
Fusion
  • Joining of light nuclei into a heavier one.
12
Reaction
  • The partner force acting on the other object.
13
Nebula
  • Cloud of gas and dust in which stars form.
14
Constant
  • Not changing.
15
Light-year
  • The distance light travels in one year, 9.5 × 10¹⁵ m.
16
Recession
  • Movement away from the observer.
17
Spectrum
  • Range of colours in white light.
18
Stable
  • Returns to its original position when tilted slightly.
19
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)

Key terms

TermMeaning
Big BangThe theory that the Universe began about 13.8 billion years ago from a hot, dense state and has expanded since.
Expanding UniverseSpace is stretching so that galaxies move apart.
RedshiftIncrease in wavelength of light from receding sources.
CMBRCosmic microwave background radiation, microwaves seen from all directions that date from the early Universe.
MicrowavesElectromagnetic waves with wavelengths of millimetres to centimetres.

Avoid these mistakes

1
Saying the Big Bang was an explosion into empty space — it was the expansion of space itself.
2
Placing the Big Bang at one point in the sky — it happened everywhere.
3
Saying the CMBR is radiation from stars — it is left over from the early Universe.
4
Saying the CMBR is visible light — it is in the microwave region.

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