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6.2.7Space Physics

Age of the Universe — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 24 of 24 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

6.2.7 Age of the Universe — IGCSE Physics Notes — IGCSE PHYSICS notes: Use Hubble's law to estimate the age of the Universe from 1 / H₀ and explain what this shows about the Big Bang.Hubble's law — recession speed is proportional to distance, v = H₀ d.Rearranged — 1 / H₀ = d / v.…

  • Distance cancels
  • Hubble constant
  • Age in seconds
  • Convert to years
  • Reverse time
  • Single point

Part 1 of 7Hubble's law and age

Hubble's law
  • Hubble's law recession speed is proportional to distance, v = H₀ d.
Rearranged
  • 1 / H₀ = d / v.
Meaning
  • Meaning since time = distance ÷ speed, 1 / H₀ is an estimate of the age of the Universe.
Distance cancels
  • Distance cancels every galaxy gives the same answer, which supports the idea that all started from one place.

Part 2 of 7Estimating the age

1Hubble constant
  • H₀ = 2.2 × 10-18 per second.
2Age in seconds
  • Age in seconds age = 1 / H₀ = about 4.5 × 1017 s.
3Convert to years
  • Convert to years divide by 3.15 × 107 s per year.
4Result
  • Result about 14 billion years, and the accepted estimate is 13.8 billion years.

Part 3 of 7Hubble's law graph

Graph
  • Graph recession velocity against distance for many galaxies.
Shape
  • Shape a straight line through the origin, with the furthest galaxies moving away fastest.
Gradient
  • Gradient it represents the Hubble constant, and its reciprocal gives the age.
Origin
  • Origin the graph passes through a single point, the singularity of the Big Bang.

Part 4 of 7Evidence for the Big Bang

Reverse time
  • Reverse time galaxies would have been closer together in the past.
Single point
  • Single point the Universe must have originated from one point and has been expanding ever since.
Measurements
  • Measurements they come from galactic redshift and the brightness of supernovae.

Part 5 of 7Assumptions and limitations

Constant speed
  • Constant speed the simple calculation assumes every galaxy has moved at a constant speed since the start.
Gravity
  • Gravity it would slow the expansion.
Dark energy
  • Dark energy observations show the expansion has been speeding up in recent billions of years.
Measuring H₀
  • Measuring H₀ it needs accurate distances to galaxies, which are hard to find.
Effect of H₀
  • Effect of H₀ a smaller H₀ gives an older Universe and a larger H₀ gives a younger one.

Part 6 of 7Comparison with other ages

Comparison with other ages
Solar System
  • Solar System about 4.6 billion years old, so about a third of the age of the Universe.
Oldest stars
  • Oldest stars about 13 billion years old, and the Universe cannot be younger than its oldest objects.
Other methods
  • Other methods the cosmic microwave background gives 13.8 billion years.

Part 7 of 7More from the specification

1
Cosmological expansion and Earth observations
  • Cosmological expansion and Earth observations observing distant galactic redshift from Earth provides a means to deduce the cosmic expansion cycle.
2
Balloon analogy
  • Balloon analogy as a balloon inflates, dots on it move apart, and the more distant two dots are, the faster they separate.
3
Balloon analogy
  • Balloon analogy as a balloon inflates, dots on it move apart, and the more distant two dots are, the faster they separate.
4
Stable
  • Returns to its original position when tilted slightly.
5
White dwarf
  • Small, hot, dense core left by a low-mass star.
6
Planetary nebula
  • The shell of gas ejected by a red giant.
7
Fusion
  • Joining of light nuclei into a heavier one.
8
Radiation
  • Transfer by electromagnetic waves, requiring no medium
9
Wavelength
  • Distance between a point on one wave and the same point on the next wave
10
Reaction
  • The partner force acting on the other object.
11
Nebula
  • Cloud of gas and dust in which stars form.
12
Helium
  • Product of the fusion of hydrogen in stars.
13
Light-year
  • The distance light travels in one year, 9.5 × 10¹⁵ m.
14
Spectrum
  • Range of colours in white light.
15
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)
16
Balloon analogy
  • Balloon analogy as a balloon inflates, dots on it move apart, and the more distant two dots are, the faster they separate.

Key terms

TermMeaning
Hubble constant (H₀)Ratio of recession speed to distance, 2.2 × 10^{-18} per second.
Age of the UniverseTime since the Big Bang, estimated as 1 / H₀.
SingularityThe single point from which the Universe began.
Dark energyThe unknown cause of the accelerating expansion.
Recession speedThe speed at which a galaxy moves away from the Earth.

Avoid these mistakes

1
Leaving the age in seconds — divide by 3.15 × 10^{7} to get years.
2
Using H₀ instead of 1 / H₀ for the age — the age is the reciprocal.
3
Giving the age as 4.6 billion years — that is the age of the Solar System.
4
Thinking the age depends on which galaxy is used — in the simple model all give the same value.

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