IGCSE Physics Universe Questions and Worked Solutions (6.2.3) | 0625

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IGCSE Physics 0625 · Topic 6.2.3

IGCSE Physics Universe Questions and Worked Solutions

50 topic-wise practice questions on The Universe (Unit 6: Space Physics), written to the Cambridge IGCSE Physics 0625 syllabus for 2026–2028, each with a step-by-step worked solution.

50 questions13 Core27 Extended10 Challenge125 marksTextbook §6.2 · pp. 286–294
Questions tagged Core are for all candidates (Papers 1 & 3). Questions tagged Extended are Supplement content (Papers 2 & 4). Questions 41–50 are harder Challenge questions. Try each question first, then tap Show solution to check your working.

Topic guide: The Universe

The Milky Way is one of billions of galaxies. Light from distant galaxies is redshifted, showing the Universe is expanding — evidence for the Big Bang. (Extended) The CMBR and the Hubble constant give further evidence and an estimate of the Universe's age.

Key formulas

H₀ = v ÷ dHubble constant (s⁻¹), recession speed v (m/s), distance d (m)Extended
H₀ ≈ 2.2 × 10⁻¹⁸ s⁻¹current estimateExtended
age of Universe ≈ d ÷ v = 1 ÷ H₀ExtendedExtended

Prerequisite knowledge

  • Light-years (6.2.2)
  • EM spectrum and wavelength (3.3)
  • Standard form

Exam tips & common mistakes

  • Redshift = longer wavelength, not 'turning red'
  • Convert light-years to metres before using H₀
  • 1/H₀ gives seconds; divide by 3.15 × 10⁷ for years

Key facts from the syllabus (6.2.3)

  • The Milky Way is about 100 000 light-years across and is one of billions of galaxies
  • Redshift: an increase in the observed wavelength of light from receding stars and galaxies
  • Light from distant galaxies is redshifted compared with light emitted on Earth
  • Redshift is evidence that the Universe is expanding and supports the Big Bang Theory
  • Extended CMBR — microwave radiation observed from all directions, produced shortly after the Universe formed and stretched to microwaves by expansion
  • Extended a galaxy's speed comes from its redshift; its distance from the brightness of a supernova in it
  • Extended 1/H₀ estimates the age of the Universe — evidence all matter was once at a single point

Textbook: §6.2 Stars and the Universe, pp. 286–294

1
Core[1 mark]

State what the Milky Way is part of.

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Method: Universe of galaxies.

One of many billions of galaxies making up the Universe
2
Core[1 mark]

State the approximate diameter of the Milky Way.

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Method: Syllabus.

About 100 000 light-years
3
Core[2 marks]

Define redshift.

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Method: Wavelength increase.

An increase in the observed wavelength of electromagnetic radiation from receding stars and galaxies
4
Core[2 marks]

State what redshift in light from distant galaxies is evidence for.

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Method: Expansion.

The Universe is expanding; it supports the Big Bang Theory
5
Core[2 marks]

Compare light from distant galaxies with light emitted on Earth.

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Method: Redshifted.

It appears redshifted (shifted to longer wavelengths)
6
Core[2 marks]

Explain how redshift shows that galaxies are moving away from us.

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Method: Receding source.

Light from a receding source has its wavelength stretched, so the observed wavelength is longer
7
Core[2 marks]

State what the Universe is made up of.

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Method: Billions of galaxies.

Many billions of galaxies
8
Core[2 marks]

State the name of our galaxy and its approximate diameter.

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Method: Milky Way.

The Milky Way, about 100 000 light-years
9
Core[2 marks]

State what is observed in the spectrum of light from a distant galaxy compared with the Sun.

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Method: Redshift.

The spectral lines are shifted towards the red end (longer wavelengths)
10
Core[2 marks]

State what redshift tells us about the motion of distant galaxies.

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Method: Moving away.

They are moving away from us
11
Core[2 marks]

State the theory supported by redshift evidence.

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Method: Big Bang.

The Big Bang Theory
12
Core[2 marks]

Explain how the Big Bang Theory accounts for the observed redshift.

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Method: Expansion.

The Universe began from a single point and has been expanding, so galaxies are moving apart
13
Core[2 marks]

Explain the difference between a galaxy and the Universe.

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Method: Part vs whole.

A galaxy is a group of billions of stars; the Universe contains all the billions of galaxies
14
Extended[1 mark]

State what CMBR stands for.

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Method: Cosmic microwave background radiation.

Cosmic microwave background radiation
15
Extended[2 marks]

Explain the origin of the CMBR.

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Method: Early universe, stretched.

It was produced shortly after the Universe formed and has been stretched into the microwave region as the Universe expanded
16
Extended[2 marks]

State how the speed of a receding galaxy is found.

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Method: Change in wavelength.

From the change in wavelength (redshift) of its starlight
17
Extended[1 mark]

State how the distance to a far galaxy can be found.

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Method: Supernova brightness.

Using the brightness of a supernova in that galaxy
18
Extended[1 mark]

Define the Hubble constant.

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Method: H₀ = v/d.

The ratio of a galaxy's speed of recession to its distance from Earth
19
Extended[1 mark]

State the current estimate of H₀.

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Method: Syllabus.

2.2 × 10⁻¹⁸ per second
20
Extended[3 marks]

A galaxy is 5.0 × 10²⁴ m away. Calculate its speed of recession (H₀ = 2.2 × 10⁻¹⁸ /s).

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Method: v = H₀d.

v = 2.2×10⁻¹⁸ × 5.0×10²⁴ = 1.1 × 10⁷ m/s
21
Extended[3 marks]

A galaxy recedes at 2.0 × 10⁷ m/s. Calculate its distance (H₀ = 2.2 × 10⁻¹⁸ /s).

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Method: d = v/H₀.

d = 2.0×10⁷ ÷ 2.2×10⁻¹⁸ = 9.1 × 10²⁴ m
22
Extended[3 marks]

Use H₀ = 2.2 × 10⁻¹⁸ /s to estimate the age of the Universe in seconds.

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Method: t = 1/H₀.

t = 1 ÷ 2.2×10⁻¹⁸ = 4.5 × 10¹⁷ s
23
Extended[3 marks]

Convert the age 1/H₀ = 4.5 × 10¹⁷ s into years (1 year = 3.2 × 10⁷ s).

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Method: Divide.

Age ≈ 4.5×10¹⁷ ÷ 3.2×10⁷ = 1.4 × 10¹⁰ years
24
Extended[2 marks]

Explain why d/v = 1/H₀ is evidence that all matter in the Universe was once at a single point.

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Method: Same time for all galaxies.

Every galaxy gives the same value of d/v, so all galaxies would have been at the same point at the same time in the past
25
Extended[3 marks]

Galaxy A is twice as far away as galaxy B. Compare their recession speeds.

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Method: v ∝ d.

Galaxy A recedes twice as fast
26
Extended[3 marks]

A galaxy is 3.0 × 10⁹ light-years away. Calculate its distance in metres and its recession speed (H₀ = 2.2 × 10⁻¹⁸ /s).

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Method: d in m, then v = H₀d.

d = 2.8 × 10²⁵ m; v = 6.3 × 10⁷ m/s
27
Extended[2 marks]

Explain why the CMBR is now in the microwave region.

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Method: Wavelength stretched.

As the Universe expanded, the radiation's wavelength was stretched from shorter wavelengths into microwaves
28
Extended[2 marks]

State where CMBR comes from in the sky.

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Method: All directions.

From all directions in space
29
Extended[2 marks]

State the region of the electromagnetic spectrum of CMBR.

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Method: Microwave.

Microwave
30
Extended[2 marks]

Explain why CMBR supports the Big Bang Theory.

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Method: Remnant radiation.

It is radiation left over from the early hot Universe, stretched by expansion, as the theory predicts
31
Extended[2 marks]

State what measurement gives the speed of a galaxy.

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Method: Redshift.

The change in wavelength (redshift) of its light
32
Extended[2 marks]

State what is used to measure the distance to a far galaxy.

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Method: Supernova.

The brightness of a supernova in it
33
Extended[3 marks]

A galaxy is 1.0 × 10²⁵ m away. Calculate its recession speed (H₀ = 2.2 × 10⁻¹⁸ /s).

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Method: v = H₀d.

v = 2.2 × 10⁷ m/s
34
Extended[3 marks]

A galaxy recedes at 6.6 × 10⁶ m/s. Calculate its distance.

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Method: d = v/H₀.

d = 3.0 × 10²⁴ m
35
Extended[3 marks]

Calculate the distance in light-years of a galaxy receding at 1.1 × 10⁷ m/s.

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Method: d = v/H₀ then ÷ LY.

d = 5.0 × 10²⁴ m = 5.3 × 10⁸ light-years
36
Extended[3 marks]

A galaxy is 4.0 × 10²⁵ m away and recedes at 8.8 × 10⁷ m/s. Calculate H₀ from these data.

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Method: H₀ = v/d.

H₀ = 2.2 × 10⁻¹⁸ /s
37
Extended[3 marks]

Use H₀ = 2.2 × 10⁻¹⁸ /s to estimate the age of the Universe in years (1 year = 3.15 × 10⁷ s).

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Method: t = 1/H₀.

t = 1.4 × 10¹⁰ years
38
Extended[2 marks]

State why the estimate 1/H₀ is called the age of the Universe.

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Method: Time since all at one point.

It is the time since all galaxies would have been together at a single point
39
Extended[2 marks]

Explain why more distant galaxies show greater redshift.

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Method: Faster recession.

They are moving away faster (v ∝ d)
40
Extended[2 marks]

State the effect of a larger value of H₀ on the estimated age of the Universe.

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Method: Younger.

The estimated age would be smaller

Challenge questions

Harder, multi-step questions at the top end of the exam. Attempt these once you are confident with the questions above.

41
ChallengeCore[4 marks]

Explain what redshift is and how observations of distant galaxies support the idea that the Universe is expanding.

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Method: Redshift evidence.

Light from receding galaxies is shifted to longer wavelengths; almost all distant galaxies show redshift, so they are moving away — the Universe is expanding
42
ChallengeCore[4 marks]

Explain how the Big Bang Theory accounts for the fact that more distant galaxies have larger redshifts.

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Method: Expansion.

If the Universe expanded from a single point, galaxies further away are moving away faster, so their light is more redshifted
43
ChallengeExtended[5 marks]

A galaxy is 2.0 × 10⁹ light-years away. Calculate its distance in metres and its recession speed (H₀ = 2.2 × 10⁻¹⁸ s⁻¹).

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Method: d then v = H₀d.

d = 1.9 × 10²⁵ m; v = 4.2 × 10⁷ m/s
44
ChallengeExtended[4 marks]

A galaxy recedes at 3.3 × 10⁶ m/s. Calculate its distance in metres and light-years.

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Method: d = v/H₀.

d = 1.5 × 10²⁴ m = 1.6 × 10⁸ light-years
45
ChallengeExtended[4 marks]

Use H₀ = 2.2 × 10⁻¹⁸ s⁻¹ to estimate the age of the Universe in seconds and years (1 year = 3.15 × 10⁷ s). Explain the reasoning.

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Method: t = 1/H₀.

t = 4.5 × 10¹⁷ s = 1.4 × 10¹⁰ years; d/v is the time for galaxies to reach their present distance if they started together at one point
46
ChallengeExtended[4 marks]

Data for two galaxies: A at 1.5 × 10²⁵ m receding at 3.3 × 10⁷ m/s; B at 6.0 × 10²⁵ m receding at 1.3 × 10⁸ m/s. Calculate H₀ from each and comment.

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Method: H₀ = v/d.

A: 2.2 × 10⁻¹⁸ s⁻¹; B: 2.2 × 10⁻¹⁸ s⁻¹ — consistent, about 2.2 × 10⁻¹⁸ s⁻¹
47
ChallengeExtended[4 marks]

Explain what the CMBR is, how it was produced and why it is now detected as microwaves.

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Method: Origin and stretching.

Radiation from shortly after the Universe formed; as the Universe expanded, its wavelength was stretched into the microwave region; it is detected from all directions
48
ChallengeExtended[4 marks]

Describe how astronomers find (a) the speed and (b) the distance of a far galaxy.

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Method: Redshift and supernovae.

(a) Measure the change in wavelength (redshift) of its starlight; (b) observe a supernova in it and use its brightness
49
ChallengeExtended[4 marks]

Explain why the CMBR is considered strong evidence for the Big Bang Theory.

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Method: Predicted relic.

The theory predicts a hot early Universe whose radiation would now be stretched to microwaves and seen in all directions — exactly what is observed
50
ChallengeExtended[3 marks]

If a larger value of H₀ were measured, explain the effect on the estimated age of the Universe.

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Method: t = 1/H₀.

The estimated age would be smaller, since age ≈ 1/H₀
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