Ch 5 · 6.2.3 The Universe · O Level Physics 5054
Topic 5 of 5 in Space Physics — O Level Physics 5054 — 49 lessons.
Introduction
Our galaxy is so big that light takes 100 000 years to cross it, and it is only one of billions. Even more surprising: almost every other galaxy is rushing away from us. That discovery led to the idea that the Universe began with a Big Bang. (Syllabus section 6.2.3.)
Key ideas — The scale of the Universe
🧑🏫 Teacher's explanation
Imagine you are standing on a sidewalk and an ambulance drives past you. As it approaches, the siren sounds high-pitched, but as it moves away, the pitch drops to a lower sound. This is the Doppler Effect. Light behaves similarly. If a galaxy is moving away from us, the light waves it emits get 'stretched out'. Since longer wavelengths correspond to the red end of the visible spectrum, we call this 'redshift'.
📖 Definitions
🔍 Worked example — Easy
A galaxy is observed to have a redshifted spectrum. If the light was emitted at a wavelength of 650 nm and is observed at 670 nm, calculate the change in wavelength Δλ.
🔍 Worked example — Standard
A student observes two galaxies, A and B. Galaxy A shows a small redshift, while Galaxy B shows a much larger redshift. Compare the distance and speed of these galaxies from Earth.
🔍 Worked example — Exam-style
Explain how the observation of redshift in distant galaxies supports the Big Bang theory.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Define 'redshift'. [1] (b) State how the observed redshift of a galaxy relates to its distance from the Earth. [1] (c) Explain how the observation of redshift provides evidence for the Big Bang theory. [2] [4 marks]
Key ideas — Redshift
When a source of waves is moving away from an observer, the waves it emits are stretched out, so the observer detects a longer wavelength than was emitted.
🧑🏫 Teacher's explanation
Imagine you are standing on a sidewalk and an ambulance drives past you. As it approaches, the siren sounds high-pitched, but as it moves away, the pitch drops to a lower, deeper sound. This happens because the sound waves are being 'stretched out' by the movement of the vehicle.
📖 Definitions
🔍 Worked example — Easy
A star emits light with a wavelength of 600 nm. If the star is moving away from Earth, will the observed wavelength be greater than or less than 600 nm?
🔍 Worked example — Standard
A galaxy shows a redshift. If Galaxy A has a redshift of 20 nm and Galaxy B has a redshift of 50 nm, which galaxy is moving away from Earth at a higher speed?
🔍 Worked example — Exam-style
Explain how the observation of redshift in distant galaxies provides evidence for the Big Bang theory.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Define redshift. [2] (b) Explain how this observation supports the Big Bang theory. [2] [4 marks]
Key ideas — Galaxies are moving away from us
Astronomers find that:
🧑🏫 Teacher's explanation
Imagine you are standing on a sidewalk and an ambulance drives past you with its siren blaring. As it approaches, the pitch is high, but as it moves away, the pitch drops to a lower tone. This is the Doppler effect for sound. Light behaves similarly. When a galaxy moves away from us, the light waves it emits are 'stretched out'. Since red light has a longer wavelength than blue light, this stretching makes the light appear more red than it actually is. This phenomenon is called redshift.\n\nThink of the Universe like a giant balloon being inflated. If you draw dots on the balloon to represent galaxies, as you blow air in, the dots move further apart. The more you blow, the faster they move away from each other. In the same way, the further a galaxy is from Earth, the more space is expanding between us, meaning it appears to be moving away faster and showing a greater redshift.
📖 Definitions
🔍 Worked example — Easy
A galaxy is observed to have its spectral lines shifted towards longer wavelengths. What does this indicate about the motion of the galaxy relative to Earth?
🔍 Worked example — Standard
Galaxy A shows a small redshift. Galaxy B shows a much larger redshift. Compare the distance and speed of Galaxy B relative to Galaxy A.
🔍 Worked example — Exam-style
Explain how the observation of redshift in distant galaxies supports the Big Bang theory.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Evidence for the Big Bang
If all distant galaxies are moving away, and the furthest are moving fastest, then the Universe is expanding. Run the expansion backwards in time, and all the galaxies come together. This suggests that the Universe began from a single, extremely small, hot, dense point, and has been expanding ever since: the Big Bang theory.
🧑🏫 Teacher's explanation
Imagine you are standing on a sidewalk and an ambulance drives past you with its siren on. As it approaches, the sound waves are squashed together, making a high-pitched sound. As it moves away, the sound waves are stretched out, making a lower-pitched sound. Light behaves similarly. When a galaxy moves away from us, the light waves it emits are 'stretched' out, shifting the color toward the red end of the spectrum. This is called redshift.\n\nThink of the Universe like a giant balloon with dots drawn on it. As you blow air into the balloon, the dots move further apart. If you filmed this process and played it in reverse, all the dots would eventually meet at a single point. This is the core logic of the Big Bang: because we see galaxies moving away (redshift), we conclude the Universe is expanding from a single, tiny, hot, dense point.
📖 Definitions
🔍 Worked example — Easy
A star is observed to have its light shifted towards the red end of the spectrum. State what this tells us about the motion of the star relative to Earth.
🔍 Worked example — Standard
Galaxy A is 10 × 10^6 light-years away and shows a certain amount of redshift. Galaxy B is 50 × 10^6 light-years away. Compare the redshift and speed of Galaxy B to Galaxy A.
🔍 Worked example — Exam-style
Explain how the observation of redshift in distant galaxies provides evidence for the Big Bang theory.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Light from galaxy A shows a small redshift; light from galaxy B shows a much larger redshift. What can you conclude?
🔍 Worked example — Easy
Galaxy X shows a small redshift, while Galaxy Y shows a large redshift. Compare their speeds and distances from Earth.
🔍 Worked example — Standard
A star shows a redshift corresponding to a velocity of v = 500 km/s. Another galaxy shows a redshift corresponding to v = 1500 km/s. How many times faster is the second galaxy moving?
🔍 Worked example — Exam-style
Explain how the observation of redshift in distant galaxies supports the Big Bang theory.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) Many billions; about 100 000 light-years. (2) An increase in the observed wavelength of electromagnetic radiation from stars and galaxies that are moving away from us. (3) The further away the galaxy, the greater the redshift (and the faster it is moving away). (4) Distant galaxies all show redshift, so they are moving away, and the furthest move fastest; so the Universe is expanding, and if we trace the expansion back, everything started from a single point: the Big Bang.