RED-SHIFT: light from distant galaxies is shifted towards LONGER WAVELENGTHS (red end of the spectrum).
This is a DOPPLER EFFECT — similar to how a siren sounds lower-pitched as an ambulance drives away.
When a light source moves AWAY from an observer, wavelengths are STRETCHED → shifted to red.
When a light source moves TOWARDS an observer, wavelengths are COMPRESSED → blue-shift.
OBSERVATIONS:
Nearly all galaxies show red-shift → nearly all galaxies are moving AWAY from us.
More distant galaxies show GREATER red-shift → moving away FASTER.
This relationship (velocity proportional to distance) was discovered by Edwin Hubble (1929) — HUBBLE'S LAW.
HUBBLE'S LAW:
Recession speed = Hubble constant × distance
v = H₀ × d
H₀ ≈ 70 km/s/Mpc (current best estimate).
KEY CONCLUSION:
The universe is EXPANDING — all galaxies moving away from all others.
The Big Bang Theory
The expanding universe implies that in the past, all matter was closer together.
Extrapolating backwards → there was a point in time when all matter and energy was concentrated in an incredibly hot, dense state — THE BIG BANG.
BIG BANG TIMELINE:
~13.8 billion years ago: Big Bang — universe begins from an extremely hot, dense singularity.
First 380,000 years: universe too hot for neutral atoms — opaque plasma.
380,000 years: universe cools enough for hydrogen atoms to form → universe becomes transparent.
First light (CMBR emitted).
Hundreds of millions of years: first stars and galaxies form.
~4.6 billion years ago: Solar System forms.
~3.8 billion years ago: life on Earth begins.
Now: observable universe ~93 billion light-years in diameter.
IMPORTANT: The Big Bang was not an explosion IN space — it was an expansion OF space itself. There is no centre of the universe — all points are moving away from all others.
Evidence for an Expanding Universe
EVIDENCE FOR THE BIG BANG:
1. RED-SHIFT OF GALAXIES:
All distant galaxies show red-shift — moving away.
More distant → greater red-shift → faster recession (Hubble's Law).
First and most direct evidence for expansion.
2. COSMIC MICROWAVE BACKGROUND RADIATION (CMBR):
Faint microwave radiation detected uniformly from all directions.
Predicted by Big Bang theory — the afterglow of the hot early universe, now cooled to ~2.7 K.
Discovered accidentally by Penzias and Wilson (1964) — confirmed the Big Bang.
The most compelling evidence for the Big Bang theory.
3. ABUNDANCE OF HYDROGEN AND HELIUM:
Big Bang nucleosynthesis predicted ~75% hydrogen, ~25% helium (by mass).
Observations of the oldest stars match this prediction remarkably well.
4. LARGE-SCALE STRUCTURE:
The distribution of galaxies in the universe matches the predicted evolution from the Big Bang.
UNANSWERED QUESTIONS:
Dark matter: ~27% of universe's mass-energy is 'dark matter' — does not interact with light.
Dark energy: ~68% — mysterious energy causing accelerating expansion.
Original cause of the Big Bang remains unknown.
⚠️ Common Mistake
Red-shift does NOT mean galaxies are moving THROUGH space away from us — it means SPACE ITSELF is expanding between galaxies. The Earth is not at the centre of the universe — all observers in any galaxy would see other galaxies receding. The Big Bang was not an explosion in an existing space — it was the beginning of space and time itself.
📐 Variables
vRecession speed (v) is measured in km/s (km/s)
H₀Hubble constant (H₀) is measured in km/s/Mpc (km/s/Mpc)
dDistance to galaxy (d) is measured in Mpc (Mpc)
📐 Key Equations
v = H₀ × d (Hubble's Law)
📌 Key Note
Red-shift: distant galaxies' light stretched to longer wavelengths → moving away → universe expanding. Hubble's Law: v = H₀d (faster recession = further away). Big Bang: ~13.8 billion years ago, hot dense singularity. Evidence: red-shift, CMBR (2.7 K microwave background), H/He abundance. CMBR = strongest evidence.
🎯 Matching Activity — Evidence for the Big Bang
Match each piece of evidence to the Big Bang theory. — drag the symbols on the right to match the component names on the left.
Red-shift of galaxies
Drop here
Cosmic Microwave Background Radiation
Drop here
H/He abundance
Drop here
Hubble's Law
Drop here
Nearly all galaxies moving away — greater red-shift for more distant galaxies — universe expanding
Faint microwave radiation from all directions — afterglow of the hot early universe at 2.7 K
Recession speed ∝ distance (v = H₀d) — first observational evidence for expansion
~75% hydrogen, ~25% helium — matches Big Bang nucleosynthesis predictions
⭐ Higher Tier Only
Apply Hubble's Law: v = H₀d. Calculate recession speed from red-shift data. Explain how CMBR is strong evidence for the Big Bang. Evaluate the significance of dark matter and dark energy for the Big Bang model and the future of the universe.
🔬 Triple Science Only
Red-shift and Big Bang (physics only) — not in Combined Science.
🎯 Test Yourself
Question 1 of 2
1. What does the red-shift of distant galaxies tell us about the universe?
2. Why is the Cosmic Microwave Background Radiation (CMBR) considered the strongest evidence for the Big Bang?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
Don't get itGetting thereNailed it!
🤖 Ask Mr Badmus AI
Stuck? Just ask! 💬
I'll use FIFA for calculations and flag Higher/Triple content clearly.