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
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Cosmic Microwave Background Radiation
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H/He abundance
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Hubble's Law
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~75% hydrogen, ~25% helium — matches Big Bang nucleosynthesis predictions
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
Nearly all galaxies moving away — greater red-shift for more distant galaxies — universe expanding
🔬 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?
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