Red Shift & Big Bang, Knowledge Organiser

Key Terms

• Red shift — wavelength increases, galaxy moving away
• Doppler effect — wave change due to relative motion
• CMBR — 2.7 K microwave radiation from all directions
• Big Bang — origin of universe 13.8 billion years ago
• Expanding universe: space is stretching, galaxies move apart
• Absorption spectrum — dark lines used to measure red shift

Evidence for Big Bang

• Red shift: galaxies moving away, universe expanding
• More distant = greater red shift = faster recession
• CMBR: predicted then discovered, from all directions
• CMBR temperature: about 2.7 K
• Amounts of hydrogen and helium match Big Bang predictions

Key Distinctions

• Red shift: longer wavelength, galaxy receding
• Blue shift: shorter wavelength, galaxy approaching
• Andromeda: blue-shifted (coming towards us)
• Space expands — galaxies carried with it, not through it
• Big Bang: created space itself, not explosion in space

CMBR Key Facts

• Temperature: approximately 2.7 K
• Detected from all directions equally
• Cooled-down afterglow of hot early universe
• Predicted before it was discovered (1965)
• Originally high-energy radiation, now stretched to microwaves

Key Ideas

• Greater distance → greater red shift → greater speed away from us
• Light from a galaxy moving away has a longer wavelength
• The universe is about 13.8 billion years old

Common Mistakes

• Saying galaxies move through space away from us: Space itself is expanding — galaxies are carried apart by the expansion of space, not moving through it like objects in a room
• Confusing red shift and blue shift: Red shift means a galaxy is moving away (wavelength stretched, frequency reduced); blue shift means moving towards us — all distant galaxies show red shift
• Saying the Big Bang happened at a single point in space: The Big Bang was an expansion of space itself from an extremely hot, dense state — it did not happen "somewhere" in pre-existing space
• Thinking CMBR is visible light: The cosmic microwave background radiation is in the microwave part of the EM spectrum — it was originally high-energy radiation stretched by the expansion of the universe
• Saying red shift only proves the universe is expanding: Red shift AND CMBR together provide the strongest evidence for the Big Bang — red shift alone could have other explanations; CMBR is the "smoking gun"

Memory Aid

Red Shift Direction — "Red means Running Away" Red = longer wavelength = source moving away. Think of a red flashing light on the back of a retreating ambulance — red light means going away from you. Red shift = galaxy running away. Two Pieces of Evidence for Big Bang — "Red CMBR" The two main pieces of evidence are: Red shift (galaxies moving apart — universe expanding) and CMBR (afterglow radiation from hot early universe). Remember them together as a pair: "Red and Warm" — Red shift shows expansion, CMBR shows the warmth that remains. CMBR Temperature — "3 Degrees" The CMBR temperature is about 2.7 K — just remember "nearly 3 degrees above absolute zero." It is incredibly cold because the universe has been cooling and expanding for nearly 14 billion years since the Big Bang.