Light from distant galaxies is shifted towards the red end of the spectrum: its wavelength is longer than expected. This red-shift shows that the galaxies are moving away from us. The further away a galaxy is, the greater its red-shift, so the faster it is moving away. This means the universe is expanding.
The Big Bang theory says the universe began about 13.8 billion years ago from an extremely small, hot and dense region and has been expanding ever since. Further evidence is the cosmic microwave background radiation, the cooled remains of energy from the early universe, detected from every direction. Observations of supernovae suggest the expansion is speeding up, which scientists explain with dark energy; dark matter is proposed to explain how galaxies rotate. Both are not yet understood.
Worked example
Galaxy A has a larger red-shift than galaxy B. What can you conclude?
- Greater red-shift means faster recession.
- Faster-moving galaxies are further away.
Answer:Galaxy A is further away and moving away faster than B.
Key words
- red-shift
- increase in wavelength of light from a receding source
- Big Bang
- theory that the universe began from a tiny, hot, dense point
- CMBR
- cosmic microwave background radiation
- dark matter
- unseen matter inferred from galaxy rotation
New questions every time.
NGSS-referenced: HS-ESS1-2