MEPX
Chapter 11 of 12All chapters
  1. 1The scale of things
  2. 2The Sun
  3. 3The rocky planets
  4. 4The giant planets
  5. 5Moons
  6. 6Asteroids and comets
  7. 7Gravity and orbits
  8. 8Light and distance
  9. 9Stars and their lives
  10. 10Galaxies
  11. 11The expanding universe
  12. 12Observing for yourself

Chapter 11 of 12

The expanding universe

How we know it started.

Expansion

Distant galaxies are moving away from us, and the further off they are the faster they recede. Running that backwards gives a hot dense beginning about 13.8 billion years ago.

  • Space itself expands, so galaxies are not flying through space away from a centre.
  • The cosmic microwave background is the leftover glow of that early heat.

What we cannot see

Dark matter is detected only by its gravity and outweighs ordinary matter several times over. Dark energy is the name for whatever is accelerating the expansion, and both remain unexplained.