E6 Galaxies and the expanding universe (HL only)

1. E6 Galaxies and the expanding universe (HL only)

2. Galactic cluster

3. Galactic supercluster

4. Galactic motion • The red-shift in the light from distant galaxies shows that they are moving away from us and thus the Universe is expanding.

5. Doppler calculations Δλ = v λ c where Δλ is the change in wavelength λ is the original wavelength v is the speed of the galaxy/star c is the speed of light (3 x 108 m.s-1)

6. Example Δλ = v λ c • A hydrogen line has a wavelength of 434 nm. When received from a distant galaxy this line has been red-shifted to a wavelength of 486 nm. What is the recession speed of the galaxy? • Δλ = 486 – 434 = 52 nm • Δλ/ λ = 52/434 = 0.12 • Hence v = 0.12c = 3.6 x 107 m.s-1

7. Hubble’s law The velocity of a galaxy’s recession is directly proportional to its distance from the earth V = H0d

8. Hubble’s Law v = H0d H0 = 72 km.s-1Mpc-1 This value is difficult to measure due to the difficulties and uncertainties in measuring the distance to distant galaxies.

9. Age of the Universe? • Assuming that the galaxies have moved with an approximate constant speed since the big bang, the age of the Universe can be estimated from time = distance/speed • = d/H0d = 1/H0 • = 1/(72 x 103 m.s-1.Mpc-1) • = 1/(72 x 103 m.s-1) x 106pc • = 1/(72 x 103 m.s-1) x 106 x 3.09 x 1016 m • = 4.29 x 1017 s = 13.6 x 109 years

10. Formation of atoms • The early Universe (<300,000 years) was so hot that atoms could not form and only elementary particles could exist (n, p and e). • As the Universe expanded it cooled meaning that first atomic nucei could form (after 3 minutes) and later electrons could be bound to nuclei to form atoms (after 300 000 years)

11. Questions Page 541 Questions 3, 7, 9, 10,