Origin And Future Of The Universe
- 1. Origin and future of the Universe
- 2. Olbers’ paradox Mid 1920s Hubble’s distance measurements had confirmed stars were grouped into galaxies. Local clusters of galaxies were distributed uniformly in space.
- 3. Although galaxies 500 million light years away were to faint to see, there was no reason to doubt that the universe didn’t continue beyond that point.
- 4. Astronomers had established a cosmological model which agreed with Newton’s static, uniform and infinite universe existing in a framework of absolute space.
- 5. Could this stand up?
- 6. No! Olbers showed that in an infinite universe, the sky at night would be very bright. Why could this be true?
- 7. Consider a shell distance r away from the Earth. Stars in this shell send light to the Earth . Intensity I. Stars in shell 2r have 4 times the volume
- 8. So there are 4 times as many stars Average intensity is ¼ that from stars in shell of radius r. This gives overall intensity I same as before. So adding up all the shells gives an overall intensity on the Earth = to that of the sun’s surface.
- 9. This contradiction Universe must be infinite otherwise it would collapse under its gravitational forces. Yet cannot be infinite otherwise the sky would be bright at night . Is called Olbers paradox.
- 10. Hubble found in 1929 that the Universe was expanding. Goodbye static model!
- 11. Doppler effect Consider a source emitting light at a wavelength λ travelling with velocity c Time for 1 wave to be emitted is λ / c sec If the source is travelling at velocity v. It will have moved away from the observer a distance v x λ / c during the time it takes for one wave to be emitted.
- 12. The change in wavelength will be Δ λ = v x λ / c Δ λ / λ = v / c
- 13. Hubble showed that galaxies are moving apart at speeds that are directly proportional to the distances between them. Evidence of big bang.
- 14. A galaxy 5 x 10 9 light years distant receding at 0.25 times the speed of light. Time taken = distance velocity Time = 5 x 10 9 0.25 = 2 x 10 10 years
- 15. So big bang occurred 20 billion years ago Overestimate as we assumed galaxies are travelling at constant speed They are probably slowing down due to gravitational attraction So 15 billion years is a better estimate.
- 16. To get an accurate value the gradient of the Hubble graph is the Hubble constant.
- 17. Hubble Constant Speed of recession = H 0 Distance H 0 = 0.25 5 x 10 9 H 0 = 5 x 10 -11 year -1