The Runaway Universe

1. The Runaway Universe Dr Martin Hendry Dept of Physics and Astronomy University of Glasgow

2. Cosmology – the study of the Universe as a whole: • Origin • Evolution • Eventual Fate

3. Cosmological theories depend on the available data

4. Ptolemy 90 – 168 AD Almagest (c 140 AD) Earth-Centred Universe

5. Nicolaus Copernicus 1473 – 1543 AD “In the true centre of everything resides the Sun” De Revolutionibus Orbis (1543)

6. Galileo Galilei: 1564 – 1642 AD

7. “I have observed the nature and the material of the Milky Way. With the aid of the telescope this has been scrutinized so directly and with such ocular certainty that all the disputes which have vexed philosophers through so many ages have been resolved, and we are at last freed from wordy debates about it. The galaxy is, in fact, nothing but a collection of innumerable stars grouped together in clusters. Upon whatever part of it the telescope is directed, a vast crowd of stars is immediately presented to view. Many of them are rather large and quite bright, while the number of smaller ones is quite beyond calculation.” Galileo Galilei: 1564 – 1642 AD from The Starry Messenger (1610)

8. Herschel’s Milky Way (1790)

9. The stars are VERY far away. The nearest star (after the Sun) is about 40 million million km from the Earth. It takes light more than 4 years to cross this distance (travelling at a speed of 300,000 km per second) If the Earth-Sun distance were the width of this screen, the nearest star would be inRome !!!

10. Measuring Astronomical Distances: Parallax

11. Measuring Astronomical Distances: Parallax Even the nearest star shows a parallax shift of only 1/2000th the width of the full Moon

12. Measuring Astronomical Distances: Inverse Square Law Brightness falls off with the square of the distance, because surface area of a sphere increases with the square of the radius

13. Measuring Astronomical Distances: Inverse Square Law

14. Cepheid Variables: Cosmic Yardsticks Henrietta Leavitt 1908-1912

15. Herschel’s Milky Way (1790) Shapley’s Model (1915)

16. Early 20th Century The nature of the nebulae?… Gas clouds within the Milky Way, or Island Universes?….

17. The Great Debate, 1920 Shapley vs Curtis at the National Academy of Sciences

18. The Great Debate, 1920 Shapley vs Curtis at the National Academy of Sciences Shapley argues successfully that the nebulae are within the Milky Way

19. 1922: Hubble finds Cepheids in the Great Nebula in Andromeda

20. Hubble measured distances to dozens of nearby nebulae Even the nearest, in Andromeda, was millions of light years distant

21. Hubble also measured the shift in colour, or wavelength, of the light from distant galaxies. Galaxy

22. Hubble also measured the shift in colour, or wavelength, of the light from distant galaxies. Galaxy Laboratory

23. Hubble also measured the shift in colour, or wavelength, of the light from distant galaxies. Galaxy Laboratory

24. Hubble’s Law: 1922 Distant galaxies are receding from us with a velocity proportional to their distance

25. Hubble’s Interpretation ‘Recession of the Nebulae’ caused not by the motion of galaxies through space, but the expansion of space itself between the galaxies

26. Einstein’s General Relativity “Matter tells spacetime how to curve, and spacetime tells matter how to move”

27. How fast is the Universe expanding? H0?

28. Hubble’s Law V = H0 d H0has units of (time)-1 – usually measured in kilometres per second per Megaparsec

29. Hubble’s Law V = H0 d H0has units of (time)-1 – usually measured in kilometres per second per Megaparsec 1 pc = 3.26 light years = 3 1016 m x

30. Hubble’s Law V = H0 d H0has units of (time)-1 – usually measured in kilometres per second per Megaparsec 1 pc = 3.26 light years = 3 1016 m x H0-1 = Hubble time = timescale for the expansion age of the Universe

31. Hubble’s Law V = H0 d Hubble’s original work gave H0= 500 (in conflict with Geological timescale) ‘Modern’ values: dichotomy between H0= 50, and H0= 100 (with small statistical error) ????

32. Principal difficulty has been local distortions in ‘Hubble flow’ e.g. spectrum of M31 is blueshifted

33. Galaxies are clustered Structure in the Universe assembled by gravity

34. Galaxies are clustered Structure in the Universe assembled by gravity Locally, gravity sufficient to overcome cosmic expansion

35. Galaxies are clustered Structure in the Universe assembled by gravity Locally, gravity sufficient to overcome cosmic expansion On larger scales, expansion diluted: galaxies have peculiar velocity on top of Hubble velocity

36. Main local distortion due to Virgo cluster

37. Problem: Need to determine H0 from remote galaxies, where peculiar motions are less important…. ….but…. We cannot use primary distance indicators to measure their distance Need Distance Ladder!!

39. HST Key Project, led by Wendy Freedman Measure Cepheid distances to ~30 nearby galaxies, Link Cepheids to Secondary distance indicators

40. HST has ‘bypassed’ one stage of the Distance Ladder, by observing Cepheids beyond the Local Group of galaxies

41. HST has ‘bypassed’ one stage of the Distance Ladder, by observing Cepheids beyond the Local Group of galaxies This has dramatically improved measurements of H0

42. Virgo Cluster galaxy M100, 60 million light years distant…..

44. HST data also allows correction for the dimming effects of DUST

45. Mustensure that remote galaxy data are free from Selection Effects e.g. intrinsically brighter or bigger?…

46. Mustensure that remote galaxy data are free from Selection Effects e.g. intrinsically brighter or bigger?… Malmquist Bias

47. Mustensure that remote galaxy data are free from Selection Effects e.g. intrinsically brighter or bigger?… Malmquist Bias

48. Mustensure that remote galaxy data are free from Selection Effects e.g. intrinsically brighter or bigger?… Malmquist Bias H0= 70 7

49. Age of the Universe = 10 – 16 billion years (depends if the expansion is speeding up or slowing down)

50. Will the Universe continue to expand forever? To find out we need to compare the expansion rate now with the expansion rate in the distant past… Is the Universe speeding up or slowing down?