1. The Cosmological Distance Ladder: the key to understanding the universe Michael Rowan-Robinson Imperial College

2. Aug 11th 2008Dublin Understanding our universe our understanding of the universe we inhabit has always been our understanding of the universe we inhabit has always been intimately connected with our ability to measure distance intimately connected with our ability to measure distance this was true for the Greeks, and it is true of the most recent this was true for the Greeks, and it is true of the most recent discoveries based on fluctuations in the cosmic microwave discoveries based on fluctuations in the cosmic microwave background radiation, which is the relic of the hot Big Bang background radiation, which is the relic of the hot Big Bang

3. Aug 11th 2008Dublin First steps on the distance ladder Aristotle (384-322 BC) - estimated the size of the earth - estimated the size of the earth (+ Eratosthenes, Poseidonius, 10%) Hipparcos (2 nd C BC) - estimated distance of the moon - estimated distance of the moon (59 R E, cf modern value 60.3) Aristotle, by Raphael

4. Aug 11th 2008Dublin The Copernican revolution Copernicus (1473-1543) - gave the correct relative distances of the sun and planets - gave the correct relative distances of the sun and planets - absolute value not determined accurately till the 19 th century - absolute value not determined accurately till the 19 th century - stars had to be much further away than for earth-centred model - stars had to be much further away than for earth-centred model

5. Aug 11th 2008Dublin The first steps outside the solar system Bessel 1838 - discovered parallax of nearby star 61 Cyg, its change in - discovered parallax of nearby star 61 Cyg, its change in apparent direction on the sky due to the earth’s orbit round the sun (the final proof of the Copernican system)

6. Aug 11th 2008Dublin The key modern distance indicator – Cepheid variable stars Delta Cephei is the prototype of the Cepheid variable stars, massive stars which pulsate and vary their light output

7. Aug 11th 2008Dublin Henrietta Leavitt’s breakthrough In 1912, Henrietta Leavitt, working at the Harvard Observatory, discovered from her studies of Cepheids in the Small Magellanic Cloud that the period of Cepheid variability was related to luminosity

8. Aug 11th 2008Dublin The distances of the galaxies In 1924 Edwin Hubble used Leavitt’s discovery to estimate the distance of the Andromeda Nebula. It clearly lay far outside the Milky Way System System. This opened up the idea of a universe of galaxies.

9. Aug 11th 2008Dublin The expansion of the universe Five years later he announced, based on distances to 18 galaxies, that the more distant a galaxy, the faster it is moving away from us velocity/distance = constant, H o (the Hubble law) This is just what would be expected in an expanding universe. The Russian mathematician Alexander Friedmann had shown that expanding universe models are what would be expected according to Einstein’s General Theory of Relativity, if the universe is homogeneous (everyone sees the same picture) and isotropic (the same in every direction).

10. Aug 11th 2008Dublin The history of the Hubble constant Hubble’s estimate of the H o, the Hubble constant, was 500 km/s/Mpc, which gave an age for the universe of only 2 billion years. This was soon shown to be shorter than the age of the earth. From 1927 to 2001 the value of the Hubble constant was a matter of fierce controversy.

11. Aug 11th 2008Dublin The cosmological distance ladder Astronomers have used a ladder of distance estimators to reach out to the distant galaxies. These have often given inconsistent results.

12. Aug 11th 2008Dublin The Hubble Space Telescope Key Program Following the first HST servicing mission, which fixed the telescope aberration, a large amount of HST observing time was dedicated to measuring Cepheids in distant galaxies, to try to measure the Hubble constant accurately.

13. Aug 11th 2008Dublin Some of the galaxies studied by the Hubble Space Telescope

14. Aug 11th 2008Dublin The HST Key program final result log V H o = 72 km/s/Mpc uncertainty 10% (2001)

15. Aug 11th 2008Dublin Implications of the Hubble constant H o is (velocity/distance) so has the dimensions of (1/time). 1/H o is the expansion age of the universe (how old the Universe would be if no forces acting) = 13.6 billion yrs For simplest model universe with only gravity acting, age of universe would be 9.1 billion years (gravity slows expansion)

16. Aug 11th 2008Dublin The age of the universe We can use the colours and brightnesses of the stars in globular clusters to estimate the age of our Galaxy ~ 12 billion years ~ 12 billion years Long-lived radioactive isotopes give a similar answer Allowing time for our Galaxy to form, the age of the universe is ~ 13 billion years ~ 13 billion years

17. Aug 11th 2008Dublin The age of the universe problem n A problem for the simplest models, where gravity slows down the expansion n To get consistency between the HST Key Program value of H o and the observed age of the universe, we need to reverse the deceleration of the universe n Uncertainties in H o are - (1) distance of Large Magellanic Cloud, - (2) corrections for dust extinction, - (3) corrections for local flow

18. Aug 11th 2008Dublin How much matter is there in the universe ? The light elements D, He, Li are generated from nuclear are generated from nuclear reactions about 1 minute reactions about 1 minute after the Big Bang. The after the Big Bang. The abundances turn out to abundances turn out to depend sensitively on the depend sensitively on the density of ordinary matter density of ordinary matter in the universe. in the universe. density ~ 4.10 -28 kg/cu m density ~ 4.10 -28 kg/cu m  b ~ 0.04  b ~ 0.04

19. Aug 11th 2008Dublin Evidence for Dark Matter The speed at which stars orbit round a galaxy points to the existence of a halo of dark matter. Sensitive surveys show that this can not be due to stars, or gas.

20. Aug 11th 2008Dublin Evidence for Dark Matter 2 Images of clusters of galaxies with HST show arcs due to gravitational lensing. These can be used to weigh the cluster. Again, the cluster is dominated by dark matter. Abell 2218

21. Aug 11th 2008Dublin Search for Dark Matter The most likely candidate for dark matter is the neutralino, a particle matter is the neutralino, a particle predicted in ‘supersymmetric’ theories predicted in ‘supersymmetric’ theories Several searches are under way world-wide, including this UK world-wide, including this UK experiment at Boulby Potash mine experiment at Boulby Potash mine (Imperial, Rutherford Lab) (Imperial, Rutherford Lab) Some anomalous events found, but probably not the neutralino probably not the neutralino

22. Aug 11th 2008Dublin Mapping the Universe

23. Aug 11th 2008Dublin Large scale structure The 3-dimensional distribution of distribution of galaxies shows galaxies shows structure on structure on different scales. This can be used to estimate the to estimate the average density average density of the universe of the universe    ~ 0.27    ~ 0.27

24. Aug 11th 2008Dublin Need for Dark Matter So there is far more matter (    ~ 0.27 ) out there than can be accounted for by the stuff we are made of (    ~ 0.04). 90% of the matter in the universe is ‘dark’ matter (the neutralino ?) Particle Physicists hope to detect this at the Large Hadron Collider (switch-on later this year)

25. Aug 11th 2008Dublin Tycho Brahe’s supernova NRAO NASA/Chandra Tycho Brahe observed a supernova in Casseiopeia in 1572.

26. Aug 11th 2008Dublin Supernova 1987A The nearest supernova of modern times - supernova 1987A 1987A in the Large in the Large Magellanic Magellanic Cloud Cloud

27. Aug 11th 2008Dublin The Large Magellanic Cloud: a satellite of the Milky Way

28. Aug 11th 2008Dublin Supernovae as Standard candles Type Ia supernovae (explosion of white dwarf star) seem to be remarkably uniform in their light curves. They behave like ‘standard candles’ and can be used to estimate distances.

29. Aug 11th 2008Dublin Distant Type Ia supernovae Recently a breakthrough in search techniques, using 4-m telescopes to locate new supernovae, and using 4-m telescopes to locate new supernovae, and 8-m telescopes plus the Hubble Space Telescope to 8-m telescopes plus the Hubble Space Telescope to follow them up, has resulted in the detection follow them up, has resulted in the detection of Type Ia supernovae at huge distances. of Type Ia supernovae at huge distances.

30. Aug 11th 2008Dublin examples of Supernovae examples of Supernovae

31. Aug 11th 2008Dublin Evidence for dark energy Over 100 Type Ia supernova have been supernova have been found at redshifts 0.5-1.5 found at redshifts 0.5-1.5 Comparing these to nearby supernova, we find that in supernova, we find that in cosmological models with cosmological models with matter only, the distant matter only, the distant supernovae are fainter than supernovae are fainter than expected for their redshift. expected for their redshift.(2002) redshift

32. Aug 11th 2008Dublin Will the mutual gravitational attraction of galaxies & clusters be sufficient to slow down the expansion of the Universe enough to make it contract to a `Big Crunch’? Or will it expand for ever? The Fate of the Universe

33. Aug 11th 2008Dublin Mean distance between galaxies today fainter  M = 1 Time Closed  M > 1 Open  M < 1  M = 0 - 14- 9- 7 billion years

34. Aug 11th 2008Dublin Galaxies are further from us than the simple decelerating models, with just gravity acting, would predict: the deceleration is slowing. The Universe is accelerating!! What causes the acceleration?

35. Aug 11th 2008Dublin What is causing the Universe to accelerate? Dark Energy

36. Aug 11th 2008Dublin What is Dark Energy ? According to Einstein’s General Theory of Relativity, According to Einstein’s General Theory of Relativity, there can be an extra term in the equation for there can be an extra term in the equation for gravity, which on large scales turns gravity into a gravity, which on large scales turns gravity into a repulsive force (the ‘cosmological repulsion’) repulsive force (the ‘cosmological repulsion’) This extra term, denoted , behaves like the energy density of the vacuum, hence ‘dark energy’ This extra term, denoted , behaves like the energy density of the vacuum, hence ‘dark energy’ So far there is no particle physics explanation for this So far there is no particle physics explanation for this dark energy dark energy

37. Aug 11th 2008Dublin The discovery of the Cosmic Microwave Background The discovery of the Cosmic Microwave Background (CMB) by Penzias and Wilson in 1965, and the confirmation of its blackbody Penzias and Wilson in 1965, and the confirmation of its blackbody spectrum by COBE in 1991, showed that we live in a hot Big spectrum by COBE in 1991, showed that we live in a hot Big Bang universe, dominated by radiation in its early stages. Bang universe, dominated by radiation in its early stages.

38. Aug 11th 2008Dublin History of the universe

39. Aug 11th 2008Dublin the deepest image of the universe, the Hubble Deep field, with galaxies seen only 2 billion years after the Big Bang. Today many of the objects in this image would have merged into a single big galaxy the deepest image of the universe, the Hubble Deep field, with galaxies seen only 2 billion years after the Big Bang. Today many of the objects in this image would have merged into a single big galaxy

40. Aug 11th 2008Dublin The CMB is incredibly smooth, to one part in 100,000, but the very small fluctuations, or ‘ripples’, are the precursors of the structure we see today. They also tell us about the matter and energy present in the early universe. Mapping the Cosmic Microwave Background (CMB)

41. Aug 11th 2008Dublin What the CMB structure tells us The most prevalent scale in the structure is literally in the structure is literally an echo of the ‘Bang’ an echo of the ‘Bang’ (the acoustic horizon). (the acoustic horizon). The angular scale of this peak tells us that the peak tells us that the universe is close to being universe is close to being spatially flat. In General spatially flat. In General Relativity, this implies Relativity, this implies    ~ 1    ~ 1

42. Aug 11th 2008Dublin angular diameter distance test courtesy: Paniez Paykari

43. Aug 11th 2008Dublin The current cosmological consensus Type Ia supernova need  ~ 0.73 Type Ia supernova need  ~ 0.73 Large scale structure needs Large scale structure needs  o ~ 0.27 CMB structure needs  o +  ~ 1 CMB structure needs  o +  ~ 1 - ( all these with uncertainty of 0.05) so we seem to have a consensus ! so we seem to have a consensus !

44. Aug 11th 2008Dublin Dark energy Audit of the mass-energy of the Universe: 4% ordinary matter 23% dark matter 73% Dark energy

45. Aug 11th 2008Dublin Origin of the universe there are speculations about the origin of the universe there are speculations about the origin of the universe theoretical physicists are trying to unify gravitation (ie General Relativity) and theoretical physicists are trying to unify gravitation (ie General Relativity) and quantum theory into a single unified ‘theory of everything’ quantum theory into a single unified ‘theory of everything’ current favourite is ‘string theory’, but so far this makes no predictions about current favourite is ‘string theory’, but so far this makes no predictions about the observed universe, instead have the ‘string landscape’ the observed universe, instead have the ‘string landscape’ one popular idea is ‘chaotic inflation’ - our universe arose out of a vacuum one popular idea is ‘chaotic inflation’ - our universe arose out of a vacuum fluctuation in an infinite fluctuating void fluctuation in an infinite fluctuating void in this picture there might be many parallel universes, each with different in this picture there might be many parallel universes, each with different properties - the ‘multiverse’ properties - the ‘multiverse’ currently no evidence to support this idea, or the ‘anthropic principle’, which is currently no evidence to support this idea, or the ‘anthropic principle’, which is supposed to select which type of universe we find ourselves in supposed to select which type of universe we find ourselves in

46. Aug 11th 2008Dublin Fate of the universe if the current consensus model, with a dominant role for dark energy, is if the current consensus model, with a dominant role for dark energy, is correct, the fate of the universe is a bleak one correct, the fate of the universe is a bleak one the distances between galaxies will increase at an ever-accelerating rate, but the distances between galaxies will increase at an ever-accelerating rate, but the horizon will remain fixed at more or less its current size, 13 billion light yrs the horizon will remain fixed at more or less its current size, 13 billion light yrs eventually, after 100 billion years, our Galaxy will have merged with eventually, after 100 billion years, our Galaxy will have merged with Andromeda and our other neighbours in the Local Group into a single large Andromeda and our other neighbours in the Local Group into a single large and dying galaxy and dying galaxy there will be no other galaxies within our observable horizon there will be no other galaxies within our observable horizon eventually all star formation will cease, all stars will die, black holes will eventually all star formation will cease, all stars will die, black holes will evaporate, and finally protons and neutrons will decay evaporate, and finally protons and neutrons will decay as the Greek poet Sappho put it: ‘nothing will remain of us’ as the Greek poet Sappho put it: ‘nothing will remain of us’

47. Aug 11th 2008Dublin Prospects for the immediate future PLANCK A future European Space Agency mission to map the Cosmic mission to map the Cosmic Microwave Background, Microwave Background, PLANCK Surveyor, due for PLANCK Surveyor, due for launch in December this year, launch in December this year, will determine cosmological will determine cosmological parameters with exquisite parameters with exquisite accuracy. accuracy.

48. Aug 11th 2008Dublin The unanswerable questions Is the universe spatially finite or infinite ? - there is a horizon defined by how far light has travelled since the Big Bang light has travelled since the Big Bang What was there before the Big Bang ? What was there before the Big Bang ? -our theories break down before we can extrapolate to the Big Bang itself extrapolate to the Big Bang itself