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Climbing the Cosmological Distance Ladder ROWAN-ROBINSON: PRESIDENTIAL ADDRESS ROWAN-ROBINSON: PRESIDENTIAL ADDRESS (a) (b) (c) Climbing the cosmological distance ladder In his Presidential Address for 2008, Michael Rowan-Robinson ABSTRACT describes the steps taken to extend our knowledge of cosmological Humankind’seffortstomeasurethe distance – towards redshift 1000! distancesoftheplanets,starsand galaxiesarecloselyboundupwiththe ristotle (384–322 BC) was the first to motion of all stars on the sky due to the Earth’s evolutionofourideasabouttheuniverse estimate the size of the Earth, using the motion, a century earlier. wefindourselvesin.Thislinkstretches angle of the shadow of a pole at noon at fromclassicaltimestotoday,withthe A verylatestanalysisofthefluctuationsin a location 100 miles south of the equator. Era- Cepheids, M31 and the Hubble Law tosthenes and Poseidonius later used a similar The next crucial step on the distance ladder, thecosmicmicrowavebackground. method. All these estimates are within about still of prime importance today, was the dis- 10% of the modern value. In the 2nd century BC covery by Henrietta Leavitt in 1912, working Hipparcos used an eclipse method to estimate at the Harvard Observatory, that the periods Controversy over H0 the distance of the Moon and deduced a value of Cepheid variable stars in the Small Magel- Hubbles’s estimate of H0 was 500 km/s/Mpc. –1 59 RE, compared to the modern value of 60.3 RE. lanic Cloud are related to their luminosity: the Now H0 has the dimensions of time and so Aristarcos tried to estimate the distance of the period–luminosity relation. In 1924 Edwin 1/H0 is the expansion age of the universe, the age Sun using an eclipse method, but was out by a Hubble used Leavitt’s discovery to estimate the the universe would have if no forces were acting. factor of 20. The Greeks also gave us Euclidean distance of M31, the Andromeda Nebula. It Hubble’s value for H0 implied an age of the uni- geometry (Euclid 300 BC), the idea of absolute, clearly lay far outside our Milky Way Galaxy, verse of 2 billion years and it was soon realized uniform time (Aristotle), and the idea of an thus resolving the long-standing controversy this was shorter than the age of the Earth as infinite physical frame (the atomists, Epicurus). about the spiral nebulae and opening up the derived from radioactive isotopes. From 1927 Interestingly, and contrary to the picture held universe of galaxies. Three years later Hubble to 2001 the value of the Hubble constant was by medieval thinkers, Aristotle believed that the announced, based on the distances of 18 galax- a matter of fierce controversy. Baade pointed stars were at a range of distances. ies, that the more distant a galaxy, the faster it is out in 1952 that there were two different types A discovery of Copernicus (1473–1543) that is moving away from us (the Hubble Law): of Cepheid, so Hubble’s calibration had been less well-known than his heliocentric system is velocity/distance = constant, H0 (1) incorrect. This reduced H0 to 200 km/s/Mpc. that he worked out, for the first time, the correct This is just what would be expected in an In 1958 Sandage recognized that objects that relative distances of the Sun and planets. His expanding universe. Aleksandr Friedmann Hubble had thought were the brightest stars in values were within 5% of the modern values. had shown in 1922 that expanding universe some of his galaxies were in fact H II regions and The absolute scale of the solar system was not models are what would be expected according arrived at the first recognizably modern value of determined accurately till the 19th century. to Einstein’s General Theory of Relativity, if H0 of 75 km/s/Mpc. During the 1970s there was The Copernican picture also immediately the universe is (a) homogeneous (everyone sees an acute disagreement between Sandage and implied a much greater distance for the “immov- the same picture) and (b) isotropic (the universe Tammann, on the one hand, favouring H0 = 50, able” stars. Newton tried, unsuccessfully, to looks the same in every direction). and de Vaucouleurs, on the other, favouring estimate the distances of stars through their This unlikely assumption, the cosmological 100 km/s/Mpc. This disagreement stimulated brightness, but the first step on the distance principle, had been introduced by Einstein in me to write my monograph The Cosmologi- ladder outside the solar system was taken by 1917 when he derived a static model of the cal Distance Ladder (1985), in which I set Bessel in 1838 when he measured the parallax of universe in which gravity is balanced by a new out to review all aspects of the distance lad- 61 Cyg, its change in apparent direction on the force, the cosmological repulsion. Einstein’s der and to reconcile the systematic differences sky due to the Earth’s orbit round the Sun. This inspired guess that the universe must be very in distance estimates from different methods. was the final proof of the Copernican system. simple (homogeneous and isotropic) is con- With an objective weighting scheme based on Bradley had discovered aberration, the elliptical firmed to very high accuracy today. quoted errors, and with higher weight for purely 3.30 A&G•June2008•Vol.49 ROWAN-ROBINSON: PRESIDENTIAL ADDRESS ROWAN-ROBINSON: PRESIDENTIAL ADDRESS (d) 1: Looking out from Earth into the universe. (a) The solar system: a Clementine image of Venus and the solar corona behind the Moon lit by earthshine (NASA/JPL/USGS). (b) The galaxy: the Milky Way seen above the dome of the Gemini North telescope on Hawaii, taken during commissioning of the laser guide star system (P Michaud/Gemini Observatory). (c) Other galaxies: M31, the Andromeda Galaxy in infrared, from the Spitzer Space Telescope (NASA/JPL-Caltech/P Barmby, Harvard-Smithsonian CfA). (d) The universe as a whole: fluctuations in the cosmic microwave background measured by the Wilkinson Microwave Anisotropy Probe satellite. (WMAP) Climbing the cosmological distance ladder geometric distance methods (or those based on we shall see later, evidence from Type Ia super- impressive. A problem with the Gibson et al. theoretical arguments), I concluded that there novae presented in 1998 was already support- (2000) analysis was that it used photographic were systematic errors in the Type Ia supernova ing the idea of a positive cosmological constant. magnitudes for some of the older supernovae. method (too high distances) and in the Tully- But it was still of interest to see whether there Riess et al. (2005) used new HST–ACS observa- Fisher and H II region methods (too low) and were any possible doubts about this HST Key tions of Cepheids in galaxies with well-observed that the best overall value was Project value for H0. The uncertainties in this recent Type Ia supernovae and concluded that H0 = 67 ± 12 km/s/Mpc. value are (a) the distance of the Large Magel- H0 = 73 ± 6 km/s/Mpc. This analysis also dem- H0 = 67 would give an expansion age for the lanic Cloud, which remains uncertain by 10%, onstrated that some of the inconsistencies with universe of 15.3 billion years (Gyr). In the sim- (b) the adopted Cepheid calibration, based on earlier Type Ia supernovae can be attributed to plest, Einstein de Sitter (Ωm = 1, Λ = 0) model, OGLE Cepheids, (c) corrections for the effects systematic errors in the photographic magni- with only gravity acting to slow the expansion, of dust extinction, (d) corrections for differ- tudes. The issue of the luminosity–decline rate the age of the universe would be 10.2 billion ences in metallicity between the LMC and the relation has been addressed by Jha et al. (2007) years. This could be compared with ages of the Cepheid host galaxies, (e) corrections for the and by Nobili et al. (2005) and Wang et al. oldest stars in globular clusters, between 10 local peculiar velocity flow. Using the Freedman (2006). There are still some unresolved incon- and 15 Gyr. Chaboyer et al. (1998) estimated et al. data, my own best estimates for these cor- sistencies in the derivation of extinction, which 12.6 ± 1.1 Gyr, and the age of the galaxy derived rections and the weighting scheme of The Cos- can only be resolved with the use of more photo- from radioactive isotope abundances was also mological Distance Ladder (1985), I concluded metric bands in future supernova studies. 10–15 Gyr. Was this already a headache for the (Rowan-Robinson 2000) A consensus? Einstein de Sitter model? H0 = 63 ± 6 km/s/Mpc. With the WMAP three-year results yielding HST Key Project Type Ia supernovae H0 = 73 ± 3 km/s/Mpc (Spergel et al. 2007), it Following the launch of the Hubble Space Tele- In 1998 two teams announced that using looks as though we have a consensus around scope (HST) in 1990, and the subsequent repair Type Ia supernovae as standard candles out to H0 = 73 km/s/Mpc, Ωm = 0.25, ΩΛ = 0.75, and an mission, substantial amounts of HST time were significant redshifts (~0.5) implied that the cos- age of the universe 13.7 Gyr. However, in 2006 dedicated to measuring Cepheids in galaxies mological constant had to be greater than 0 Sandage et al. announced the results of their Λ out to distances of 20 Mpc, to try to measure (Riess et al. 1998, Perlmutter et al. 1999). There HST programme, with the Hubble constant accurately and to give the were issues with (a) the treatment of extinction H0 = 62 ± 5 km/s/Mpc (3) different distance methods a secure and consist- by dust, and (b) the consistency of the assumed This was based on a new extensive study of ent calibration.
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