The Role of Images in Astronomical Discovery Read online

  his results. “The publications of 1925 definitely settled and closed the debate on island-

  universes.”36 And Hubble started employing the descriptor “extragalactic nebulae.” It was

  never clear if by “extragalactic” Hubble meant outside the Milky Way or simply above or

  below the plane of the Milky Way. It was only after Hubble’s death in 1953 that the word

  “galaxies” began to be used broadly.

  Having opened wide a giant field of research, Hubble also provided a lasting legacy by

  developing a classification scheme for galaxies based on their shapes. Combining earlier

  schemes, he proposed his own: “ . . . nebulae are closely related members of a single family.

  They are constructed on a fundamental pattern that varies systematically through a limited

  range. The nebulae fall naturally into an ordered sequence of structural forms. . . . ”37 I come

  back to this topic in Chapters 9 and 10.

  Galaxies and the Expanding Universe

  Once distances to galaxies could be reliably established, several staggering discoveries

  quickly unfolded. Indeed, these measurements opened up a box of treasures. First, the

  universe was found to be expanding; this was an awesome aftermath, one of the greatest

  findings by the human mind in its quest to determine the nature of the universe.

  Unknowingly and unwillingly at first, Albert Einstein (1879–1955) was the scout on

  the path to that stupendous discovery. In 1915 and 1916, Einstein had proposed and devel-

  oped a new theory of gravitation that became his most famous legacy.38 The theory of

  35 M. Bartusiak, The Day We Found the Universe, New York: Vintage Books, 2010.

  36 H. Nussbaumer and L. Bieri, Discovering the Expanding Universe, Cambridge: Cambridge University Press, 2009, p. 62.

  37 E. P. Hubble, The Realm of the Nebulae, New Haven: Yale University Press, 1936, p. 56.

  38 A. Einstein, Die Grundlage der allgemeinen Relativitätstheorie, Annalen der Physik, 1916, Vol. 49, pp. 769–822.

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  general relativity brought about a scientific revolution and a total change of paradigm with

  respect to Newtonian physics. Postulating the invariance of physical laws, Einstein surmised

  that space and time were not absolute. Daringly, he unified space, time and gravitation to

  describe a four-dimensional warping world, one with dips and ripples caused by concentra-

  tions of mass: stars, stellar clusters, galaxies and galaxy clusters. The “squashing” of time

  depended on the force of acceleration, and the curving of space hinged on the amount and

  concentration of mass.

  John Archibald Wheeler (1911–2008) summarized the new Einsteinian physics most

  eloquently: “Spacetime tells matter how to move; matter tells spacetime how to curve.”39

  Particles without mass, like photons, and particles of matter, either ordinary or dark, follow

  curved space. Everything in the universe had to obey a new geometry of spacetime. But

  there was a hidden potency in Einsteinian spacetime: it is dynamic, an astonishing property

  that Einstein did not want to see at first.

  To Einstein’s surprise and dismay, the equations describing the new universe also indi-

  cated that the new spacetime he had invented could not be in equilibrium. Just as a pencil

  on its tip falls one way or another, Einstein’s weird universe had to expand or to contract.

  Wanting a static solution at all costs, Einstein introduced a locking term, , in the equations

  that described the structure and energy content of the universe. For Einstein, the universe

  had to be in a steady state, with the present moment being an instant between an infinite

  past and an infinite future. Like several of his contemporaries, Einstein had remained New-

  tonian in his thinking. However, two “young wolves” working independently with Ein-

  stein’s equations soon developed different views and pulled out an unexpected but amazing

  result.

  Russian physicist and mathematician Alexander Alexandrovitch Friedmann (1888–

  1925) was a brilliant student of general relativity. He accepted the static solution, but

  explored the dynamic solutions of the equations that Einstein had ignored for philosophical

  reasons. Friedmann’s main work was published in German in 1922 and 1924. It showed that

  Einstein’s field equations allowed a dynamic solution for the structure of space. He did not

  try to connect his findings with astronomical observations of the apparent recession of the

  bulk of galaxies that Slipher had revealed a few years earlier. Instead, he proposed a dynamic

  universe where “the world’s radius of curvature . . . is constantly increasing in time; cases

  are also possible when the radius of curvature changes periodically: the Universe contracts

  into a point (into nothing) and then again increases its radius from a point up to a certain

  value, then again, diminishing its radius of curvature, transforms itself into a point, etc.”

  To illustrate his unorthodox viewpoint, Friedmann reminded his readers of the Hindu

  mythology of cyclic universes and of “the creation of the world from nothing.”40 Unfortu-

  nately, Friedmann passed away, victim of the typhoid fever, when only 37 years of age. But

  his rather fantastic view of a swinging universe was not lost for long.

  39 J. A. Wheeler (with Kenneth Ford), Geons, Black Holes and Quantum Foam, New York: W. W. Norton & Company, 1998, p. 235.

  40 Cited in E. A. Tropp, V. Ya. Frenkel and A. D. Chernin, Alexander A. Friedmann: The Man who Made the Universe Expand, Cambridge: Cambridge University Press, 1993, p. 157.

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  Part II – Images as Galaxy Discovery Engines

  Fig. 5.9 Georges Lemaître. Credit: Archives Georges Lemaître, Université catholique de Louvain,

  Louvain-la-Neuve, Belgium.

  A young Belgian cosmologist, unknowingly and independently of Friedmann, was

  conducting his own exploration of general relativity. He also had something phenome-

  nal to tell about the structure and origin of the universe. Independently of Friedmann,

  Georges Lemaître (1894–1966) found the expanding or contracting universe solutions to

  the equation of general relativity (Fig. 5.9). In 1927, that is two years before Hubble’s first

  announcement of the velocity–distance relation, the young Belgian priest published his

  work in French. Lemaître had given the following (translated) title to his seminal paper: “A

  homogeneous universe of constant mass and increasing radius accounting for the radial

  velocity of extragalactic nebulae.”41 The last words clearly indicated that Lemaître was

  fully aware of the observational results on the recession of “nebulae.” Using the data set on

  the velocities of galaxies measured by Slipher, he concluded that the universe was expand-

  ing, not contracting. Lemaître’s work was at first ignored, until the British astronomer

  and physicist Arthur Stanley Eddington (1882–1944) had Lemaître’s paper translated and

  published in the British journal, Monthly Notices of the Royal Astronomical Society, in

  41 Lemaître’s paper appeared in French in the Annals of the Scientific Society of Brussels, 1927, Vol. 47, pp. 49–59.

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  1931.42 This time Lemaître’s work aroused wide interest. In his groundbreaking paper,

  the young cosmologist had derived an approximation of what was to be called later the

  “Hubble law” (Figs. 5.10 and 5.11). As commented by the science historian Helge Kragh:

  “The famous Hubble law is clearly in Lemaître’s paper. It could as well have been named

  Lemaître’s law.”43 It can firmly be established that Georges Lemaître figured out the expan-

  sion of the universe before anyone else.

  At the Mount Wilson Observatory, Hubble and Humason had initiated a massive pro-

  gram to photograph galaxies and measure their apparent velocities. They published results

  confirming the expansion of the universe in 1931 (Fig. 5.10). Following the formative paper,

  the study of the expansion of the universe became a major field of investigation for the rest

  of the twentieth century and beyond.44 Again with a groundbreaking result, the recurring

  hesitancy on the part of Hubble came into play: “The interpretation of red-shifts as actual

  velocities, however, does not command the same confidence, and the term ‘velocity’ will be

  used for the present in the sense of ‘apparent’ velocity, without prejudice as to its ultimate

  significance.”45

  The historians Harry Nussbaumer and Lydia Bieri have reviewed the papers and notes

  of Hubble and of Hubble’s colleagues. Their conclusion is inescapable: Hubble did not

  believe that the high receding velocities were real, or meant that spacetime was expanding.46

  Hubble, whom many quote incorrectly as being the discoverer of the expanding universe,

  did not believe in this expansion. Allan Sandage wrote in 2009: “The irony, of course, is that

  although the discovery of the expansion is often attributed to Hubble with his 1929 paper, he

  never believed in its reality.”47 In the end, it is fair to say that the discovery of the expanding

  universe was the collective work of several men: Einstein, Friedmann and Lemaître on the

  theoretical front; Slipher, Humason and Hubble (unwillingly) on the observational side,

  with some obvious reluctance on the part of some of the players.

  Expansion meant that, in the past, the universe was smaller and in a distant past, very,

  very small. Following on Friedmann’s idea of creation out of nothing, the young and bold

  Lemaître invoked a new quantum theory to propose a physical and rigorous explanation

  for an early, very hot universe, which he called the “primeval atom.”48 It was 1931, and

  Lemaître’s daring proposal was the birth of the Big Bang theory. Lemaître’s extraordinary

  intuition was the existence of a primeval atom, the initial phase of the universe when

  42 On the controversy around the translation, see Mario Livio, Mystery of the Missing Text Solved, Nature, 2011, Vol. 479, pp. 171–173.

  43 H. S. Kragh, Cosmology and Controversy, The Historical Development of Two Theories of the Universe, Princeton: Princeton University Press, 1996, pp. 29–30.

  44 E. P. Hubble and M. L. Humason, The Velocity–Distance Relation Among Extra-Galactic Nebulae, The Astrophysical Journal, 1931, Vol. 74, pp. 43–80.

  45 E. Hubble and M. L. Humason, The Velocity–Distance Relation Among Extra-Galactic Nebulae, The Astrophysical Journal, 1931, pp. 76–77.

  46 H. Nussbaumer and L. Bieri, Discovering the Expanding Universe, Cambridge: Cambridge University Press, 2009, pp. 119–

  120.

  47 A. Sandage, in H. Nussbaumer and L. Bieri, Discovering the Expanding Universe, Cambridge: Cambridge University Press, 2009, p. xv.

  48 G. Lemaître, The Beginning of the World from the Point of View of Quantum Theory, Nature, 1931, Vol. 127, p. 706. Lemaître’s pioneering paper made less than half a page in Nature; it is packed with insights. Lemaître’s paper shared the journal page with another short article, “Insects remains in the guts of a cobra.”

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  Part II – Images as Galaxy Discovery Engines

  Fig. 5.10 Galaxy spectra by Humason showing galaxies of increasing distances and their spectra

  displaying greater receding velocities. From Humason (1936), The Astrophysical Journal. C

  AAS.

  Reproduced with permission.

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  Fig. 5.11 The velocity–distance linear relation as originally presented by Hubble. From Hubble

  (1929), Proceedings of the National Academy of Sciences.

  density and temperature were awesomely colossal, and the fabric of spacetime sud-

  denly unfolded. Although Fred Hoyle described this beginning as a ‘big bang’ to

  ridicule the idea, Lemaître’s revolutionary concept has been vindicated multiple times by

  observations.

  The aftermath of the discovery of the world of galaxies and of the expansion of the uni-

  verse did not end here. While trying to establish the mean density or the mass of the universe

  by mapping the expansion velocity of the universe as a function of time, and expecting to

  find a weak deceleration due to gravity, groups of researchers found that the expansion

  was instead accelerating. These findings, published in 1998, were confirmed by detailed

  spectroscopy of distant supernovae of type Ia. Such supernovae result from material that

  is “dumped” by a red giant star on its companion, a degenerate star (white dwarf), trigger-

  ing the nuclear detonation of the star. Researchers now associate the acceleration with the

  ‘infamous’ of Einstein. This time, instead of locking the universe into a static state, the

  new fosters the physics for the acceleration under a term called “dark energy.” We don’t

  yet know what this is.

  A Rip-Roaring Century

  “ . . . It seems clear that the discovery of new classes of astronomical objects is an extended

  process consisting of detection, interpretation, and understanding, all of which may be

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  Part II – Images as Galaxy Discovery Engines

  preceded by a pre-discovery phase.”49 Images of “nebulae” and of galaxies as they devel-

  oped over centuries (Chapters 1 to 5) were key tools for the many conceptual transforma-

  tions and paradigm shifts the astronomers went through.

  In the last 100 years, astronomers exploring and mapping the world of galaxies have

  inflated the volume of the observable universe by 1015, or one million billion times, and have

  unveiled a new form of mysterious energy: dark energy. Invoking Stigler’s law of eponymy,

  it is safe to say that many people “discovered” galaxies; several people established the dis-

  tances to galaxies; a few people found that the universe was expanding and accelerating.

  Among these, some individuals stand out for their outstanding clarity, audacious insights

  and stunning observational findings. Their main tool was astronomical images of increas-

  ing depth and precision, including amazing images of spectra of galaxies. The next three

  chapters will explore how images and spectra unveiled galaxies.

  49 S. J. Dick, Discovery and Classification in Astronomy, Controversy and Consensus, Cambridge: Cambridge University Press, 2013, p. 189.

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  6

  Galaxies in Focus

  Galaxies are the largest sing
le aggregates of stars in the universe. They

  are to astronomy what atoms are to physics.

  Allan Sandage 1

  The absorption is effective in all galactic longitudes but seems to take

  place mainly in a thin layer extending along the galactic plane.

  Robert J. Trumpler 2

  How were Key Properties of Galaxies Discovered?

  German astronomer Wilhelm Heinrich Walter Baade (1893–1960) was a visitor to the

  Mount Wilson Observatory from 1926 (Fig. 6.1), and he moved to work there from 1931

  to 1959. Allan Sandage and Halton Arp were his two best known doctoral students, and

  their roles and influences will be discussed later. Having neglected to refresh the citizen-

  ship papers he had lost before World War II, he was declared an “enemy alien” of the United

  States at the start of World War II. During the war, Baade was restricted to the Mount Wil-

  son and Pasadena area in southern California, but was allowed to continue working. He

  used the 100-inch telescope under unusual night-sky conditions, as Los Angeles and the

  neighboring towns were under a strict blackout.

  Baade was a superb and talented observer, outstanding at getting the best photographic

  material, an ability he had already demonstrated when observing at the Hamburg Zeiss

  40-inch reflector back in Germany.3 His work was always of exquisite quality. Halton Arp

  gave a colourful description of how meticulous Baade was with his photographic work: “He

  insisted on having separate dark rooms in the telescopes [sic]. One for people who did direct

  photography like himself and the other for what he called the pigs, the spectroscopists, who

  slopped the dark room up and made messes and shouldn’t be allowed in a top flight dark

  room.”4

  1 A. R. Sandage, The Hubble Atlas of Galaxies, Washington: Carnegie Institution of Washington, 1962, p. viii.

  2 R. J. Trumpler, Preliminary Results on the Distances, Dimensions and Space Distribution of Open Star Clusters, Lick Observatory Bulletin, 1930, No. 42, p. 188.

  3 D. E. Osterbrock, Walter Baade, A Life in Astrophysics, Princeton: Princeton University Press, 2011.

  4 H. C. Arp, in an American Institute of Physics oral interview by Paul Wright, 29 July, 1975.