The Role of Images in Astronomical Discovery
Page 20
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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Journey
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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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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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5. The One-Thousand-Year Journey
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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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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.