Brilliant Blunders: From Darwin to Einstein - Colossal Mistakes by Great Scientists That Changed Our Understanding of Life and the Universe (28 page)

Einstein had introduced the assumption of the large-scale homogeneity and isotropy of space in 1917, but this simplifying conjecture was elevated to the status of a fundamental principle in a paper published in 1933 by the English astrophysicist Edward Arthur Milne. Milne called his principle the “extended principle of relativity,” requiring that
“not only the laws of nature but also the events occurring in nature, the world itself, must appear the same to all observers, wherever they be.” Today the stipulation of homogeneity and isotropy is known as the cosmological principle (a name coined by the German astronomer Erwin Finlay-Freundlich), and the most powerful direct evidence for its validity comes from observations of the “afterglow of creation”: the cosmic microwave background radiation. This radiation is a relic of the primeval hot, dense, and opaque fireball. It comes from all directions and is isotropic to better than one part in ten thousand. (In the words of astronomer Bob Kirshner: “much smoother than a baby’s bottom.)” Large-scale galaxy surveys also indicate a high degree of homogeneity. In all surveys that encompass a large enough slice of the cosmos to constitute a “fair sample,” even the most conspicuous structural features are dwarfed and smoothed out.

Since the cosmological principle has proven to be so effective when applied to different positions in space, it was only natural to wonder whether it could be extended to apply to
time
as well. That is, could one argue that the universe is unchanging in its large-scale appearance as well as in its physical laws? This was the big
question raised by Hoyle, Bondi, and Gold in 1948. Amusingly enough, the illustrious trio may have been inspired to ask this question by a British horror film called
Dead of Night.
(Figure 24 shows the original poster of the film.) Here is how Hoyle himself described the sequence of events:

Figure 24

In a sense, the steady-state theory may be said to have begun on the night that Bondi, Gold, and I patronized one of the cinemas in Cambridge . . . It [the film
Dead of Night
] was a sequence of four ghost stories, seemingly disconnected as told by the several characters in the film, but with the interesting property that the end of the fourth story connected unexpectedly with the beginning of the first, thereby setting-up the potential for a never-ending cycle.

When the three colleagues returned to Trinity College, Gold asked suddenly, “What if the universe is like that?” meaning that the universe could be eternally circling on itself without a beginning or an end. The idea was certainly intriguing, except that at first blush it appeared to be at odds with the discovery by the Belgian priest and cosmologist Georges Lemaître and astronomer Edwin Hubble that the universe was expanding. The cosmic expansion seemed to be pointing rather to a linear evolution, starting from a dense and hot beginning (the big bang) and indicating a clear direction for the arrow of time. Hoyle, Bondi, and Gold were fully aware of these findings, since Hubble’s discovery and its potential implications had already featured frequently in the discussions of the trio. In an interview in 1978, Gold reminisced about those intense analyses:

What happened was that there was a period when Hoyle and I would sit around in Bondi’s rooms in college a substantial amount of the time and discuss, as Hoyle always insisted, what does the Hubble thing really mean? . . . all those galaxies, all this flying apart, would the space be terribly empty afterwards? Has it been very dense in the past?

All of those contemplations had led to an unexpected outcome: Hoyle, Bondi, and Gold started to think seriously about the problem of whether the observed cosmic expansion could somehow be accommodated in the context of a theory of an unchanging universe.

But before delving into that fascinating topic, let’s go back to the 1920s for a moment. The discovery of the expanding universe is not only the greatest astronomical discovery of the twentieth century, it plays such a crucial role both in Hoyle’s blunder and Einstein’s that it would be instructive to take a short detour to review the history of this breakthrough. This story is especially pertinent, since a new, very intriguing twist in the chronicle of events created a huge buzz in the astronomical and history of science communities in 2011.

When cosmologists say that our universe is expanding, they base this statement primarily on evidence that comes from the apparent motion of galaxies. A highly simplified, oft-used example can help visualize the concept.

Imagine a two-dimensional world that exists only on the surface of a rubber sphere (figure 25). That is, galaxies in this world are simply small, round chads (like those created with a hole punch) glued on the surface. Neither the inside of the sphere nor the space outside it exists for the inhabitants of this world; their entire universe is just the surface. Note that this world has no center; no chad on the surface is different from any other chad. (Remember that the center of the sphere itself is not a part of this world.) This universe also has no boundary or edge. If a point were to move in a certain direction on the spherical surface, it would never reach an edge.

Figure 25

Now, what would happen if this sphere were being inflated? Irrespective of which chad on the surface you happen to belong to, you will see all the other chads receding and rushing away from you. Moreover, chads that are more distant will be receding faster: A chad that is twice as far as another will be moving twice as fast (since it will cover twice the distance during the same period of time). In other words, the speed of recession will be proportional to the distance. Einstein’s theory of general relativity articulates that the fabric of space-time (the combination of space and time into a single continuum) in our universe behaves in such a way that we can turn around this simplified example. That is, the discovery that all the distant galaxies are receding from us, combined with the fact that the speed of recession is proportional to the distance, imply that space in our universe is
stretching. (We shall return to this topic in chapter 10.) Note that the expansion of the universe cannot be compared with an exploding hand grenade. In the latter case, the explosion occurs within a preexisting space, and it has a definite center (and an edge). In the universe, the receding motion arises because the fabric of space itself is stretching. No galaxy is any different from any other galaxy; from every location, you will see all the other galaxies rushing away in all directions.

The key figure with whom the discovery of cosmic expansion is usually associated is astronomer Edwin Hubble, after whom the Hubble Space Telescope was named. Hubble is commonly credited with having measured (in collaboration with his assistant, Milton Humason) the distances and recession velocities to a few dozen galaxies, and having established,
in a paper published in 1929, the law that bears his name, stating that galaxies recede from us at speeds that are proportional to their distance. From that “Hubble’s law,” Hubble and Humason derived an overall current expansion rate suggesting that with every 3.26 million light-years of distance, the
recession speed of the galaxies increases by about 500 kilometers per second, or about 311 miles per second.

Given the relatively small-distance range of Hubble’s original observations, it would have been a real leap of faith to infer from them a universal expansion were it not for some supporting theoretical ideas, a few of which had even preceded the observations. In fact, as early as 1922,
the Russian mathematician Aleksandr Friedmann showed that general relativity allowed for an expanding, matter-filled, unbounded universe. While few took notice of Friedmann’s results (other than Einstein himself, who eventually acknowledged their mathematical correctness but dismissed them, since he thought that “a physical significance can hardly be ascribed to them”), the notion of a dynamical universe was starting to gain influence during the 1920s. Consequently, the interpretation of Hubble’s observations in terms of an expanding universe became popular fairly fast.

Physicists sometimes tend to ignore the history of their subject. After all, who cares who discovered what as long as the discoveries are made widely known. Only totalitarian regimes have been obsessed with insisting that all good ideas are homegrown. In an old joke about the Soviet Union, an important visitor is brought to the science museum in Moscow. In the first room, he sees a giant picture of a Russian man he had never heard of. When he asks who that person is, he is told, “This is so-and-so, the inventor of the radio.” In the second room: another giant portrait of a complete stranger. “The inventor of the telephone,” his host informs him. And so it continues for about a dozen rooms. In the final room, there is a picture that dwarfs by comparison all of the other pictures. “Who is this?” the visitor asks in astonishment. The host smiles and answers, “This is the man who invented all of those other men in the previous rooms.”

In a few cases, however, discoveries are of such magnitude that understanding the path that had led to these insights—including the correct attribution—can be of great value. There is very little doubt that the discovery of the expansion of the universe falls into this category, even if for no other reason than the fact that the expansion suggests that our universe had a beginning.

During 2011,
a passionate debate flared up about who actually deserves the credit for discovering the cosmic expansion. In particular, a few articles even raised the suspicion that some improper censorship practices may have been applied in the 1920s to ensure Edwin Hubble’s priority on the discovery.

Here are, very briefly, the background facts that are most relevant for this debate.

By February 1922,
astronomer Vesto Slipher had measured the radial velocities (velocities along the line of sight from us) for forty-one galaxies. In a book published in 1923,
Arthur Eddington listed those velocities and remarked, “The great preponderance of positive [receding] velocities is very striking; but the lack of observations of southern nebulae is unfortunate, and forbids a final conclusion.” (Galaxies were initially called nebulae [from Latin for “mist,” or “cloud”] because of their fuzzy appearance.) In 1927
Georges Lemaître published (in French) a remarkable paper whose title read (in its English translation): “A Homogeneous Universe of Constant Mass and Increasing Radius Accounting for the Radial Velocity of Extra-Galactic Nebulae.” Unfortunately, it was published in the little-read
Annals of the Brussels Scientific Society
. In it, Lemaître first discovered dynamic (expanding) solutions to Einstein’s general relativity equations, from which he derived the theoretical basis for what is now known as Hubble’s law: the fact that the velocity of recession is directly proportional to the distance. But Lemaître went beyond mere theoretical calculations. He actually used the velocities of the galaxies as measured by Slipher—and approximate distances as determined
from brightness measurements by Hubble in 1926—to discover the existence of a tentative “Hubble’s law” and to determine the rate of expansion of the universe. For the numerical value of that rate, today called the Hubble constant, Lemaître obtained 625 (in the common units of kilometers per second for every 3.26 million light-years of distance). Two years later,
Edwin Hubble obtained a value of about 500 for this same quantity. (Both values are known today to have been wrong by almost an order of magnitude.) In fact, Hubble used essentially the same recession velocities—the
ones determined by Slipher—without ever mentioning in his paper that these were the latter’s work. Hubble did use superior distances, which were based in part on better stellar distance indicators. Lemaître was fully aware of the fact that the distances he had used were only approximate. He concluded that the accuracy of the distance estimates available at the time seemed insufficient to assess the validity of the linear relation he had discovered.