B00B7H7M2E EBOK (28 page)

CHAPTER 6

The Demise of Constancy and Stability

1929–1992

On philosophical grounds too I cannot see any good reason for preferring the Big Bang idea. Indeed it seems to me in the philosophical sense to be a distinctly unsatisfactory notion, since it puts the basic assumption out of sight where it can never be challenged by a direct appeal to observation.

Fred Hoyle

IN THE LATE
1920s, a sea-change was about to take place in the way men and women envisioned the universe. All of Edwin Hubble’s earlier contributions might seem enough for one lifetime, but when it came to the impact he was to have on the history of human thought, Hubble had only barely begun.

Hubble was continuing his observational work in collaboration with Milton Humason, whose background, like Hubble’s, was not in science. Humason was a janitor and mule driver at Mount Wilson whose formal education had ended at the age of 14 when he’d come to summer camp there and decided he didn’t want to leave. By 1919, Humason, now 28, was a night assistant, tending the telescopes, assisting the astronomers, and
occasionally
doing a little observing on his own. So obvious was his extraordinary innate skill with delicate instruments and the large telescopes that George Ellery Hale, ignoring the opposition of those who thought someone of Humason’s background should not be part of the academic staff, decided to appoint him Assistant Astronomer. In 1928 Humason began working with Hubble on the measurement of red shifts of faint distant galaxies.

In 1929, having established beyond doubt that there are many galaxies besides our own, the two men made the first announcement since Copernicus’s
De revolutionibus
that rivalled that book’s significance in the history of astronomy. Hubble and Humason had found that except for galaxies clustered close to ours every galaxy in the universe appears to be receding from us. What’s more, on the large scale, every galaxy appears to be receding from every other. These discoveries had a much more immediate impact on the scientific community and the wider public than Copernicus’s book, precipitating rapid change in ideas about what the universe is like, about its history, and even about ourselves.

It hadn’t escaped Hubble’s notice that there were connections between the observations that Slipher, Humason and he were making and the solutions that physicists such as Willem de Sitter, Alexander Friedmann and Abbé Georges Henri Lemaître were getting from the equations of Albert Einstein – solutions that implied that the universe must be either expanding or contracting.

The debate about whether the universe is expanding, shrinking or just holding its own has a history that goes back long before the 20th century. Ancient and medieval thinkers regarded the Earth as the region of the universe where change, decay and evil held sway, while all beyond the Moon was unchanging and perfect. Newton echoed these sentiments to the extent of believing that a universe created by God could not be changing dramatically over time, for constancy and stability
reflected
the nature of God, while change (implying decay and conflict) did not. Newton was also of the opinion that if a system goes far awry, as he realized the planetary orbits would over time, God would set it right again, so things would never be allowed to change too drastically.

As for the specific question whether the universe might be expanding or contracting, Newton decided on logical grounds that it couldn’t be doing either. He reasoned that if it were expanding or contracting, there would have to be a centre to the motion – in other words, a point away from which it was expanding or towards which it was contracting. But matter distributed uniformly through an
infinite
space (as Newton believed it was) has no centre. Newton couldn’t foresee that others nearly three centuries later would find that his own equations led to the prediction that the universe must be expanding or contracting. In the 18th century, Kant, who took off from Thomas Wright’s picture of the universe as a flattened slab of stars, thought that if the universe were not perfectly balanced between the orbital motion of stars and their gravitational attraction for each other, it would end in destruction and chaos and lack ‘the character of that stability which is the mark of the choice of God’.

Although it was out of fashion from the mid-19th century onwards to include God in scientific statements, the feeling that there was something sublimely rational and sacred about an unchanging universe and something shifty and distasteful about one that changed had by no means disappeared. It had become a doctrine of science rather than of religion. In an interesting turnabout, one of the 20th-century reasons for clinging doggedly to the notion of a static universe (one that isn’t expanding or contracting) was that an expanding universe – which almost surely must have had a beginning – seemed more likely to require a creator. That was a possibility some had considered safely put to rest.

Albert Einstein resisted the idea of an expanding or
contracting
universe for reasons having to do with his scientific intuition. Soon after he produced his general theory of relativity in 1915, Einstein and the Dutch astronomer Willem de Sitter realized that solutions to Einstein’s equations implied that the universe was either expanding or contracting. Einstein was not necessarily one to cling to old assumptions, but at this juncture he did, and he dug in his heels. Annoyed by the ridiculous upshot of his equations, he wrote, ‘To admit such a possibility seems senseless.’ Such strong aversion did he feel that he decided to adjust his theory to cancel out the offensive prediction. He put in a new constant of nature – a ‘cosmological constant’, a mathematical term that would allow the universe to be static. He was later to regret this move, calling it ‘the biggest blunder of my life’. But the notion of a cosmological constant didn’t disappear when Einstein reneged on it. It still haunts physics.

While Einstein was tinkering with his equations, Russian mathematician Alexander Friedmann decided instead to take Einstein’s theory at face value. Friedmann insisted that if there is a cosmological constant, its value is probably nothing else but zero. He pointed out in one of his first papers dealing with Einstein’s theories that the assumption that the universe is static had always been only an assumption. No observations required one to believe it. Einstein himself was well aware that this was the case.

Friedmann proceeded to find not one, but a number of solutions to the cosmological equations of general relativity. Each solution described a different sort of universe. See
Figure 6.1.

Friedmann predicted that regardless of where we were to situate ourselves in the universe, in any galaxy, we would find the other galaxies receding from us. The further away a galaxy is from us, the faster it’s receding, twice as far away, twice as fast. For an analogy, imagine a loaf of raisin bread rising in the oven. Sitting on any raisin while the dough rises and expands between
the
raisins, we would see every other raisin moving away from us, twice as far, twice as fast. We cannot of course directly observe the universe from any vantage point except our solar system, but at least from here that is the sort of recession that Hubble observed in 1929 with the 100-inch telescope at Mount Wilson. The outward-bound speed of a galaxy is directly proportional to its distance from us. Twice as far away, twice as fast.

Figure 6.1

Three models of the universe: (a) The universe expands to a maximum size and then recollapses; (b) The universe expands rapidly and never stops expanding; (c) The universe expands at exactly a critical rate to avoid recollapse.

Belgian astrophysicist and theologian Abbé Georges Henri Lemaître discovered solutions to Einstein’s equations that were similar to Friedmann’s. What intrigued Lemaître most was what the equations and solutions could reveal about the origin of the universe. It was Lemaître who first described something like what was soon to be dubbed the ‘Big Bang’, though he didn’t give it that name. His suggestion was that there must have been a time when everything that makes up the present universe was compressed into a space only about 30 times the size of our Sun – a ‘primeval atom’. Partly because he was a priest and theologian as well as an astrophysicist, some of Lemaître’s colleagues greeted his idea with derision. It smacked too much of Genesis.

Though Friedmann’s theoretical work ended prematurely – he died at the age of 37 – and he remained largely unknown except among mathematicians, Lemaître’s work came to the attention of observational astronomers, largely through British physics giant Arthur Eddington, whose student Lemaître had been at Cambridge, and another of Eddington’s students, George McVittie.

Thinking about the universe as an expanding lump of raisin bread dough led to interesting speculation. Would it be possible, assuming the necessary technology existed, to travel to the surface of the loaf and find the border of the universe? What would be beyond that? Unfortunately those questions probably have no real meaning. Eddington fielded them by providing an analogy of a balloon and an ant:

The balloon has dots painted all over it. The ant crawls on the surface of the balloon. All that exists for this ant is that surface. It can’t look outward from the balloon’s surface or conceive of an interior to the balloon. Air is let into the balloon and the balloon expands. The ant sees every dot on the surface of the balloon moving away. Anywhere the ant crawls on the balloon, every dot is moving away. The ant may wander forever, like the Flying Dutchman, but it will never find an edge or a border to this universe. Our situation in our own universe is probably similar to the ant’s, but with more dimensions. There is no edge from which we would see galaxies in one direction and absolutely nothing in the other.

The question of a ‘centre’ has cropped up often in this book, most recently in Newton’s objections to an expanding or contracting universe. Where in the universe
did
the expansion begin? From what centre point is everything retreating? The Big Bang was an explosion that sent everything flying outwards. Even granting that there are no absolute directions in the universe, it would seem that beings riding on a piece of debris from this explosion would have the right to assume there is an answer to the question: Where did the explosion take place in relation to where we are now?

Eddington’s balloon analogy helps with that question as well: Can the ant ask where on the balloon’s surface the expansion began? No. From our vantage point, watching the ant, we can see that that question would be meaningless. No dot on the balloon represents the ‘centre’ of the expansion. Newton failed to imagine a situation in which all points in the universe are moving away from all other points, with no ‘centre’ to the expansion, no direction towards which we can look and insist that it all began there.

Paradoxically, living in an expanding universe means that there is a direction in which we
can
peer and see something different, perhaps even see an ‘edge’. That direction is the past. What’s more, in any space direction we look, we look towards
the
origin of the universe, for
any
direction is towards the past. That is true not only when we gaze deep into space with telescopes. It’s true even in the small area of the room in which I write this paragraph. What I see of the opposite wall is old news. Of course the delay with which the picture of that wall reaches my eyes is not worth considering because light, and thus any picture that comes into my eyes, travels extremely fast, 186,000 miles or 300,000 kilometres per second.

When speaking of cosmic distances – where measurement in light years is more meaningful than measurement in miles or kilometres – light speed
isn’t
terribly fast, and the delay can’t be ignored. As the history of cosmic measurement continued in the 20th century, measurement of distance in space was to become inextricably bound up with measurement of distance in time. One can no longer ask how far away something is without also implying the other question: How far in the past is it? Questions about what is meant by an ‘edge’ or ‘outside the universe’ become entangled with questions about what is meant by a ‘beginning’ or ‘before the universe’.