Farewell to Reality (18 page)

This cosmic microwave background (CMB) radiation was first detected in 1964. A succession of satellite surveys has mapped the CMB in exquisite detail, and provides much of the observational evidence on which theories of the origin and evolution of the universe are constructed. The quantum fluctuations that rippled through spacetime as the universe ballooned in size were the seeds for the subsequent formation of gas, clouds, stars, galaxies and clusters of galaxies. So, the pattern of points of light that we see in a night sky is a reflection of those quantum ripples from the dawn of time.

Despite this astonishing progress, the universe remains an almost complete mystery. But it remains a mystery for all the
right
reasons. More evidence from observational astronomy led physicists to conclude that there must exist an extraordinary form of matter presently unknown to the standard model of particle physics. We know next to nothing about this form of matter. Whatever it is, it cannot be affected by the electromagnetic force, since then it would become visible to us (in the form of radiation). It cannot be affected by the strong nuclear
force, otherwise we would be able to observe its effects on visible matter. We know it exerts gravitational effects, and may also be susceptible to the weak force. It is utterly mysterious, and truly deserving of the name ‘dark matter'.

There's more. Observational astronomy also suggests that the expansion of the universe is (rather counter intuitively)
accelerating.
Present theoretical structures can accommodate an accelerating expansion by assuming that the universe is filled with an invisible energy field, which has inevitably become known as ‘dark energy'.

Dark matter and dark energy are no mere quirks. These are not mildly curious phenomena at the edges of our understanding waiting, like undotted i's and uncrossed t's, for the tidy pen strokes of explanation. When placed in a theoretical structure called the standard model of big bang cosmology, the most recent data from observational astronomy indicate that the density of dark matter represents about 22 per cent of all the mass-energy in the universe. The density of dark energy accounts for a further 73 per cent.

Visible physical matter and radiation — everything we can see in the universe and everything we are — accounts for just 5 per cent of everything there must be.

The universe is mostly missing.

Einstein's biggest blunder

The development of the ΛCDM (lambda: cold dark matter) model of the universe, also known as the standard model of big bang cosmology, is a triumph of modern physics. It will come as no surprise to learn that this is a development whose origins can be traced back to Einstein.

General relativity is all about the large-scale motions of planets, stars and galaxies and the structure of the universe within a four-dimensional spacetime. It deals with the universe in all its vastness.

At first, the task of applying general relativity to develop a theory of the entire universe (in other words, a cosmology) seems impossibly difficult. It's hard enough to keep track of the subtle interplay between gravitational forces and planetary motions within our own solar system. How then could it ever be possible to apply the theory to all the stars and galaxies in the observable universe?

The simple answer is: by making a few auxiliary assumptions. Although we can clearly see that the patterns of stars and galaxies are quite different in different parts of the night sky, we can nevertheless draw some simplifying conclusions about the ‘coarse-grained' structure of the universe.

For one thing, there are no large patches of night sky that are completely devoid of starlight. The universe looks more or less the same in all directions, in the sense that we see roughly the same numbers of stars and galaxies, with roughly the same brightness. Secondly, the stars and galaxies that we see are not vastly different from one another in composition. There are certainly differences in the sizes of stars, galaxies and clusters of galaxies, and this leads to differences in their physical behaviour, but they are all made of the same kind of ‘stuff, mostly' hydrogen and helium.

So, we can assume that the universe is roughly uniform in all directions and uniform in composition. We must also further assume something called the
cosmological principle,
which states that stargazers on earth occupy no special or privileged position in the universe.
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What we see from our vantage point on earth (or earth orbit) accurately reflects the way the universe appears from any or all such vantage points. What we see is a ‘fair sample' of the universe as a whole.

With these assumptions in place, in 1917 Einstein applied the general theory of relativity to the entire universe. But he immediately hit a major problem. He expected that the universe that should emerge from his calculations would be consistent with prevailing scientific prejudice — a universe that is stable, static and eternal. What he got instead was a universe that is unstable and dynamic.

Gravity is the weakest of nature's forces, but it is cumulative and inexorable and acts only in one ‘direction' — it attracts but does not repel. Einstein realized that the mutual gravitational attraction between all the masses in the universe would inevitably result in a universe that
collapses in on itself. This was a disastrous result, quite inconsistent not only with prevailing scientific opinion but also arguably with simple observation. Several centuries of astronomy had yielded no evidence that all the stars in the universe were rushing towards each other in a catastrophic collapse.

This was a problem that was neither new nor a particular feature of general relativity. When applied to the universe as a whole, Newton's gravity also predicts a collapsing universe. Newton had resolved the problem by suggesting that God acts to keep the stars apart: ‘… and lest the systems of the fixed stars should, by their gravity, fall on each other mutually, he hath placed those systems at immense distances one from another'.
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Einstein felt he needed something a little more scientific than this.

His solution was to modify arbitrarily the equations of general relativity as applied to the universe by introducing a ‘cosmological constant', usually given the symbol Λ (lambda). This is the ‘extension to the field equations' referred to in the title quotation.

In essence, the cosmological constant imbues space itself with a kind of anti-gravitational force, a negative pressure which increases in strength over longer distances. By carefully selecting the value of this constant, Einstein found that he could counterbalance the gravitational attraction that tended to pull everything together with a space that tended to push everything apart. The result was equilibrium, a static universe.

It was a relatively neat solution. Introducing the cosmological constant didn't alter the way general relativity works over shorter distances, so the successful predictions of the perihelion of Mercury and the bending of starlight were preserved. But it was, nevertheless, a rather unsatisfactory ‘fudge', one that was ‘not justified by our actual knowledge of gravitation'. There was no evidence for the cosmological constant, other than the general observation that the universe
seems
to be stable, and static.

Einstein found it all rather ugly and would come to regret his decision, as he later revealed to Ukrainian-born theoretical physicist George Gamow: ‘When I was discussing cosmological problems with Einstein he remarked that the introduction of the cosmological term was the biggest blunder he ever made in his life.'
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The expanding universe

Einstein had taken great pains to ensure that the solutions to the gravitational field equations of general relativity yielded a universe that conformed to physical experience. But, of course, equations are just equations — the fact that they can be applied to physical problems doesn't necessarily mean that the only solutions are physically realistic or sensible ones.

In 1922, Russian physicist and mathematician Alexander Friedmann offered three models based on solutions of Einstein's field equations. These were essentially descriptions of three different kinds of ‘imaginary' universe.

In the first, the density of mass-energy is high (lots of stars in a given volume of space) and spacetime is
expanding,
although the rate of expansion is modest. Such a universe is said to be ‘closed': it would expand for a while before slowing, grinding to a halt and then turning in on itself and collapsing. In the second, the density of mass-energy is low (fewer stars) and the effects of gravity are insufficient to overcome expansion. Such a universe is said to be ‘open', and would expand for ever.

In the third model, the density of mass-energy and the rate of expansion are finely balanced, such that gravity can never quite overcome the expansion. Such a universe is said to be ‘flat'. The rate of expansion slows but it never stops. And a slow rate of expansion would give the appearance of a static universe.

Each of these different universes is characterized by the value of a density parameter, given the symbol Ω (omega), the ratio of the density of mass-energy to the critical value required for a flat universe. A closed universe has Ω greater than 1, an open universe has Ω less than 1 and a flat universe has Ω equals 1.

Friedmann's model universes were very different to Einstein's. They were dynamic, not static. Einstein initially rejected Friedmann's solutions as wrong, and was quickly obliged to retract when he realized that, mathematically speaking, the solutions were perfectly acceptable:

I am convinced that Mr Friedmann's results are both correct and clarifying. They show that in addition to the static solutions to the field equations there are time varying solutions with a spatially symmetric structure.
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But the retraction referred only to the mathematical rigour of Friedmann's analysis. Einstein was convinced that the idea of an expanding universe had nothing to do with reality.

Tragically, Friedmann died in 1925. His expanding-universe solutions were independently rediscovered two years later by Belgian theorist (and ordained priest) Georges Lemaître. But Lemaître published his results in French in a rather obscure Belgian journal, and they attracted little attention.

Hubble's law

By 1931, everything had changed. Einstein was forced to accept that an expanding universe was not only possible, but appeared to describe the universe we inhabit. He publicly acknowledged that Friedmann and Lemaître had been right, and he had been wrong.

What had convinced him were the results of a series of observations reported by American astronomer Edwin Hubble and his assistant Milton Humason. The results pointed to a relatively unambiguous conclusion: most of the galaxies we observe in the universe are moving away from us.

Hubble had already radically transformed our understanding of the universe in the early 1920s. He had shown that what had appeared to be wispy patches of interstellar gas and dust — called nebulae — were in fact vast, distant galaxies of stars much like our own Milky Way. This revolutionary revision in our understanding that the Milky Way is but one of a huge number of galaxies not only greatly increased the size of the known universe; it also begged questions concerning a growing mystery surrounding their speeds.

Starting in 1912, American astronomer Vesto Slipher at Lowell Observatory in Flagstaff, Arizona, had used the Doppler effect to investigate the speeds of what were then still judged to be nebulae. The technique works like this. When we receive a wave signal (light or sound) from a moving object, we find that as the object approaches, the waves become bunched and their pitch (frequency) is detected to be higher than the frequency that is actually emitted. As the object moves away, the waves become spread or stretched out, shifting the pitch to lower frequencies. The effect is familiar to anyone who has listened to the siren of an ambulance or police car as it speeds past.

If we know the frequency that is emitted by the source, and we measure the frequency that is detected, then we can use the difference to calculate the speed at which the source is moving, towards or away from us, the receiver.

Stars are composed mostly of hydrogen and helium atoms. Hydrogen atoms consist of a single proton orbited by a single electron. Depending on its energy, the electron wave particle may be present in any one of a number of ‘orbitals' inside the atom, forming discrete shapes or clouds of probability relating to where the electron actually is. Each orbital has a characteristic, sharply defined energy. Consequently, when an excited electron releases energy in the form of a photon, the frequency of the photon lies in a narrow range, determined by the difference in the energies of the two orbitals involved.

The result is an atomic
spectrum,
a sequence of ‘lines' with each line representing the narrow range of radiation frequencies absorbed or emitted by the various electron orbital states inside the hydrogen atom. These frequencies are fixed by the energies of the orbitals and the physics of the absorption or emission processes. They can be measured on earth with great precision.

But if the light emitted by a hydrogen atom in a star that sits in a distant galaxy is moving relative to our viewpoint on earth, then the spectral frequencies will be shifted by an amount that depends on the speed with which the galaxy is moving.

In his observations, Slipher actually used two light frequencies characteristic of calcium atoms. He discovered that light from the Andromeda nebula (soon to be relabelled the Andromeda galaxy) is blueshifted (higher frequencies), suggesting that the galaxy is moving at high speed
towards
the Milky Way.
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However, as he gathered more data on other galaxies, he found that most are redshifted (lower frequencies), suggesting that they are all moving away.

Hubble and Humason now used the more powerful 100-inch telescope at Mount Wilson near Pasadena, California, to gather data on more galaxies. What they found was that the majority of galaxies are indeed receding from us. Hubble discovered, in what appears to be an
almost absurdly simple relationship, that the speed at which each galaxy is receding is proportional to the galaxy's distance. This is
Hubble's law.
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