B00B7H7M2E EBOK (31 page)

A third stubborn puzzle plaguing Big Bang theorists after Wilson and Penzias’s discovery was how a universe that looked so uniform when it was 300,000 years old had become so diverse and clumpy all these years later. In repeated measurements, researchers found that the cosmic microwave background radiation was disappointingly uniform in temperature. The temperature was the same in readings taken out to the end of observability in every direction. This meant the early universe must have been extremely smooth, without lumps, clumps or irregularities that would show up as fluctuations in that temperature. How then could the universe have evolved to have galaxy clusters, galaxies, stars, planets – even such small clumps of matter as people? Somewhere back there must lie the seeds of those developments, but where?

Here is the problem: picture every particle of matter in the universe attracting every other by means of gravitational attraction. The closer to one another the particles are, the stronger they feel each other’s gravitational pull. If all particles of matter in the universe are equidistant, and there are no areas in which
a
few particles have drawn together even slightly more densely, then every particle will feel equal pull from every direction and none will budge to move closer to any other particle.

It was this sort of gridlock researchers seemed to have discovered in the early universe, where matter appeared to have been distributed so evenly that it could never yield to form the structure evident in the universe today. If this were not so, why couldn’t anyone find even the tiniest fluctuation in the background radiation – the ‘photograph’ of how matter was distributed back then?

Finally, in April 1992, astrophysicist George Smoot at Lawrence Berkeley Laboratory and the University of California at Berkeley announced that he and his cohorts at several other institutions had found the long-sought ‘wrinkles’. New data from a satellite called the Cosmic Background Explorer (COBE) had revealed the fluctuations in the cosmic microwave background radiation that astrophysicists had been seeking for a quarter of a century. The temperature fluctuations measured no more than a hundred-thousandth of a degree, but that was enough, the researchers felt, to explain what had happened to the universe. These minuscule variations in its topography when it was only 300,000 years old were evidence of a gravitational situation in which matter could have attracted matter into larger and larger clumps.

How did the tiny fluctuations get there? You and I in our innocence might think that in any real-life situation, having a few irregularities is something to be expected. It would be much more surprising
not
to have them. Surely this is the sort of non-problem only a mathematical physicist who hasn’t looked at the real world for a while would dream up! Even Isaac Newton commented that a situation in which matter is evenly spread is far less probable than getting a needle to stand on its point on a looking glass. But you and I, and Newton as well, for that matter, have been conditioned in a universe that is full of asymmetries and irregularities – the very situation modern
theorists
find suspicious. Why should the universe be like this?

There have been some stabs at answering the question of how the tiny fluctuations got there, but no observational evidence to help. Inflation theorists point out that according to the so-called Heisenberg uncertainty principle of quantum mechanics, what we call ‘empty space’ cannot actually be empty. Instead, always and everywhere in the universe there are tiny energy fluctuations. During the inflation period, the peaks and troughs generated by these fluctuations in the newborn universe would have been inflated large enough to serve as the seeds of all the irregularity to come.

In the year 2000, NASA was scheduled to launch its Microwave Anisotropy Probe. ‘MAP’ would measure the background radiation 30 times more precisely than the Cosmic Background Explorer did. Even more precise observations were expected to come from the European Space Agency’s Planck Satellite scheduled for in 2004. There were also several balloon-based missions in the offing.

Meanwhile, a wealth of evidence pointed to the fact that we do live in a Big Bang universe. To sum up its history, according to the theory as it stands amended by inflation theory: all that we observe or ever will be able to observe started out compressed in a state of almost unimaginable density. That exploded and everything – space itself – began to expand. After a short interval of extremely rapid inflation, the expansion slowed down and continued more sedately. Everything thinned out and cooled. All was smooth and virtually uniform except for some faint wrinkles, or ‘density fluctuations’. While expansion continued, the gravitational attraction of areas where matter was already concentrated more densely pulled in more matter, and thus matter began clumping, eventually forming stars, galaxies, clusters and superclusters of galaxies gravitationally bound to one another.

While observations and experiments were confirming earlier Big Bang predictions, theorists had been raising new
questions
about the moment of ‘beginning’ itself. One in particular was: Does an expanding universe that is not a Steady State universe have to have had a beginning?

Both everyday logic and mathematics seemed to indicate that in a universe where on a large scale everything is moving further and further from everything else, if we could reverse the direction of time and travel back towards the beginning, we would find things getting closer and closer together. Eventually everything would be in precisely the same place. Is there any other possible conclusion?

As early as 1963, Russian scientists Evgenii Lifshitz and Isaac Khalatnikov proposed another possible ending to this time-reversed story. They ran the history-of-the-universe film backwards, imagining a scenario in which the universe contracts and all the galaxies draw closer to one another, appearing to be on collision course. But Lifshitz and Khalatnikov pointed out that the galaxies have other motion in addition to the motion that brings them towards one another. Could it be that this additional motion might cause them, as they approach one another, to miss one another and fly past with a nod, so to speak? Was this the way to avoid having a beginning? If we kept watching the movie backwards, might we simply see the universe expand again? Or must everything have begun in the same spot?

It was this question that engaged Stephen Hawking of Cambridge and Roger Penrose of Oxford in the middle and late 1960s.

Hawking’s story is well known. He has become a legend in his time – the physicist who explores the frontiers of scientific knowledge and speculation, who works by spectacular leaps of intuition, who often seems to reverse himself, who writes best-selling books that readers struggle to understand. Motor neurone disease has locked Hawking’s body motionless in a wheelchair, but he is the most nimble-minded of men.

Roger Penrose also thinks outside the envelope. In his youth he discovered an ‘impossible object’ – which means a figure that
can’t
really exist because it contradicts itself. His father helped him turn the idea into the ‘Penrose Staircase’, and Maurits Escher used it in two of his famous lithographs:
Ascending and Descending
and
Waterfall
. Penrose also managed to visualize an impossible object in four-dimensional space. As he matured he never gave up ‘playful’ mathematics. He’s become one of the world’s most imaginative mathematicians, physicists and authors, and has discovered two shapes (‘Penrose tiles’) that in their three-dimensional forms may underlie a new kind of matter.

In 1965 Penrose, building on earlier work of John Archibald Wheeler, Subrahmanyan Chandrasekhar and others, showed that if the universe obeys general relativity and several other constraints, when a very massive star has no nuclear fuel left to burn and collapses under the force of its own gravity, it will be crushed to a point of infinite density and infinite spacetime curvature – a ‘singularity’. General relativity predicts the existence of singularities. In the early sixties some physicists speculated that a star of great enough mass undergoing gravitational collapse might form a singularity at the centre of a black hole, but very few took this prediction seriously. Penrose calculated that this will happen even if the collapse isn’t perfectly smooth and symmetrical. No ‘might’ about it. It must.

Hawking, in his doctoral thesis at Cambridge in 1965, reversed the direction of time and applied the same concept to the entire universe, suspecting that if he could watch the expansion of the universe run backwards he would discover something similar to what Penrose had found with black holes. Once the collapse (the expansion of the universe run in reverse) had proceeded far enough, whatever additional motions the galaxies had would make no difference to the history of the universe. By 1970 Hawking and Penrose were able to demonstrate, in Hawking’s words, ‘that if general relativity is correct, any reasonable model of the universe must start with a singularity’. Everything that was to be the matter/energy
of
the universe that human beings might eventually be able to observe would have been compressed not to the sphere Lemaître envisioned (the primeval atom) but to something much smaller than that – to a point of infinite density.

Well, that did it! Physical theories can’t work with infinite numbers. When the theory of general relativity predicts a singularity of infinite density and infinite spacetime curvature, it also predicts its own breakdown. All the theories of classical physics are useless at a singularity. There is no possibility of predicting what will emerge; one can only wait to observe what it will be. Indeed, why should anything emerge at all? There is no way to find out why this singularity suddenly ceases to be a singularity and becomes a universe. And what happened before the singularity? It’s not even clear whether that question has any meaning.

Hawking and Penrose’s discovery did not, however, put an end to attempts to devise an origin-of-the-universe story that would be more palatable to those unwilling to accept impenetrable obstacles and hints of a Creator. Hawking would soon be one of those most eagerly trying to untie the Gordian Knot he and Penrose had discovered.

CHAPTER 7

Deciphering Ancient Light

1946–1999

When people on airplanes ask me what I do, I used to say I was a physicist, which ended the discussion. I once said I was a cosmologist, but they started asking me about makeup, and the title ‘astronomer’ gets confused with astrologer. Now I say I make maps.

Margaret Geller

IN THE SECOND
half of the 20th century, the cosmic distance ladder became increasingly sturdy. Measurements using old and new techniques served as checks on one another, previous estimates were re-examined with improved technology and fresh theoretical understanding. It became possible to discern as never before the structure of the Galaxy and of the universe as a whole.

In 1946, researchers had first bounced a radar signal off the Moon. After that, astronomers were able to fine-tune distance measurements within the solar system by bouncing signals off the planets and the outer atmosphere of the Sun and timing how long it took the radar echo to come back. Later, unmanned missions visited the planets. If you send a spacecraft to Mars, and it gets there with only minor course corrections along the
way
, you know you have Mars’s distance and orbit more or less right. That settles any argument between you and Cassini.

Between 1838 – when Bessel, Henderson and von Struve first measured stellar parallax – and 1900, astronomers used the parallax method to measure approximate distances to no more than about 100 stars. Because the Earth’s atmosphere refracts light rays passing through, blurring the images of stars, the best ground-based telescopes, even today, measure accurate parallaxes of the brightest stars out to only about 300 light years, a minuscule distance by the standards of the Galaxy. The potential for considerable extension of that range came in the late 1950s with the ability to put telescopes beyond the atmosphere. By the early 1990s, astronomers had reasonably accurate parallaxes for close to 10,000 stars. In 1989, the European Space Agency launched the satellite Hipparcos – or High Precision Parallax Collecting Satellite. Hipparcos still measures parallaxes from the base line provided by the Earth’s orbit, but it can measure them several times further away than earth-bound telescopes, in a volume of space 100 times as large. By the mid-1990s, Hipparcos had swelled the catalogue of precisely measured distances to 120,000 stars. Before it came on line there was still no more accurate way to determine the distance to the nearest star cluster, the Hyades, than the old ‘moving cluster method’. Hipparcos measures its parallax directly.

Historically, one of the most important advances in cosmic measurement was finding that Cepheid variables could serve as ‘standard candles’, distance calibrators to measure beyond the parallax range. However, when it came to finding absolute distances, rather than how the distance of one star relates to another, this rung in the cosmic distance ladder depended on there being Cepheids close enough to be measured directly by parallax. There were none. The best that could be done was to measure their distance by a version of statistical parallax, which meant that the Cepheid rung was a less reliable part of the
ladder
than it might have been if there were closer Cepheids. Again the Hipparcos satellite finally promised a breakthrough. It has now measured a few Cepheid parallaxes directly. There is some question about the reliability of these measurements, but astronomers hope these uncertainties will be resolved by a project called the Space Interferometry Mission in the first decade of the 21 st century.