by Neil Turok
It would fall to two very unusual people to see what Einstein could not: that his theory describes an expanding universe.
THE FIRST WAS ALEXANDER Friedmann, a gifted young Russian mathematical physicist who had been decorated as a pilot in the First World War. Due to the war and the Russian Revolution that followed it, news of Einstein’s theory of general relativity did not reach St. Petersburg, where Friedmann worked, until around 1920. Nevertheless, within two years, Friedmann was able to publish a remarkable paper that went well beyond Einstein. Like Einstein, Friedmann assumed the universe, including ordinary matter and the cosmological term, to be uniform across space and in all directions. However, unlike Einstein, he did not assume that the universe was static. He allowed it to change in size, in accordance with Einstein’s equation.
What he discovered was that Einstein’s static universe was completely untypical. Most mathematical solutions to Einstein’s equations described a universe which was expanding or collapsing. Einstein reacted quickly, claiming Friedmann had made mathematical errors. However, within a few months, he acknowledged that Friedmann’s results were correct. But he continued to believe they were of exclusively mathematical interest, and would not match the real universe. Remember, at the time of these discussions, very little was known from observations. Astronomers were still debating whether our own Milky Way was the only galaxy in the universe, or whether the patchy clouds called “nebulae,” seen outside its plane, were distant galaxies.
Einstein’s reason for disliking Friedmann’s evolving universe solutions was that they all had singularities. Tracing an expanding universe backward in time, or a collapsing universe forward in time, you would typically find that at some moment all of space would shrink to a point and its matter density would become infinite. All the laws of physics would fail at such an event, which we call a “cosmic singularity.”
Friedmann nevertheless wondered what would happen if you followed the universe through a singularity and out the other side. For example, in some models he studied, the universe underwent cycles of expansion followed by collapse. Mathematically, Friedmann found he could continue the evolution through the singularity and out into another cycle of expansion and collapse. This idea was again prescient, as we will discuss later.
Today, Friedmann’s mathematical description of the expansion of the universe provides the cornerstone of all of modern cosmology. Observations have confirmed its predictions in great detail. But Friedmann never saw his work vindicated. In the summer of 1925, he made a record-breaking ascent in a balloon, riding to 7,400 metres, higher than the highest mountain in all Russia. Not long afterwards, he became ill with typhoid and died in hospital.
TWO YEARS LATER, UNAWARE of Friedmann’s work, a Belgian Jesuit, Abbé Georges Lemaître, was also considering an evolving universe. Lemaître added a new component: radiation. He noticed that the radiation would slow the expansion of the universe. He also realized that the expansion would stretch out the wavelength of electromagnetic waves travelling through space, causing the light emitted from distant stars and galaxies to redden as it travelled towards us. The U.S. astronomer Edwin Hubble had already published data showing a reddening of the starlight from distant galaxies. Lemaître interpreted Hubble’s data to imply that the universe must be expanding. And if he traced the expansion back in time, billions of years into our past, he found the size of the universe reached zero: it must have started at a singularity.
Once more, Einstein resisted this conclusion. When he met Lemaître in Brussels later that year, he said, “Your calculations are correct, but your grasp of physics is abominable.”64 However, he was once more forced to retract. In 1929, Hubble’s observations confirmed the reddening effect in detail and were quickly recognized as confirming Lemaître’s predictions. Still, many physicists resisted. As Eddington would say, the notion of a beginning of the world was “repugnant.”65
Lemaître continued to pursue his ideas, trying to replace the singular “beginning” of spacetime with a quantum phase. What he had in mind was to use quantum theory to prevent a singularity at the beginning of the universe, just as Bohr had quantized the orbits of electrons in atoms to prevent them from falling into the nucleus. In a 1931 article in Nature, Lemaître stated: “If the world has begun with a single quantum, the notions of space and time would altogether fail to have any meaning at the beginning: they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time.” 66 Lemaître called his hypothesis “the Primeval Atom,” and as we’ll see, it prefigured the ideas of the 1980s.
In January 1933, there was a charming encounter between Einstein and Lemaître in California, where they had both travelled for a seminar series. According to an article in the New York Times,67 at the end of Lemaître’s talk, Einstein stood and applauded, saying, “This is the most beautiful and satisfactory explanation of creation to which I have ever listened.” And they posed together for a photo, which appeared with the caption: “Einstein and Lemaître — They have a profound admiration and respect for each other.”
If ever one needed confirmation of the idea that diversity feeds great science, it is provided by these strange encounters between Einstein, and, first, a Russian aviator, and then a Belgian priest. The meetings must have been intense: our whole understanding of the cosmos rested in the balance.
SHORTLY AFTER THE SECOND World War, George Gamow, a former student of Friedmann’s at St. Petersburg, made the next big step forward by bringing nuclear physics into cosmology. “Geo,” as he was known to his friends, was a Dionysian personality — fond of jokes and pranks, a heavy drinker, larger than life. His great strength as a scientist was his audacious insight: he wasn’t one to bother with details, but enjoyed the big picture. He did much to encourage others to work on interesting problems, especially applications of nuclear physics. For example, in 1938, he and Edward Teller, best known as “the father of the hydrogen bomb,” organized a conference called “Problems of Stellar Energy Sources,” bringing physicists and astronomers together to work out the nuclear processes that power the sun and other stars. This historic meeting launched the modern description of stars in the universe, one of the most successful areas of modern science.
After the Russian Revolution, Gamow had been the first student to leave on an international exchange, spending time in Copenhagen with Bohr and then in Cambridge with Rutherford. Eventually, and with many regrets, he defected to the West and became a professor at George Washington University in Washington, D.C. No doubt due to his Russian connections, Gamow was not cleared to work on the atomic bomb, but he did consult for the U.S. Navy’s Bureau of Ordnance. In this capacity he played the role of go-between, carrying documents up to Princeton for Einstein to examine at his home there. Apparently, they would work on these documents in the mornings and discuss cosmology in the afternoons.68
Gamow was an expert in nuclear physics. In 1928, he had explained the radioactive decay of heavy atomic nuclei, discovered by Curie, as being due to the quantum tunnelling of subatomic particles out of the nuclei’s interiors. When Gamow started thinking about cosmology, his goal was typically ambitious: to explain the abundances of every chemical element in nature, from hydrogen through the whole periodic table.
His idea was simplicity itself: if the early universe was hot, it would behave like a giant pressure cooker. At very high temperatures, space would be filled with a plasma of the most basic particles — like electrons, protons, and neutrons. At those temperatures, they would be flying around so fast that none of them could stick together. As the universe expanded, it would cool, and the neutrons and protons would stick together to form atomic nuclei.
His student Ralph Alpher, and another young collaborator, Robert Herman, worked out the details, combining Friedmann and Lemaître’s equations with
the laws of nuclear physics to figure out the abundances of the chemical elements in the universe today. The approach worked well for the lightest elements — hydrogen and its heavier isotopes, and helium and lithium — and is now the accepted explanation for their relative abundances. But it failed to explain the formation of heavier elements, like carbon, nitrogen, and oxygen, later understood to have formed in stars and supernovae, and for this reason did not attract the interest it deserved.
Lying buried in Alpher, Gamow, and Herman’s papers was a most wonderful prediction: the hot radiation that filled space in the early universe would never entirely disappear. One second after the singularity, when the first atomic nuclei started to form, the radiation’s temperature would have been billions of degrees. As the universe expanded, the temperature of the radiation would fall, and today it would be just a few degrees above absolute zero.
Alpher, Gamow, and Herman realized that today’s universe should be awash with this relic radiation — the cosmic microwave background radiation. In terms of its total energy density, it would greatly exceed all of the energy ever radiated by every star formed in the universe. And its spectrum — how much energy there is in the radiation at each wavelength — would be the same as that of a hot object, the very same spectrum described by Max Planck in 1900 when he proposed the quantization of light.
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UP TO THIS POINT, discussions of the hot big bang theory were almost entirely theoretical. That situation was about to change dramatically. In 1964, working in Holmdel, New Jersey, Arno Penzias and Robert Wilson unintentionally detected the cosmic microwave background radiation. Their instrument was a giant, ultrasensitive radio antenna that had been built at Bell Labs, AT&T’s research laboratory in New Jersey, for bouncing radio signals off a large metallic balloon in space, called Echo 1. The experiments with Echo 1 were part of the effort to develop global communications technologies following the wartime development of radar.
The trials with Echo 1 were followed by the deployment of the first global communications satellite, Telstar. Launched in 1962 and looking a bit like R2-D2, the 170-pound satellite was blasted into space on the back of a modified missile. Telstar relayed the first transcontinental live TV show, watched by millions. It inspired a hit song, titled “Telstar,” by a British band called the Tornados. The song opens with sounds of radio hiss, crackle, and electronic blips that give way to a synthesizer tune with an optimistic melody line. Even now it sounds futuristic. The song was an instant hit — the first British record to reach number one in the U.S., it ultimately sold five million copies worldwide. Ironically, Telstar was felled by atmospheric nuclear tests — the increases in radiation in the upper atmosphere overwhelmed its fragile transistors. According to the U.S. Space Objects Registry, its corpse is still in orbit.69
Telstar transformed TV’s grasp of the world. In parallel with NASA’s plans for the first lunar landing in 1969, a global network of communications satellites was placed into geosynchronous orbit. The network was ready just in time for the world to watch on TV as the Apollo 11 astronauts stepped onto the moon.70
With Telstar’s deployment, the giant radio antenna for receiving signals from Echo 1 was no longer required. Penzias and Wilson, who both earned Ph.D.s in astronomy before joining Bell Labs, were allowed to use the antenna as a radio telescope, and they jumped at the chance. But as they collected their first data, they discovered a persistent hiss from the antenna, at microwave wavelengths, no matter which direction it faced on the sky. Try as they might, they could not get rid of the noise. Famously, they even tried scrubbing pigeon guano off the antenna, but the noise still came through.
Eventually, Penzias and Wilson’s attention was drawn to a lecture that had just been given by James Peebles, a young theorist working with Robert Dicke nearby at Princeton, predicting radiation from the cosmos at just the wavelengths that would account for the hiss in their antenna. Unaware of Alpher, Gamow, and Herman’s earlier calculations, Dicke and Peebles had reproduced their prediction of cosmic background radiation at a temperature of a few degrees Kelvin, corresponding to millimetre wavelengths — microwaves. So Penzias and Wilson phoned Dicke, then planning an experiment to search for the background radiation. As soon as Dicke put the phone down, he told his younger colleagues, “We’ve been scooped.” Penzias and Wilson’s discovery was the “shot heard around the world,” immediately convincing almost all physicists that the universe had started with a hot big bang.
IN 1989, A DEDICATED NASA satellite called the Cosmic Background Explorer (COBE) took these background radiation measurements to a whole new level of precision. At the time, I was a professor at Princeton and had been working for a decade on how to test unified theories through cosmological observations. It was an exciting field, using the universe itself as the ultimate laboratory to test ideas about physics at extremely high energies, well beyond the reach of any conceivable experiment. However, I was worried about two things: first, that many of the theories we were discussing were too contrived; and second, that the data was too limited to tell which theory was right.
Then I went to the most dramatic seminar I have ever attended. Held in the basement of the physics department in Princeton, it was the first unveiling of data from the COBE satellite. The speaker was my colleague in the department, David Wilkinson. Dave had been part of Bob Dicke’s original team that had been narrowly scooped by Penzias and Wilson. By now he had become one of the pioneers of experimental cosmology, having made increasingly refined measurements of the background radiation, culminating in his involvement with COBE. COBE marked a transition in the subject: before it, almost nothing in cosmology was known to a better accuracy than a factor of two. But after COBE, one precision measurement followed another. Theories are now routinely proved wrong on the basis of discrepancies with the data of only a few percent.
One of COBE’s main experiments was the Far Infrared Absolute Spectrophotometer (FIRAS), designed to measure the spectrum of the background radiation. The hot big bang theory predicted that this spectrum should be the Planck spectrum predicted by Max Planck when he first proposed the quantization of light to describe radiation from a hot body. The background radiation was emitted from the hot plasma of the early universe four hundred thousand years after the expansion began, when the temperature had fallen to a few thousand degrees. At that time, the radiation took the form of red light. Once released, the radiation was stretched out a thousand-fold as it travelled across space, so that today most of the energy is carried in wavelengths of a few millimetres. Today, its spectrum is that of a hot body at just a few degrees Kelvin. In fact, to calibrate its measurements, FIRAS compared the sky with a perfectly black internal cavity whose temperature was dialled up and down to match that of the sky.
As Wilkinson put up a slide showing the measured spectrum, he said: “Here’s a plot to bring tears to your eyes.” There was an audible gasp from the audience. The data looked too good to be true. After just ten minutes of data, the measurements from the sky fit the Planck spectrum perfectly. By the time FIRAS completed its work, at every single frequency of radiation measured, the intensity matched the Planck spectrum to better than one part in a hundred thousand, and the temperature of the sky was measured at 2.725 degrees Kelvin. The measurements showed, in the most convincing way possible, that we live in a universe full of radiation left over from a hot big bang.
All I could think was, “Wow. The whole universe is shining down on us, saying, ‘Quantum mechanics, quantum mechanics, quantum mechanics.’”
COBE HAD MORE IN store. Its measurements would indicate that quantum mechanics not only governed the cosmic radiation, it controlled the structure of the universe.
Following Penzias and Wilson’s discovery, astrophysicists such as Peebles had been trying to work out how galaxies and other structures could have been formed from small density variations present in the very early universe. Einstein’s equations predict how such var
iations grow with time. Places where there is greater density expand more slowly, and their gravity pulls more mass towards them. And places where there is less density expand more rapidly and empty out. In this way, gravity is the great sculptor of the universe, shaping planets, stars, galaxies, galaxy clusters, and all the other structures within the cosmos. Using Einstein’s equations, we can trace this shaping process in reverse to figure out how the universe looked at earlier times. In other words, we can use the equations to retrace cosmic evolution. And then we can work out how the density variations should appear in the background radiation’s temperature as it is mapped across the sky.
The COBE satellite carried another experiment called the Differential Microwave Radiometer (DMR). It was designed to scan for tiny differences in the temperature of the background radiation across the sky. The principal investigator of that experiment was George Smoot of the University of California, Berkeley. Smoot, like Wilkinson, had pioneered measurements of the cosmic background radiation throughout his career.
I first met Smoot in 1991 at a summer school in Italy. Everyone wanted to know whether COBE’s DMR experiment had detected any variations in the temperature across the sky (click to see photo). In Smoot’s lectures, he showed a perfectly uniform map of the temperature. In private, he showed me slides of the actual measurements, which showed faint milky patches. Smoot was at that time still skeptical about whether the patches were real, and thought they might be an artefact of the experiment. And he had little confidence in theory: ever since the 1960s, theorists had been steadily lowering their predictions for the level of the expected temperature variations — from one part in a thousand, to a part in ten thousand, to one part in a hundred thousand, making them harder and harder for experimentalists like Smoot to detect.
The theorists had a reason to lower their predictions: they did so because of the growing evidence for dark matter (click to see photo). Dark matter’s existence was first inferred in the 1930s by Swiss astronomer Fritz Zwicky, from observations of galaxies orbiting other galaxies in clusters. Their high velocities implied that more mass was present than could be accounted for by the visible stars. In the 1970s, U.S. astronomer Vera Rubin observed a similar effect in the outskirts of galaxies, where stars are seen to orbit so fast that if it were not for some extra gravity holding them in, they would escape from the galaxies.