Origin Story, page 4
A lot more matter consists of dark matter, stuff we don’t yet understand, though we know it exists because its gravitational pull determines the structure and distribution of galaxies. So, a few minutes after the big bang, our universe consisted of vast clouds of dark matter in which were embedded crackling plasmas of protons and electrons with photons of light flowing through them. Today, we find plasmas only in the centers of stars.
Now we must pause and wait about 380,000 years (almost twice as long as our species has existed on Earth). During this time, the universe kept cooling. When temperatures fell below ten thousand degrees Celsius, there was one more phase change, like steam turning into water. To explain this phase change, we need to understand that heat is really a measure of the motion of atoms. All particles of matter are constantly jiggling about with energy, like nervous children, and temperature is a measure of the average degree of jiggling. The jiggling is real. In a famous paper published in 1905, Einstein showed that the jiggling of atoms causes the random gyrations of dust particles in the air. As temperatures drop, particles jiggle less, until eventually they can link up. As the universe cooled, the electromagnetic force tugged negatively charged electrons toward positively charged protons until the electrons calmed down enough to fall into orbits around protons. And voilà! We had the first atoms, the basic constituents of all the matter around us.
Normally, isolated atoms are electrically neutral, because the positive and negative charges of their protons and electrons cancel each other out. So when the first atoms of hydrogen and helium formed, most of the matter in the universe suddenly went neutral, and the tingling plasma evaporated. Photons, the carriers of the electromagnetic force, could now flow freely through an electrically neutral mist of atoms and dark matter. Today, astronomers can detect the results of this phase change, because photons that escaped the plasma generated a thin background hum of energy (the cosmic microwave background radiation) that still pervades the entire universe.
Our origin story has crossed its first threshold. We have a universe. Already it has some structures with distinctive emergent properties. It has distinct forms of energy and matter, each with its own personality. It has atoms. And it has its own operating rules.
What’s the Evidence?
Bizarre as this story may seem when you hear it for the first time, we have to take it seriously, because it is supported by vast amounts of evidence.
The first clue that the big bang really happened was the discovery that the universe is expanding. If it’s expanding now, logic tells us that at some time in the remote past, it must have been infinitesimally small. We know the universe is expanding because we have instruments and observational techniques that were not available to the people of Lake Mungo, even though we can be sure they were superb naked-eye astronomers.
Most astronomers since Newton’s time assumed that the universe must be infinite, because if it was not, the laws of gravity should have gathered its contents into a single gluggy mass, like oil in a sump. By the nineteenth century, astronomers had instruments precise enough to start mapping the distribution of stars and galaxies, and the astronomical maps they created began to hint at a very different picture of the universe.
The mapping began with nebulae, fuzzy blurs that popped up on all their star charts. (We now know that most nebulae are entire galaxies, each with billions of stars.) How far away were the nebulae? What exactly were they? Were they moving? Over time, astronomers have learned how to tease out more and more information about stars from the light they emit. That information includes their distance from us and whether they are heading closer or moving away.
One of the cleverest methods to study the movement of stars and nebulae uses the Doppler effect (named after the nineteenth-century Austrian mathematician Christian Andreas Doppler) to measure the speed at which stars or nebulae are moving toward or away from us. Energy travels in waves, and waves, like those at the beach, have a frequency. They reach peaks at a regular pace that you can measure. But the frequency changes if you move. If you get in the ocean and swim out, the frequency at which you encounter waves will seem to increase. The same thing happens with sound waves. If an object, such as a motorbike, is making a noise and moving toward you, the frequency of the sound waves will seem to increase, and your ears will interpret the higher frequency as a higher pitch. After it passes you, the pitch will seem to drop, because now the waves are being stretched out. The rider, of course, is not moving relative to the motorbike and keeps hearing the same pitch. The Doppler effect is the apparent change in frequency of electromagnetic emissions as objects move toward or away from each other.
The same principle works with starlight. If a star or galaxy is moving toward Earth, the frequency of its light waves will seem to increase. Our eyes interpret higher-frequency visible light as blue light, so we say it has shifted toward the blue end of the electromagnetic spectrum. But if it is moving away from Earth, the frequency of its light will seem to shift toward the red end of the spectrum; astronomers say it is redshifted. And we can tell how fast a star or galaxy is moving by measuring how much the frequency has shifted.
In 1814, a young German scientist, Joseph von Fraunhofer, created the first scientific spectroscope, a specialized prism that splits up the frequencies of starlight just as a glass prism splits light into the colors of the rainbow. Fraunhofer found that spectra from sunlight had thin dark lines at particular frequencies, like cosmological bar codes. Two other German scientists, Gustav Kirchhoff and Robert Bunsen, eventually showed in the lab that particular elements emit or absorb light energy at specific frequencies. It seemed that the dark lines were the result of light from the sun’s core being absorbed by atoms of different elements in the sun’s cooler outer regions. This reduced the energy at those frequencies, leaving dark lines on the emission spectrum. We call these dark lines absorption lines, and different elements generate different patterns of absorption lines. For example, there are lines that are typical of carbon and iron. If starlight is redshifted, then all these lines shift to the red end of the spectrum, and we can even measure exactly how far they have shifted. This is the astronomer’s equivalent of a police speed trap.
In the early twentieth century, an American astronomer, Vesto Slipher, used these techniques to show that a surprising number of astronomical objects were redshifted—that is, they were moving away from Earth, and quite fast. That scattering was very strange. Its real meaning became clear only when another American astronomer, Edwin Hubble, combined these findings with measurements of the distance to these remote objects.
Estimating the distance to stars and nebulae is tricky. In principle, as the Greeks understood, you could use the parallax method, like a surveyor. Over the months, as Earth swings around the sun, watch to see if some stars in the night sky seem to move relative to other stars. If they do, you can use trigonometry to figure out how far away they are. Unfortunately, even the nearest star, Proxima Centauri, is so distant (about four light-years from Earth) that you cannot detect any motion without fancy equipment. Not until the nineteenth century were astronomers able to measure the distance to nearby stars using parallax. But in any case, the objects Vesto Slipher was studying were much more distant.
Fortunately, in the early twentieth century, Henrietta Leavitt, a Harvard Observatory astronomer, found a way to measure the distance to remote stars and nebulae using a particular type of star known as a Cepheid variable, a star whose brightness varies with great regularity (the polestar is a Cepheid). She found a simple correlation between the frequency of the variations and the star’s luminosity, or brightness, so she could calculate a Cepheid’s absolute brightness. Then, by comparing that with the apparent brightness the star had when seen from Earth, she could calculate how far away it was, because the amount of light from a star diminishes by the square of the distance through which it travels. This wonderful technique provided the astronomical standard candles that Edwin Hubble needed to make two profound discoveries about our universe.
Early in the twentieth century, most astronomers believed that the entire universe was contained within our galaxy, the Milky Way. In 1923, Hubble used one of the world’s most powerful telescopes, at the Mount Wilson Observatory in Los Angeles, to show that Cepheid variables in what was then known as the Andromeda nebula were so far away that they could not be in our own galaxy. This proved what some astronomers had suspected: that the universe was much larger than the Milky Way and consisted of many galaxies, not just our own.
Hubble made an even more astonishing discovery as he began to measure the distance to large numbers of distant objects using Cepheid variables. In 1929, he demonstrated that almost all galaxies appeared to be moving away from us and that the most remote objects seemed to have the largest redshifts. In other words, the more distant an object was, the faster it was moving away. And that seemed to mean that the entire universe was expanding. The Belgian astronomer Georges Lemaître had already suspected this on purely theoretical grounds. And, as Lemaître pointed out, if the universe was currently expanding, at some time in the past, everything in it must have been compressed into a tiny space, something he described as the primordial atom.
Most astronomers were shocked by the idea of an expanding universe and assumed there was an error in Hubble’s calculations. Hubble himself was not at all sure about it, and Einstein was so convinced the universe was stable that he fiddled with the equations of general relativity so they would predict a stable universe, by adding what he called a cosmological constant.
Astronomers were skeptical partly because there really were problems with Hubble’s estimates. He calculated that the expansion of the universe had begun just two billion years ago, yet astronomers already knew that Earth and its solar system were much older than that. That is one reason why, for several decades, most astronomers regarded the idea of an expanding universe as intriguing but probably wrong. Many preferred the alternative idea of a steady-state universe, proposed in 1948 by Hermann Bondi, Thomas Gold, and Fred Hoyle. Yes, agreed the steady-staters, galaxies seemed to be moving apart, but new matter was being created at the same time, so at large scales, the universe remained at about the same density and changed little.
Eventually, though, the evidence tipped in favor of an expanding universe. In the 1940s, Walter Baade, working at the Mount Wilson Observatory in LA (the same observatory at which Hubble had worked), showed there were two types of Cepheid variable stars, and they yielded different estimates of distance. Baade’s revised calculations suggested that the big bang might have happened more than 10 billion years ago (current best estimates suggest it occurred as much as 13.82 billion years ago). This eliminated the chronology problem. Today we know of no astronomical objects older than 13.82 billion years, which is a strong argument in favor of big bang cosmology. After all, if the universe were unchanging and eternal, there really should be lots of objects more than 13.8 billion years old.
The clinching evidence came in the mid-1960s, and it involved the discovery of cosmic microwave background radiation (CMBR). This is the radiation released when the first atoms formed, about 380,000 years after the big bang. The CMBR turned out to be the crucial proof of an expanding universe. Why?
By the 1940s, some astronomers and physicists were impressed enough by Hubble’s data that they tried to figure out what might have happened if there really had been a big bang. What would the universe have been like at the start if everything was crushed into a primordial atom? If Hubble and Lemaître were right, the early universe would have been extremely dense and hot, and it must have been expanding and cooling fast. How would matter and energy behave under such extreme conditions? During the Second World War, the Manhattan Project to build an atomic bomb had encouraged research into the physics of very high temperatures. In the late 1940s, the Russian-born physicist George Gamow used insights from the Manhattan Project to figure out what had probably been going on in the universe just after the big bang. With a colleague, Ralph Alpher, he predicted that the universe would have eventually cooled enough for atoms to form, and when the first atoms formed, there should have been a huge release of energy as photons escaped the charged plasma of the preatomic era and began to flow freely through an electrically neutral universe. Further, they argued that this flash of energy should still be detectable, though its frequency would have fallen to near zero as it was stretched across an expanding universe. If scientists looked carefully enough, they would find radiation at temperatures close to absolute zero coming from all directions. To many this seemed a crazy idea, which was why no one started looking for low-temperature radiation pervading the entire universe.
In 1964, Gamow’s flash of radiation was detected by accident. At Bell Labs in Holmdel, New Jersey, two radio astronomers, Arno Penzias and Robert Wilson, were building a high-precision radio antenna to communicate with artificial satellites. To eliminate interference, they cooled down the receiver to about 3.5 degrees Celsius above absolute zero, but there remained a puzzling hum of low-temperature energy. It seemed to come from all directions, so they knew it was not from some massive stellar explosion. Suspecting a glitch in their receiver, they removed a pair of pigeons roosting in the hornlike antenna and cleaned out the droppings, but it made no difference. (Sadly, the pigeons kept trying to return to the antenna and eventually had to be shot.) Nearby, in Princeton, a team of astronomers led by Robert Dicke had just started to look for Gamow’s background radiation when they heard what Penzias and Wilson had found. They immediately realized they had been scooped. The two teams decided to collaborate on papers describing the discovery. They argued that it was probably the energy from just after the big bang that Gamow had predicted.
The discovery of cosmic microwave background radiation persuaded most astronomers that the big bang was real because no other theory could explain this all-pervading radiation. Making an odd but ultimately successful prediction like this is one of the most powerful ways of persuading scientists that your theory is sound. The universe, it seemed, really was expanding, and it really had been created in a big bang.
Today, the evidence that our universe began in a big bang is overwhelming. Lots of details remain to be worked out, but for the time being, the core idea is firmly established as the first chapter of a modern origin story. That’s the bootstrap. And, as quantum physics allows things to appear from a vacuum, it seems that the entire universe really did pop out of a sort of nothingness full of potential.14
CHAPTER 2
Stars and Galaxies: Thresholds 2 and 3
Mankind is made of star stuff.
—HARLOW SHAPLEY, VIEW FROM A DISTANT STAR
The big bang gave us a universe, but for several hundred million years the universe was extremely simple. Beneath the surface, though, interesting new possibilities were stirring, and eventually, stars and galaxies began to light up the darkness. They added an entirely new cast of characters, new emergent properties, and new forms of complexity, and they led the universe across a second threshold of increasing complexity. To explain how these majestic new objects emerged, we need to go back to the beginning.
Free Energy: The Driver of Complexity
In the seconds and minutes after the big bang, the universe was in thermodynamic free fall. For a dazzling few moments, there was enough energy to make and unmake exotic new forms of energy and matter. But as temperatures plummeted, energy and matter froze into a few simple structures. In the kiln of the big bang, forces and particles stabilized like fired pottery. Together, the violent energies of the big bang and a few simple operating rules had created structures such as protons and electrons that would prove remarkably stable, because the temperatures that created them would rarely appear again in a cooling universe.
Then the rapid descent slowed, rather as if the universe were falling down a thermodynamic mountain into a valley. Gradients flattened, temperatures dropped less precipitously, and the pace of change decreased as the thermodynamic cliff face of the early universe gave way to a flatter, undulating landscape in which temperatures could rise as well as fall. Now it got harder to lock new structures in because they could be unraveled by even modest increases in heat. Atoms, for example, fell apart inside the first stars when temperatures rose above about ten thousand degrees Celsius.
In these less predictable environments, complex structures needed extra bracing if they were to stabilize. That bracing was provided by controlled, nonrandom flows of energy. Stars are held together by flows of energy generated in their cores. Living organisms, including you and me, are held together by delicate and precisely directed flows of energy managed by intricate metabolic processes in our cells. In a post–big bang universe, it takes work to build and maintain new complex structures. This is why there is a deep link between form, complexity, and directed or structured flows of energy.
Structured flows of energy is an intuitive description rather than a piece of scientific jargon. But here’s the idea it’s getting at: Thermodynamic theory distinguishes between energy flows that are completely random and energy flows that have direction, structure, and coherence so they can do work. Structured flows of energy are known as free energy, and unstructured flows are known as heat energy. The difference is not absolute. We’re really talking about degrees of coherence or randomness. Nevertheless, the distinction between free energy and heat energy is fundamental to our origin story.
The first law of thermodynamics tells us that the total amount of energy in the universe never changes. It is conserved. Our universe seems to have arrived with a fixed potential for things to happen. So the first law is really telling us about the primordial ocean of possibilities. The second law of thermodynamics tells us that the things that emerge from the ocean of possibilities can be more or less structured, like the ripples in a stream. But we should expect most of them to be less structured and become even less structured over time. That is because most possible arrangements of matter and energy have little or no structure, and if by chance you do find structure, expect it to decay fast.
