We have followed the history of physics up to its current frontiers, taking us deep into the first instants of the universe. Now we're ready to start our narrative of the natural history of the universe, beginning in those first instants. First, we need to get an idea of the kinds of timescales we are talking about. Timeline 1, the first of many timelines which the narrative part of the book is built around, shows the branching of particles and forces in the Big Bang. This figure summarizes this chapter, so I urge you to refer to it often. It covers the entire age of the universe, which has recently been established as very close to 13,700 million years. Notice, however, that the timeline is given in seconds. This may seem like a rather small increment for discussing the history of the universe. But it really isn't, because most of the events in this chapter take place within the first second. The reason I've extended the timeline to the current age of the universe, showing its age in seconds, is to highlight how very tiny some of the increments of time are that we will discuss. As the timeline shows, the universe is currently about 1017 seconds old. Yet scientists today are fairly confident about what happened back to the first 10-15 (.000000000000001) second or so, and they can tell a plausible story about what happened back to 10-43 of the first second - the smallest increment of time known according to current theories. One second, then, is an increment closer to the current age of the universe than to the smallest increments of time. In this chapter, we'll follow the history of the universe to its first 380,000 years. Most of this story, however, occurred in those astonishingly tiny increments of the first second.
There are two big things we have to remember about the beginning of the universe: 1. It began in an incredibly condensed, hot state, from which it expanded and thus cooled. 2. Matter can be created from energy, according to Einstein's equation: E=mc2. Let's look at these two features, and at how they help explain the Big Bang..
If you compress a gas, it gets hotter. Heat is simply the energy of vibrating atoms in a certain volume, so if you reduce the volume, more energy will be contained in a smaller volume - the atoms will vibrate and bounce off of each other more and more frantically. Now, if the universe is expanding today, it must have been more compressed in the past. The first law of thermodynamics says that the total energy in the universe is constant, so if the universe was more compressed in the past, its energy must have been concentrated in a smaller volume, making it unimaginably hot.
Now, if you compress a gas enough, you won't be able to compress it any more - to do so would take more energy than you can provide. With the universe at large, however, as we look back, imagining it getting smaller and smaller, such limits don't show up. It just keeps getting smaller. Running the film backward, the visible universe contracts to the size of a galaxy, a star, a football field, a football, an atom, a proton... And it keeps going, shrinking smaller than a proton by 18 orders of magnitude, to the Plank length - a distance of 10 -33 meters. This is the smallest unit of space that physicists can make much sense of. Beyond this outrageously tiny point, the known laws of physics break down. We don't know what happened when the universe was smaller, or if it ever was smaller. General relativity suggests that it could shrink to a point of zero size and infinite density, called a singularity. But this probably just means that general relativity has broken down in these extreme conditions. There are reasons to believe that the Plank length is actually the smallest size that things can shrink to. And really, that seems quite extreme enough. The Planck length is as much smaller than a proton as the average bacterium is smaller than the Earth's orbit. It's the smallest length known, and we are talking about compressing the largest thing known - the whole universe - to this size. It doesn't get much more extreme than that.
If the universe were shrunk to the Plank length, its temperature would be around 1032 Kelvin - about 1,000,000,000,000,000,000,000,000,000,000 degrees Celsius, give or take a few degrees. Luckily for us, it didn't stay that hot and dense. It began to expand. And just as a can of spray paint gets cool when you press the button and let it expand out of the can, the universe began to cool. In the process, its original ultra-symmetrical state began to fragment, just as the symmetry of liquid water fragments when it turns to ice. One consequence of this breaking of symmetry was the splitting of the forces of nature into the four forces we know today. Another consequence was the evolution of matter.
We are made of matter, so we tend to think in material terms. But matter is really rather scarce in the universe at large. Most of space is just that - empty space. On average, there is about one atom of matter for every square meter of space in the universe. All of those square meters, though, are filled with photons - a few energetic ones from glowing cosmic gas and shining stars, and hordes of weak ones comprising the background radiation from the beginning of the universe. There are millions of photons for every proton, neutron and electron. As we will see, this is because important periods in the early universe were not dominated by matter, but by light.
Today, the background radiation is cold, about 2.7 degrees k. Now, we know that heat is the vibration of atoms and molecules, if we are talking about matter. What does it mean to say how hot light is? As we have seen, the energy of a photon is proportional to its frequency, and high-frequency photons are more energetic than low-frequency ones. We've also seen that the expansion of space causes photons to become redshifted; in effect stretching out their wavelengths. This means that when the universe was smaller, these photons had shorter wavelengths, and thus, higher frequencies and higher energies. Today, the background radiation is that of a blackbody object at 2.7 k. This is how we can assign a temperature to light - it's the temperature of an object that would radiate light with the same spectrum.
The background radiation today is weak enough to be almost unnoticeable. But it wasn't always so. During the Big Bang, all those photons were incredibly powerful gamma rays, capable of doing something that we don't see light doing today - creating matter. Just as matter is converted into energy in stars and nuclear reactors, energy can be converted into matter. The way this works is shown in Figure 5.1. Two energetic photons collide with each other, annihilate, and produce a particle of anti-matter and a particle of matter - in this case, a positron and an electron. Einstein's equation shows that in order to create the particles, the energy of the photons must equal the mass of the particles, times the speed of light squared. A little mass requires a whole lot of energy.
Figure 5.1. Creation and Annihilation of Matter and Antimatter
Since matter and antimatter are produced at the same time, what usually happens is that the two particles simply collide with each other and annihilate back into photons. So, the very early universe was roiling soup of matter (and anti-matter) and photons. At first, there was an equilibrium between energy and matter - on average, there were about as many matter and anti-matter particles as there were photons.
Because it takes more energy to produce heavy particles, the earlier we go back in time, the more heavy particles we will see popping into existence. Thus, very early on the heavy quarks and leptons were being produced and annihilated. But such particles are unstable, so they quickly decayed even if they didn't annihilate with their anti-particles. Still, the earlier we look, the more heavy, exotic particles we would find, possibly including things like the X bosons of the Grand Unified Theories, and the heavy super-partners of the Supersymmetry theories.
For the most part, we will ignore these exotic particles, focusing on the evolution of the particles that are still around today, especially the ones we are made of. Just keep in mind that this is a simplification, and that these were just a handful of the exotic and heavy particles that were around in the first instants. We'll also focus on photons, even though they were not the only energy-carrying particles. All the other bosons were around, and as we move backward, they become more alike, as all the forces merged together. So, matter was not just created by photons; it was created by whatever force-carrying particles were present at the time that had enough energy. For our purposes, we will simply say that matter was created from energy, without getting tangled up in which kind of energy it was. So, compression and heat, expansion and cooling, creation and annihilation - these are the basic dynamics of the universe at the time of the Big Bang. With these concepts in mind, we can begin our story of the universe, starting at some of its earliest moments.
THE PLANK ERA: ZERO TO 10-43 SECONDS
IN THE BEGINNING........ we don't quite know what happened. Physicists have some idea of what could have happened back to the first 10 -43 seconds, an
interval of time called the Planck Time. The first such interval, the stretch of time between zero seconds - the beginning of the universe - and the Planck time, is
called the Planck Era. This is surely the shortest segment of time to be dubbed an Era, and we know hardly anything about what happened then. In fact, it isn't
even clear that it makes sense to talk about smaller lengths of time; the Planck
time may be the smallest unit there is, which would mean that the Planck Era
can't be subdivided.
All we can say with any confidence is that by 10 -43 seconds after time zero, gravity had separated from the other three forces, which were still combined in a force called the Grand Unified Force. To describe what happened before that (if before is even the right word) we need a quantum theory of gravity. Perhaps supersymmetry theories or superstring theories will be the ones we need. They do suggest scenarios for this era, but we don't know yet whether they are more than speculations, so we won't go into them here. For now, we can assign a big question mark to the Planck Era, leaving it as a tiny but significant blank, to be filled in later.
THE GUT ERA: 10-43 SECONDS TO 10-35 SECONDS
After the Planck Era, the conceptual fog begins to clear a little. Between 10-43 seconds and 10-35 seconds, the universe possessed two different forces: Gravity and the GUT force. Consequently, this is called the GUT Era. Of course, we don't know for sure which, if any, of the Grand Unified Theories are correct, so this name may be a bit misleading. However, we can be fairly sure that the strong force, the weak force, and the electromagnetic force were unified into a single force, whether or not the GUT theories are correct. We just don't exactly know its characteristics of that force.
What else can we say about this era? We're still on fairly shaky ground here, but we can make some conjectures. The universe was (probably) extremely hot, starting at 10 32 k, (which is about 25 orders of magnitude hotter than the center of the sun) and dropping to about 1029 k. Space would have been filled with a seething gas of gravitons and GUT force-carrying bosons. If matter appeared, and if the grand unified theories are on the right track, then there would have been no distinction between quarks and leptons. Whatever particles appeared would have been some sort of hybrid of the two. That's about all we can say for sure about the GUT Era itself. Really, the part of the GUT Era that tells us the most is the way it ended. It ended, as some nations do, in a fit of inflation.
INFLATION: 10-35 TO 10-32 SECONDS
When the universe was 10-35 of a second old, it had cooled to 1029 k. At this point, the GUT force split into the Strong Nuclear Force and the still-unified
Electroweak force. This symmetry-breaking event probably happened somewhat differently in different parts of the universe, which may have broken up into
domains where the Strong and Electroweak forces split off with different strengths. These domains would be separated by domain walls - wall-like defects in spacetime.
There would have been other defects as well, such as cosmic strings and magnetic
monopoles. But today, we don't see any of these defects, because of the other result of the splitting of the GUT force
- inflation.
When the force split, in at least some parts of the universe (including all of that part we can see), it seems to have caused a false vacuum state, as we discussed in the last chapter. The energy of empty space became stuck at a high level, causing space itself to expand exponentially, doubling in size every 10-34 seconds. This means it would have doubled about 100 times before it stopped. That's an enormous increase in size. In the story of the chess playing king in Volume I, sixty doublings increased one grain of rice to more than all the rice in the world. Here we have 100 doublings, which would have increased the size of what is now the visible universe from the quantum scale to about the size of an orange.
One of the reasons we can't say much about what
happened before inflation is that inflation dramatically altered the universe,
or at least that portion of it
we live in. We don't know if the rest of the universe was involved or not, and
we don't know what percentage of the whole thing is comprised by our visible
universe. Inflation theories suggest that the entire visible universe is just a speck in a much,
much larger whole. In any case, inflation changed our portion of the universe
drastically. We are living in what, at the time of inflation, was just a
minuscule space within one of the domains of the universe. When this portion
inflated, it flattened out space-time in our region. If there was matter in our
region, such as the quark/lepton hybrids, they would have been thinned out to
practically nothing, as would all the magnetic monopoles and cosmic strings. Because of inflation, our part of the universe today is flat and homogenous on the largest
scales. We don't know much about what the universe was like before inflation because inflation erased most of the evidence. On the other hand, for our corner
of the universe after inflation, the picture becomes much more clear.
THE HADRON ERA: 10-32 TO 10-4 SECONDS
THE BIRTH OF MATTER: 10-32 TO 10-10 SECONDS
During inflation, the universe would have cooled enormously because it was expanding so much. But as soon as inflation ended, the false vacuum energy that
had been driving it was released, heating everything back up. This energy immediately began to condense into particles of matter and anti-matter. Since the
strong force had separated from the electroweak force, there was now a distinction between quarks, which feel the strong force, and leptons, which don't. The
two families of matter in the standard model had been born.
But the new particles didn't act like they do today. For one thing, because the electroweak force had not yet split, there was little distinction between the flavors
of particles. Up and Down, Charmed and Strange, and Top and Bottom quarks were more or less interchangeable, as were electrons and neutrinos in each of
the three families of leptons. Quarks had separated into their separate colors, but the temperature was so high that they didn't
come together as hadrons, such as protons or neutrons. They didn't last long
enough anyway, because as soon as particles and antiparticles appeared, they would meet and annihilate back into
energy. Luckily, a slight asymmetry in physical laws seems to have produced slightly more matter particles than anti-matter particles; about one for every
thousand million particles. This slight imbalance would become quite important later on.
THE ELECTROWEAK SPLIT: 10-10 SECONDS
The next big event happened at 10-10 seconds, when the temperature dropped enough for the next symmetry-breaking transition -
the division of the electroweak
force into the electromagnetic and the weak forces. This is when the W and Z bosons acquired mass,
while the photon remained massless. The four forces we know today had become distinct.
Matter also began to look more familiar - there were now distinct flavors, such
as Up and Down quarks, and Electrons and Neutrinos. Of course, none of them
lived long, since they were still being created and destroyed constantly.
THE DECLINE OF THE HADRONS:10-6 TO 10-4 SECONDS
Starting at about 10-6 seconds, two things started happening to the
quarks. First, the temperature had dropped enough that they began to respond to
the attraction of the strong force, and began clinging together in color-neutral groups, or hadrons. This group includes many kinds of heavy particles consisting
of three quarks, called baryons, and pairs of quarks and anti-quarks, called mesons. Two of the lightest baryons, of course, are our friends and constituents the
protons and neutrons, which made their first appearance around this time.
The second thing that happened to the quarks, and their composite particles, the hadrons, is that the energy
of the Big Bang was getting too low to keep replenishing them after
they annihilated. Heedless of this danger, the matter and antimatter particles continued annihilating each other until they were almost gone, except for the 1 in 1000 million
matter particles that could find no particles to annihilate with. Eventually, the heavier hadrons decayed, leaving just the lighter ones, mainly the protons and
neutrons. Now these were vastly outnumbered by the leptons, which were still being replenished, so we leave the Hadron Era for the Lepton Era.
THE LEPTON ERA: 10-4 TO 10 SECONDS
At the beginning of the Lepton Era, matter consisted of a few protons and neutrons swimming in a sea of leptons. Since protons and neutrons were constantly
interacting with leptons, the weak force was causing them to change back and forth into one another. As things cooled down, the leptons stopped being created,
and started to annihilate into oblivion, just as the quarks had done. The taus and muons were the first to go, annihilating with their anti-particles, while their
neutrinos stopped interacting with other particles and went their separate ways. Today they almost certainly fill the universe as a "neutrino background
radiation", even more difficult to detect than the photon background radiation. The few taus and muons that weren't annihilated decayed into electrons. But
when the temperature fell enough, even the light electrons were no longer replenished. They annihilated with the positrons, except for that tiny excess of matter
created after inflation. Because the total electric charge in the universe is a conserved quantity, the number of remaining electrons was equal to the number of
remaining protons. The electron neutrinos joined their siblings, detaching themselves from other matter to roam the universe as a ghostly background fog.
Since neutrons are a little heavier than protons, it began to be more likely for a neutron to convert into proton than the other way around. Protons thus began to
get more numerous, and by the time the Lepton Era ended, when the universe was about ten seconds old, they outnumbered neutrons by five to one. There was
about one particle of matter for every 1000 million photons. These photons were still incredibly energetic by our standards, so they began to dominate the
universe. Matter was just a bit of sediment in a furious sea of photons, and the Radiation Era had begun.
THE RADIATION ERA: 10 SECONDS TO 10,000 YEARS
NUCLEAR SYNTHESIS: 3 TO 5 MINUTES
The temperature of the photons continued to fall, and after a couple of minutes they no longer knocked protons and neutrons away from each other. These
began to stick together because of the strong force (the remainder of the color force which had earlier bound the quarks). At first,
single protons and neutrons came together (1), forming
nuclei of the isotope of hydrogen called deuterium. But deuterium is not a very stable nucleus, and these were generally torn apart by the photons as soon as they formed.
In the meantime, the minutes were ticking away, and the free neutrons were starting to feel their age. The half-life of neutrons is a little over ten minutes, so after about three minutes a fraction of the neutrons had decayed into protons and electrons. At this point the balance between neutrons and protons was about 12% neutrons and 88% protons. The temperature cooled enough for the deuterium to stay together, and it rapidly started fusing (in a couple of different reactions) into a more stable nucleus - that of helium; with two neutrons and two protons. The neutrons were stabilized, safely tucked away in helium nuclei, by the time the universe was about five minutes old. Of every hundred nucleons, 12 neutrons combined with 12 of the 88 protons, making 6 helium nuclei and 76 single protons, which would become hydrogen nuclei. Thus, by weight, the atoms eventually formed in the Big Bang were roughly 24% helium, 76% hydrogen, along with trace amounts of deuterium, helium-3 (two protons and one neutron), and lithium-7 (three protons and four neutrons). This ratio would hold for hundreds of millions of years, until the first stars began making heavier elements.
THE MATTER ERA: 10,000 YEARS TO THE PRESENT
THE THINNING OF THE PHOTONS: 10,000 YEARS
After nuclei formed in the first five minutes, the timescales involved jump from short to quite long (by human standards). For hundreds of thousands of years, the universe was a hot gas of photons, electrons, helium nuclei, and single protons. The photons were so
energetic and densely packed that they still dominated the behavior of the universe. They acted like tiny chaperones; even though electrons were attracted to protons, they couldn't pair with them because the photons kept knocking them apart. The photons, on the other hand,
couldn't travel far because they kept bouncing off of the matter particles. The universe would have been a blinding fog with zero visibility, because light couldn't travel through it unimpeded.
The first sign of change in the dominance of radiation occurred at about 10,000 years. While the photons vastly outnumbered the matter particles (and still do), both photons and material particles were becoming less densely packed as the universe expanded. Since
matter can be converted into energy, we can think of this as a decline in the density of energy in the universe. Since matter had stopped being created, and most of it had annihilated, most of the energy density of the universe was in the form of radiation. However, the
energy density of the photons was decreasing faster than the energy density represented by matter. Here's why: each particle of matter can be converted into a certain amount of energy, an amount which is the same whether the particles are densely or loosely packed.
Photons, however, have their wavelength stretched by the expansion of space, and a larger wavelength means a lower frequency, and thus, a lower energy. So, as photons grow thinner, the energy density decreases, not just because there are less of them in a certain
volume, but because they are dropping in frequency. About 10,000 years after the Big Bang, the energy density of photons fell below that of matter. The universe was dominated by matter, not radiation, and the Matter Era had begun.
ATOMS IN A TRANSPARENT UNIVERSE: 380,000 YEARS
The next change happened about 380 thousand years after the big Bang, when the temperature had cooled to about 4000 k. As the wavelengths of the photons kept stretching, and their frequency and energy kept dropping, they could no longer keep the electrons away from the protons, so these joined up in atoms of hydrogen and helium; in the same ratios as those of the nuclei, established during a few minutes long before. The process of atom formation lasted several thousand years, ending when none of the frequencies of photons could knock away the electrons. After a while, the radiation cooled so much that it stopped interacting at all. Cosmologists call this event decoupling. The furious sea of photons, which had outnumbered and dominated matter particles, had become the background radiation. When satellites scan the background radiation today, they are seeing back to that event - 99.99 % of the way to the Big Bang. Since most light was no longer scattered by atoms, the universe became transparent. One could have seen a long way, if there were anything to see. But there wasn't, because the universe had gone dark, and so it would remain until the first stars began to form, millions of years later.
EXPANSION, ATTRACTION, AND THE FALL FROM EQUILIBRIUM
Up until this time, the constant interaction between photons and matter had kept the temperature of the universe quite uniform. But when the radiation decoupled, it did more than create atoms and clear up the view-it
also marked a turning point. The universe had departed from thermal equilibrium. Ever since the radiation era had begun, the universe had been dominated by electromagnetism, in the form of the sea of photons. Now electromagnetism took a back seat, and gravity, the weakest force, took over. The
most important processes would now be caused by the gravitational attraction of matter.
In Volume I we talked about the preoccupation at the turn of the 20th century with the heat death of the universe; an imagined time when everything has fallen into thermal equilibrium and no useful work can be done. It's debatable whether such a thing will ever happen in the future, but in a way, it already happened in the past. For hundreds of thousands of years after the Big Bang, the universe was indeed in a state of thermal equilibrium. It was a bit like an early, scorching version of the heat death.
What changed? If thermal equilibrium is an absence of useful potential energy, where did the potential come from? It came from the expansion of the universe. As the universe expanded, it stretched out the photons until they couldn't keep the universe in equilibrium. It had been a temporary, high-energy equilibrium, not a terminal, lowest-energy one, because of the potential energy of expansion. Once the photons decoupled, this allowed the potential of the electromagnetic attraction between electrons and photons to go to work, snapping together the first atoms. After that happened, the electromagnetic potential was mostly used up, because the positive and negative forces more or less balanced each other.
This let the universe go further out of equilibrium, as gravity took over. Gravity may be weak, but it only works in one direction - it is always attractive, which means it can never cancel itself out like electric charge can. Gravity thus tends to work against equilibrium. It causes matter to clump, not to spread out evenly. From our perspective this is vital. As users of potential energy, we ultimately live on gravity. Gravity eventually caused matter to clump, sometimes into stars. When this happened, the potential energy of gravity compressed stars enough to liberate the energy of mass in fusion reactions. Not only did this create the elements we're made of, but it also provided the source of energy we live on - the energy of hot starlight, radiated into cold space. Our star, the sun, is so much hotter than the space around it that it is an enormous source of potential energy, one that will last for 5000 million more years. And ultimately, it's fueled by the energy of gravity.
This complicates the simple interpretation of the second law of thermodynamics; that entropy and disorder will always increase as the universe moves toward equilibrium. Gravity tends to pull the universe away from equilibrium, and today scientists are not quite sure what this means for entropy on the largest scales of space and time. Gravity may modify the second law, or it may just hold it at bay for a while. We are not quite sure which. Maybe a quantum theory of gravity will help answer this question. In any case, matter and gravity had taken the reins from the photons. To understand the next events, we need to shift our focus to larger scales and consider the ways of stars and galaxies.
1. Remember that protons and neutrons have a stronger net attraction than protons and protons, because the strong force is not opposed by the electric repulsion between protons.
