The Hidden Reality Part 2

In Einstein's hands, repulsive gravity was used for a single erroneous purpose. He proposed finely adjusting the amount of negative pressure that permeates s.p.a.ce to ensure that the repulsive gravity produced would exactly counter the attractive gravity exerted by the universe's more familiar material contents, yielding a static universe. As we've seen, he subsequently renounced this move. Six decades later, the developers of the inflationary theory proposed a kind of repulsive gravity that differed from Einstein's version much as the finale of Mahler's Eighth differs from the drone of a tuning fork. Rather than a moderate and steady outward push that would stabilize the universe, the inflationary theory envisions a gargantuan surge of repulsive gravity that's astoundingly short and thunderingly intense. Regions of s.p.a.ce had ample time before the burst to come to the same temperature, but then, riding the surge, covered the great distances necessary to reach their observed positions in the sky.

At this point, Newton would surely shoot you another disapproving look. Ever the skeptic, he would find another problem with your explanation. After catching up on the more intricate details of general relativity by racing through one of the standard textbooks, he would accept the strange fact that gravity can-in principle-be repulsive. But, he'd ask, what's all this talk of negative pressure permeating s.p.a.ce? It's one thing to use the inward pull of a stretched rubber band as an example of negative pressure. It's another to argue that billions of years ago, just around the time of the big bang, s.p.a.ce was momentarily permeated by an enormous and uniform negative pressure. What thing, or process, or ent.i.ty has the capacity to supply such a fleeting but pervasive negative pressure?

The genius of inflation's pioneers was to provide an answer. They showed that the negative pressure required for an antigravity burst naturally emerges from a novel mechanism involving ingredients known as quantum fields quantum fields. For our story, the details are crucial because the manner in which inflationary expansion comes about is central to the version of parallel universes it yields.

Quantum Fields.

In Newton's day, physics concerned itself with the motion of objects you can see-stones, cannonb.a.l.l.s, planets-and the equations he developed closely reflected this focus. Newton's laws of motion are a mathematical embodiment of how such tangible bodies move when they're pushed, pulled, or shot through the air. For more than a century, this was a wonderfully fruitful approach. But in the early 1800s, the English scientist Michael Faraday initiated a transformation in thinking with the elusive but demonstrably powerful concept of the field field.

Take a strong refrigerator magnet and place it an inch above a paper clip. You know what happens. The clip jumps up and sticks to the magnet's surface. This demonstration is so commonplace, so thoroughly familiar, that it's easy to overlook how bizarre it is. Without touching the paper clip, the magnet can make it move. How is this possible? How can an influence be exerted in the absence of any contact with the clip itself? These and a mult.i.tude of related considerations led Faraday to postulate that though the magnet proper does not touch the paper clip, the magnet produces something that does does. That something is what Faraday called a magnetic field magnetic field.

We can't see the fields produced by magnets; we can't hear them; none of our senses are attuned to them. But that reflects physiological limitations, nothing more. As a flame generates heat, so a magnet generates a magnetic field. Lying beyond the physical boundary of the solid magnet, the magnet's field is a "mist" or "essence" that fills s.p.a.ce and does the magnet's bidding.

Magnetic fields are but one kind of field. Charged particles give rise to another: electric fields, such as those responsible for the shock you sometimes receive when you reach for a metal doork.n.o.b in a room with wall-to-wall wool carpeting. Unexpectedly, Faraday's experiments showed that electric and magnetic fields are intimately related: he found that a changing electric field generates a magnetic field, and vice versa. In the late 1800s, James Clerk Maxwell put mathematical might behind these insights, describing electric and magnetic fields in terms of numbers a.s.signed to each point in s.p.a.ce; the numbers' values reflect the field's ability, at that location, to exert influence. Places in s.p.a.ce where the magnetic field's numerical values are large, for instance an MRI's cavity, are places where metal objects will feel a strong push or pull. Places in s.p.a.ce where the electric field's numerical values are large, for instance the inside of a thundercloud, are places where powerful electrical discharges such as lightning may occur.

Maxwell discovered equations, which now bear his name, that govern how the strength of electric and magnetic fields varies from point to point in s.p.a.ce and moment to moment in time. These very same equations govern the sea of rippling electric and magnetic fields, so-called electromagnetic waves electromagnetic waves, within which we're all immersed. Turn on a cell phone, a radio, or a wireless computer, and the signals received represent a tiny portion of the thicket of electromagnetic transmissions silently rushing by and through you every second. Most stunning of all, Maxwell's equations revealed that visible light itself is an electromagnetic wave, one whose rippling patterns our eyes have have evolved to see. evolved to see.

In the second half of the twentieth century, physicists united the field concept with their burgeoning understanding of the microworld encapsulated by quantum mechanics. The result, quantum field theory quantum field theory, provides a mathematical framework for our most refined theories of matter and nature's forces. Using it, physicists have established that in addition to electric and magnetic fields, there exists a whole panoply of others with names like strong strong and and weak nuclear fields weak nuclear fields and and electron, quark electron, quark, and neutrino fields neutrino fields. One field that to date remains wholly hypothetical, the inflaton field inflaton field, provides a theoretical basis for inflationary cosmology.*

Quantum Fields and Inflation.

Fields carry energy. Qualitatively, we know this because fields accomplish tasks that require energy, such as causing objects (like paper clips) to move. Quant.i.tatively, the equations of quantum field theory show us how, given the numerical value of a field at a particular location, to calculate the amount of energy it contains. Typically, the larger the value, the larger the energy. A field's value can vary from place to place, but should it be constant, taking the same value everywhere, it would fill s.p.a.ce with the same energy at every point. Guth's critical insight was that such uniform field configurations fill s.p.a.ce not only with uniform energy but also with uniform negative pressure. And with that, he found a physical mechanism to generate repulsive gravity he found a physical mechanism to generate repulsive gravity.

To see why a uniform field yields negative pressure, think first about a more ordinary situation that involves positive pressure: the opening of a bottle of Dom Perignon. As you slowly remove the cork, you can feel the positive pressure of the champagne's carbon dioxide pushing outward, driving the cork from the bottle and into your hand. A fact you can directly verify is that this outward exertion drains a little energy from the champagne. You know those vapor tendrils you see near the bottle's neck when the cork is out? They form because the energy expended by the champagne in pushing against the cork results in a drop in temperature, which, much as with your breath on a wintry day, causes surrounding water vapor to condense.

Now imagine replacing the champagne with something less festive but more pedagogical-a field whose value is uniform throughout the bottle. When you remove the cork this time, your experience will be very different. As you slide the cork outward, you make a little extra volume inside the bottle available for the field to permeate. Since a uniform field contributes the same energy at every location, the larger the volume the field fills, the greater greater the total energy the bottle contains. Which means that, unlike with champagne, the act of removing the cork adds energy to the bottle. the total energy the bottle contains. Which means that, unlike with champagne, the act of removing the cork adds energy to the bottle.

How could that be? Where would the energy come from? Well, think about what happens if the bottle's contents, rather than pushing the cork outward, pull the cork inward pull the cork inward. This would require you to pull on the cork to remove it, an exertion of effort that in turn would transfer energy from your muscles to the contents of the bottle. To explain the increase in the bottle's energy we thus conclude that, unlike champagne, which pushes outward, a uniform field sucks inward. That's what we mean by a uniform field's resulting in a negative-not positive-pressure.

Although there's no sommelier uncorking the cosmos, the same conclusion holds: if there's a field-the hypothetical inflaton field-that has a uniform value throughout a region of s.p.a.ce, it will fill that region not only with energy but also with negative pressure. And, as is now familiar, such negative pressure yields repulsive gravity, which drives an ever-quickening expansion of s.p.a.ce. When Guth slotted into Einstein's equations the likely numerical values for the inflaton's energy and pressure consonant with the extreme environment of the early universe, the mathematics revealed that the resulting repulsive gravity would be stupendous. It would easily be many orders of magnitude stronger than the repulsive force Einstein envisioned years earlier when he dallied with the cosmological constant, and would propel a spectacular spatial stretching. That alone was exciting. But Guth realized there was an indispensable bonus.

The same reasoning that explains why a uniform field has negative pressure applies as well to a cosmological constant. (If the bottle contains empty s.p.a.ce endowed with a cosmological constant, then when you slowly remove the cork the extra s.p.a.ce you make available within the bottle contributes extra energy. The only source for this extra energy is your muscles, which therefore must have strained against an inward, negative pressure supplied by the cosmological constant.) And, as with a uniform field, a cosmological constant's uniform negative pressure also yields repulsive gravity. But the vital point here is not the similarities, per se, but the manner in which a cosmological constant and a uniform field differ.

A cosmological constant is just that-a constant, a fixed number inserted on the third line of general relativity's tax form that would generate the same repulsive gravity today as it would have billions of years ago. By contrast, the value of a field can change, and generally will. When you turn on your microwave oven, you change the electromagnetic field filling its interior; when the technician flips the switch on an MRI machine, he or she changes the electromagnetic field threading the cavity. Guth realized that an inflaton field filling s.p.a.ce could behave similarly-turning on for a burst and then turning off-which would allow repulsive gravity to operate during only a brief window of time. That's essential. Observations establish that if the blistering growth of s.p.a.ce happened at all, it must have happened billions of years ago and then sharply dropped off to the statelier-paced expansion evidenced by detailed astronomical measurements. So an all-important feature of the inflationary proposal is that the era of powerful repulsive gravity be transient.

The mechanism for turning on and then shutting off the inflationary burst relies on physics that Guth initially developed but that Linde, and Albrecht and Steinhardt, refined substantially. To get a feel for their proposal, think of a ball-better still, think of nearly round Eric Cartman-perched precariously on one of South Park's snow-covered mountains. A physicist would say that because of his position, Cartman embodies energy. More precisely, he embodies potential energy potential energy, meaning that he has pent-up energy that's ready to be tapped, most easily by his tumbling downward, which would transform the potential energy into the energy of motion (kinetic energy). Experience attests, and the laws of physics make precise, that this is typical. A system harboring potential energy will exploit any opportunity to release that energy. In short, things fall.

The energy carried by a field's nonzero value is also potential energy: it, too, can be tapped, resulting in an incisive a.n.a.logy with Cartman. Just as the increase in Cartman's potential energy as he climbs the mountain is determined by the shape of the slope-in flatter regions his potential energy varies minimally as he walks, because he gets hardly any higher, while in steeper regions his potential energy rises sharply-the potential energy of a field is described by an a.n.a.logous shape, called its potential energy curve potential energy curve. Such a curve, as in Figure 3.1 Figure 3.1, determines how a field's potential energy varies with its value.

Following inflation's pioneers, let's then imagine that in the earliest moments of the cosmos, s.p.a.ce is uniformly filled with an inflaton field, whose value places it high up on its potential energy curve. Imagine further, these physicists urge us, that the potential energy curve flattens out into a gentle plateau (as in Figure 3.1 Figure 3.1), allowing the inflaton to linger near the top. Under these hypothesized conditions, what will happen?

Figure 3.1 The energy contained in an inflaton field (vertical axis) for given values of the field (horizontal axis The energy contained in an inflaton field (vertical axis) for given values of the field (horizontal axis).

Two things, both critical. While the inflaton is on the plateau, it fills s.p.a.ce with a large potential energy and negative pressure, driving a burst of inflationary expansion. But, just as Cartman releases his potential energy by rolling down the slope, so the inflaton releases its potential energy by its value, throughout s.p.a.ce, rolling to lower numbers. And as its value decreases, the energy and negative pressure it harbors dissipate, bringing an end to the period of blistering expansion. Just as important, the energy released by the inflaton field isn't lost-instead, like a cooling vat of steam condensing into water droplets, the inflaton's energy condenses into a uniform bath of particles that fill s.p.a.ce. This two-step process-brief but rapid expansion, followed by energy conversion to particles-results in a huge, uniform spatial expanse that's filled with the raw material of familiar structures like stars and galaxies.

Precise details depend on factors that neither theory nor observation has as yet determined (the initial value of the inflaton field, the exact shape of the potential energy slope, and so on)5 but in typical versions the mathematical calculations show that the inflaton's energy would roll down the slope in a tiny fraction of a second, on the order of 10 but in typical versions the mathematical calculations show that the inflaton's energy would roll down the slope in a tiny fraction of a second, on the order of 1035 seconds. And yet, during that brief span, s.p.a.ce would expand by a colossal factor, perhaps 10 seconds. And yet, during that brief span, s.p.a.ce would expand by a colossal factor, perhaps 1030 if not more. These numbers are so extreme that they defy a.n.a.logy. They imply that a region of s.p.a.ce the size of a pea would be stretched larger than the observable universe in a time interval so short that the blink of an eye would overestimate it by a factor larger than a million billion billion billion. if not more. These numbers are so extreme that they defy a.n.a.logy. They imply that a region of s.p.a.ce the size of a pea would be stretched larger than the observable universe in a time interval so short that the blink of an eye would overestimate it by a factor larger than a million billion billion billion.

However difficult it is to envision such a scale, what's essential is that the region of s.p.a.ce that sp.a.w.ned the observable universe was so small that it would easily have come to a uniform temperature before it was stretched into our grand cosmic expanse by the rapid burst. The inflationary expansion, and billions of years of subsequent cosmological evolution, resulted in this temperature cooling substantially, but the uniformity set in place early on dictates a uniform result today. This resolves the mystery of how the universe's uniform conditions came to be. In inflation, a uniform temperature across s.p.a.ce is inevitable.6 Eternal Inflation.

During the nearly three decades since its discovery, inflation has become a fixture of cosmological investigation. But to have an accurate picture of the research panorama, you should be aware that inflation is a cosmological framework, but it is not a specific theory. Researchers have shown that there are many ways to skin an inflationary cat, differing in details such as the number of inflaton fields supplying the negative pressure, the particular potential energy curves to which each field is subject, and so on. Fortunately, the sundry realizations of inflation have some implications in common, so we can draw conclusions even in the absence of a definitive version.

Among these, one first fully realized by Alexander Vilenkin of Tufts University and developed further by others, including most notably Linde, is of great importance.7 In fact, it's the very reason I've spent the first half of this chapter explaining the inflationary framework. In fact, it's the very reason I've spent the first half of this chapter explaining the inflationary framework.

In many versions of the inflationary theory, the burst of spatial expansion is not a onetime event. Instead, the process by which our region of the universe formed-rapid stretching of s.p.a.ce, followed by a transition to a more ordinary, slower expansion, together with the production of particles-may happen over and over again at various far-flung locations throughout the cosmos. From a bird's-eye view, the cosmos would appear riddled with innumerable widely separated regions, each being the aftermath of a portion of s.p.a.ce transitioning out of the inflationary burst. Our realm, what we have always thought of as the the universe, would then be but one of these numerous regions, floating within a vastly larger spatial expanse. If intelligent life exists in the other regions, those beings would just as surely have thought their universe to be universe, would then be but one of these numerous regions, floating within a vastly larger spatial expanse. If intelligent life exists in the other regions, those beings would just as surely have thought their universe to be the the universe, too. And so inflationary cosmology steers us headlong into our second variation on the theme of parallel universes. universe, too. And so inflationary cosmology steers us headlong into our second variation on the theme of parallel universes.

To grasp how this Inflationary Multiverse Inflationary Multiverse comes about, we need to engage two complications that my Cartman a.n.a.logy glossed over. comes about, we need to engage two complications that my Cartman a.n.a.logy glossed over.

First, the image of Cartman perched high on a mountaintop offered an a.n.a.logy to an inflaton field harboring significant potential energy and negative pressure, poised to roll to lower values. But whereas Cartman is perched on a single mountaintop, the inflaton field has a value at each each point in s.p.a.ce. The theory posits that the inflaton field starts off with the same value at each location within an initial region. And so we'd achieve a more faithful rendering of the science if we imagine something a little odd: numerous Cartman clones perched on numerous, closely packed, identical mountaintops throughout a spatial expanse. point in s.p.a.ce. The theory posits that the inflaton field starts off with the same value at each location within an initial region. And so we'd achieve a more faithful rendering of the science if we imagine something a little odd: numerous Cartman clones perched on numerous, closely packed, identical mountaintops throughout a spatial expanse.

Second, we've so far barely touched on the quantum quantum aspect of quantum field theory. The inflaton field, like everything else in our quantum universe, is subject to quantum uncertainty. This means that its value will undergo random quantum jitters, momentarily rising a little here and dropping a little there. In everyday situations, quantum jitters are too small to notice. But calculations show that the larger the energy an inflaton has, the greater the fluctuations it will experience from quantum uncertainty. And since the inflaton's energy content during the inflationary burst was extremely high, the jitters in the early universe were big and dominant. aspect of quantum field theory. The inflaton field, like everything else in our quantum universe, is subject to quantum uncertainty. This means that its value will undergo random quantum jitters, momentarily rising a little here and dropping a little there. In everyday situations, quantum jitters are too small to notice. But calculations show that the larger the energy an inflaton has, the greater the fluctuations it will experience from quantum uncertainty. And since the inflaton's energy content during the inflationary burst was extremely high, the jitters in the early universe were big and dominant.8 We should thus not only picture a platoon of Cartmans perched high on identical mountaintops; we should also imagine that they are all subject to a random series of tremors-strong here, weak there, very strong way over there. With this setup, we can now determine what will happen. Different Cartman clones will stay perched on their mountaintops for different durations. In some locations, a strong tremor knocks most Cartmans down their slopes; in other locations, a mild tremor coaxes only a few to tumble down; in others still, some Cartmans may have started to roll down until a strong tremor knocked them back up up. After a while, the terrain will be divided into a random a.s.sortment of domains-much as the United States is divided into states-in some of which no Cartmans are left on mountaintops, while in others many Cartmans remain securely perched.

The random nature of quantum jitters yields a similar conclusion for the inflaton field. The field begins high up on its potential energy slope at every point in a region of s.p.a.ce. The quantum jitters then act like tremors. Because of this, as ill.u.s.trated in Figure 3.2 Figure 3.2, the expanse of s.p.a.ce rapidly divides into domains: in some, quantum jitters cause the field to topple down the slope, while in others it remains high.

So far, so good. But now stay with me closely; here's where cosmology and Cartmans differ. A field that's perched high up on its energy curve affects its environment far more significantly than a similarly perched Cartman does. From our familiar refrain-a field's uniform energy and negative pressure generate repulsive gravity-we recognize that the region the field permeates expands at a fantastic rate. This means that the inflaton field's evolution across s.p.a.ce is driven by two opposing processes. Quantum jitters, by tending to knock the field off its perch, decrease decrease the amount of s.p.a.ce suffused with high field energy. Inflationary expansion, by rapidly enlarging those domains in which the field remains perched, the amount of s.p.a.ce suffused with high field energy. Inflationary expansion, by rapidly enlarging those domains in which the field remains perched, increases increases the volume of s.p.a.ce suffused with high field energy. the volume of s.p.a.ce suffused with high field energy.

Which process wins?

In the vast majority of proposed versions of inflationary cosmology, the increase occurs at least as quickly as the decrease. The reason is that an inflaton field that can be knocked off its perch too quickly typically generates too little inflationary expansion to solve the horizon problem; in cosmologically successful versions of inflation, the increase thus wins over the decrease, ensuring that the total volume of s.p.a.ce in which the field's energy is high increases over time. Recognizing that such field configurations yield yet further inflationary expansion, we see that once inflation begins it never ends.

Figure 3.2 Various domains in which the inflaton field has dropped down the slope (darker gray) or remains high (lighter gray Various domains in which the inflaton field has dropped down the slope (darker gray) or remains high (lighter gray).

It's like the spread of a viral pandemic. To eradicate the threat, you need to wipe out the virus faster than it can reproduce. The inflationary virus "reproduces"-a high field value generates rapid spatial expansion and thus infuses a yet larger domain with that same high field value-and it does so faster than the competing process eliminates it. The inflationary virus effectively resists eradication.9 Swiss Cheese and the Cosmos.

Collectively, these insights show that inflationary cosmology leads to a vastly new picture of reality's expanse, one that can be grasped most easily with a simple visual aid. Think of the universe as a gigantic block of Swiss cheese, with the cheesy parts being regions where the inflaton field's value is high and the holes being regions where it's low. That is, the holes are regions, like ours, that have transitioned out of the superfast expansion and, in the process, converted the inflaton field's energy into a bath of particles, which over time may coalesce into galaxies, stars, and planets. In this language, we've found that the cosmic cheese acquires more and more holes because quantum processes knock the inflaton's value downward at a random a.s.sortment of locations. At the same time, the cheesy parts stretch ever larger because they're subject to inflationary expansion driven by the high inflaton field value they harbor. Taken together, the two processes yield an ever-expanding block of cosmic cheese riddled with an ever-growing number of holes. In the more standard language of cosmology, each hole is called a bubble universe bubble universe (or a (or a pocket universe pocket universe).10 Each is an opening tucked within the superfast stretching cosmic expanse ( Each is an opening tucked within the superfast stretching cosmic expanse (Figure 3.3).

Don't let the descriptive but diminutive-sounding "bubble universe" fool you. Our universe is gigantic. That it may be a single region embedded within an even larger cosmic structure-a single bubble in an enormous block of cosmic cheese-speaks to the fantastic expanse, in the inflationary paradigm, of the cosmos as a whole. And this goes for the other bubbles too. Each would be as much a universe-a real, gigantic, dynamic expanse-as ours.

Figure 3.3 The Inflationary Multiverse arises when bubble universes continually form within an ever-expanding spatial environment permeated by a high-valued inflaton field The Inflationary Multiverse arises when bubble universes continually form within an ever-expanding spatial environment permeated by a high-valued inflaton field.

There are versions of the inflationary theory in which inflation is not eternal. By fiddling with details such as the number of inflaton fields and their potential energy curves, clever theorists can arrange things so that the inflaton would, in due course, be knocked off its perch everywhere. But these proposals are the exception rather than the rule. Garden-variety inflationary models yield a gargantuan number of bubble universes carved into an eternally expanding spatial expanse. And so, if the inflationary theory is on the mark, and if, as many theoretical investigations conclude, its physically relevant realization is eternal, the existence of an Inflationary Multiverse would be an inevitable consequence.

Changing Perspectives.

Back in the 1980s, when Vilenkin realized the eternal nature of inflationary expansion and the parallel universes to which it would give rise, he excitedly visited Alan Guth at MIT to tell him about it. Midway through the explanation, Guth's head drooped forward: he'd fallen asleep. This was not necessarily a bad sign; Guth is famous for nodding off during physics seminars-he's caught a few winks during talks I've given-then opening his eyes midway through to ask the most insightful of questions. But the broader physics community was no more enthusiastic than Guth was, so Vilenkin shelved the idea and moved on to other projects.

Sentiment today is very different. When Vilenkin was first thinking about the Inflationary Multiverse, the evidence in direct support of the inflationary theory itself was thin. So, to the few who paid any attention at all, ideas about inflationary expansion yielding a vast collection of parallel universes seemed like speculation piled upon speculation. But in the years since, the observational case for inflation has grown much stronger, once again thanks largely to precise measurements of the microwave background radiation.

Even though the observed uniformity of the microwave background radiation was one of the prime motivations for developing the inflationary theory, early proponents realized that rapid spatial expansion would not render the radiation perfectly perfectly uniform. Instead, they argued that quantum mechanical jitters stretched large by the inflationary expansion would overlay the uniformity with minuscule temperature variations, like tiny ripples on the surface of an otherwise smooth pond. This has proved to be a spectacular and enormously influential insight. uniform. Instead, they argued that quantum mechanical jitters stretched large by the inflationary expansion would overlay the uniformity with minuscule temperature variations, like tiny ripples on the surface of an otherwise smooth pond. This has proved to be a spectacular and enormously influential insight.* Here's how it goes. Here's how it goes.

Quantum uncertainty would have caused the value of the inflaton field to jitter. Indeed, if the inflationary theory is correct, the burst of inflationary expansion stopped here because a large and lucky quantum fluctuation, nearly 14 billion years ago, knocked the inflaton off its perch in our vicinity. Yet there's more to the story. As the inflaton's value rolled down its slope headlong toward the point of bringing inflation in our bubble universe to a close, its value would still have been subject to quantum jitters. The jitters, in turn, would have made the inflaton's value a little higher here and a little lower there, like the wavy surface of an unfurled sheet as it descends to your mattress. This would have produced slight variations in the energy the inflaton harbored across s.p.a.ce. Normally, such quantum variations are so tiny and happen over such minuscule scales that they are irrelevant over cosmological distances. But inflationary expansion is anything but normal.

The expansion of s.p.a.ce is so rapid, even during the transition out of the inflationary phase, that the microscopic would have been stretched to the macroscopic. And much as a tiny message scribbled on a deflated balloon becomes easier to read when air stretches the balloon's surface, so the influence of quantum jitters becomes visible when inflationary expansion stretches the cosmic fabric. More particularly, minute energy differences caused by quantum jitters are stretched into temperature variations that become imprinted in the cosmic microwave background radiation. Calculations show that the temperature differences wouldn't exactly be huge, but could be as large as a thousandth of a degree. If the temperature is 2.725 K in one region, the stretched-out quantum jitters would result in its being a touch colder, say 2.7245 K, or a touch hotter, 2.7255 K, at nearby regions.

Painstakingly precise astronomical observations have sought these temperature variations. They've found them. Just as the theory predicted, they measure about a thousandth of a degree (see Figure 3.4 Figure 3.4). More impressive still, the tiny temperature differences fit a pattern on the sky that is explained spot-on by the theoretical calculations. Figure 3.5 Figure 3.5 compares theoretical predictions of how the temperature should vary as a function of the distance between two regions (measured by the angle between their respective lines of sight when viewed from earth) with the actual measurements. The agreement is stunning. compares theoretical predictions of how the temperature should vary as a function of the distance between two regions (measured by the angle between their respective lines of sight when viewed from earth) with the actual measurements. The agreement is stunning.

The 2006 n.o.bel Prize in Physics was awarded to George Smoot and John Mather, who led more than a thousand researchers on the Cosmic Background Explorer team in the early 1990s to the first detection of these temperature differences. During the past decade, every new and more accurate measurement, yielding data such as those in Figure 3.5 Figure 3.5, has resulted in yet more precise verification of the predicted temperature variations.

These works have capped a thrilling story of discovery that began with the insights of Einstein, Friedmann, and Lemaitre, was pushed sharply forward by the calculations of Gamow, Alpher, and Herman, was reinvigorated by the ideas of d.i.c.ke and Peebles, was shown relevant by the observations of Penzias and Wilson, and has now culminated in the handiwork of armies of astronomers, physicists, and engineers whose combined efforts have measured a fantastically minute cosmic signature that was set in place billions of years ago.

On a more qualitative level, we should all be thankful for the blotches in Figure 3.4 Figure 3.4. At the close of inflation in our bubble universe, regions with slightly more energy (equivalently, via E=mc2, regions with slightly more ma.s.s) exerted a slightly stronger gravitational pull, attracting more particles from their surroundings and thus growing larger. The larger aggregate, in turn, exerted an even stronger gravitational pull, thus attracting yet more matter and growing larger still. In time, this s...o...b..ll effect resulted in the formation of clumps of matter and energy that, over billions of years, evolved into galaxies and the stars within them. In this way, inflationary theory establishes a remarkable link between the largest and smallest structures in the cosmos. The very existence of galaxies, stars, planets, and life itself derives from microscopic quantum uncertainty amplified by inflationary expansion.

Figure 3.4 The enormous spatial expansion in inflationary cosmology stretches quantum fluctuations from the microscopic to the macroscopic, resulting in observable temperature variations in the cosmic microwave background radiation (the darker splotches are slightly colder than the lighter ones The enormous spatial expansion in inflationary cosmology stretches quantum fluctuations from the microscopic to the macroscopic, resulting in observable temperature variations in the cosmic microwave background radiation (the darker splotches are slightly colder than the lighter ones).

Figure 3.5 The pattern of temperature differences in the cosmic microwave background radiation. Temperature variation is the vertical axis; the separation between two locations (measured by the angle between their respective lines of sight when viewed from earth-larger angles to the left, smaller angles to the right) is the horizontal axis. The pattern of temperature differences in the cosmic microwave background radiation. Temperature variation is the vertical axis; the separation between two locations (measured by the angle between their respective lines of sight when viewed from earth-larger angles to the left, smaller angles to the right) is the horizontal axis.11 The theoretical curve is solid; the observational data are given by the circles The theoretical curve is solid; the observational data are given by the circles.

Inflation's theoretical underpinnings may be rather tentative: the inflaton, after all, is a hypothetical field whose existence has yet to be demonstrated; its potential energy curve is posited by researchers, not revealed by observation; the inflaton must somehow start at the top of its energy curve across a region of s.p.a.ce; and so on. Despite all that, and even if some details of the theory are not quite right, the agreement between theory and observation has convinced many that the inflationary scheme taps into a deep truth about cosmic evolution. And since a great many versions of inflation are eternal, yielding an ever-growing number of bubble universes, theory and observation combine to make an indirect yet compelling case for this second version of parallel worlds.

Experiencing the Inflationary Multiverse.

In a Quilted Multiverse, there's no sharp divide between one parallel universe and another. All are part of a single spatial expanse whose overall qualitative features are similar from region to region. The surprise lies in the details. Most of us wouldn't expect worlds to repeat; most of us wouldn't expect, every so often, to encounter versions of ourselves, our friends, our families. But if we could journey sufficiently far, that's what we would find.

In an Inflationary Multiverse, the member universes are sharply divided. Each is a hole in the cosmic cheese, separated from the others by domains in which the inflaton's value remains high. Since such intervening regions are still undergoing inflationary expansion, the bubble universes are rapidly driven apart, with a speed of recession proportional to the amount of swelling s.p.a.ce between them. The farther apart they are, the greater the expansion's speed; the ultimate result is that distant bubbles move apart faster than the speed of light. Even with unlimited longevity and technology, there's no way to cross such a divide. There's no way to even send a signal.

All the same, we can still imagine a voyage to one or more of the other bubble universes. On such a journey, what would you find? Well, because each bubble universe results from the same process-the inflaton is knocked from its perch, yielding a region that drops out of the inflationary expansion-they are all governed by the same physical theory and so are all subject to the same set of physical laws. But, much as the behavior of identical twins can differ profoundly as a result of environmental differences, identical laws can manifest themselves in profoundly different ways in different environments.

Imagine, for example, that one of the other bubble universes looks much like ours, dotted by galaxies containing stars and planets, but with one essential difference. Permeating the universe is a magnetic field, thousands of times stronger than that created in our most advanced MRI machines, and one that can't be switched off by a technician. Such a powerful field would affect the way a great many things behave. Not only would objects containing iron have a nasty habit of flying off in the direction of the field, but even basic properties of particles, atoms, and molecules would shift. A sufficiently strong magnetic field would so disrupt cellular function that life as we know it couldn't take hold.

Yet just as the physical laws operating inside an MRI are the very same laws that operate outside, so the fundamental physical laws operating in this magnetic universe would be the same as ours. The discrepancies in experimental results and observable features would be due solely to an aspect of the environment: the strong magnetic field. Talented scientists in the magnetic universe would in time tease out this environmental factor and home in on the same mathematical laws we've discovered.

Over the past forty years, researchers have built a case for a similar scenario right here in our own universe. The most lauded theory of fundamental physics, the Standard Model of particle physics Standard Model of particle physics, posits that we are immersed in an exotic mist called the Higgs field Higgs field (named after the English physicist Peter Higgs, who with important contributions from Robert Brout, Francois Englert, Gerald Guralnik, Carl Hagen, and Tom Kibble pioneered this idea in the 1960s). Both Higgs fields and magnetic fields are invisible and hence can fill s.p.a.ce without directly revealing their presence. However, according to modern particle theory, a Higgs field camouflages itself far more fully. As particles move through a uniform, s.p.a.ce-filling Higgs field, they don't speed up, they don't slow down, they are not coaxed to follow particular trajectories, as some would in the presence of a strong magnetic field. Instead, the theory claims, they're influenced in ways more subtle and profound. (named after the English physicist Peter Higgs, who with important contributions from Robert Brout, Francois Englert, Gerald Guralnik, Carl Hagen, and Tom Kibble pioneered this idea in the 1960s). Both Higgs fields and magnetic fields are invisible and hence can fill s.p.a.ce without directly revealing their presence. However, according to modern particle theory, a Higgs field camouflages itself far more fully. As particles move through a uniform, s.p.a.ce-filling Higgs field, they don't speed up, they don't slow down, they are not coaxed to follow particular trajectories, as some would in the presence of a strong magnetic field. Instead, the theory claims, they're influenced in ways more subtle and profound.

As fundamental particles burrow through a Higgs field, they acquire and maintain the ma.s.s that experiments have revealed them to possess they acquire and maintain the ma.s.s that experiments have revealed them to possess. According to this idea, when you push against an electron or quark in an effort to change its speed, the resistance you feel comes from the particle's "rubbing" against the mola.s.ses-like Higgs field. It's this resistance that we call the particle's ma.s.s. Were you to remove the Higgs field from some region, particles pa.s.sing through would suddenly become ma.s.sless. Were you to double the value of the Higgs field in another region, particles pa.s.sing through would suddenly have twice their usual ma.s.s.*

Such human-induced changes are hypothetical, because the energy required to substantially modify a Higgs field's value in even a small region of s.p.a.ce is enormously beyond what we can muster. (The changes are also hypothetical because the existence of the Higgs fields is still up in the air. Theorists eagerly antic.i.p.ate highly energetic collisions between protons at the Large Hadron Collider chipping off small chunks of the Higgs field-Higgs particles-that may be detected in the coming years.) But in many versions of inflationary cosmology, a Higgs field would naturally have different values in different bubble universes a Higgs field would naturally have different values in different bubble universes.

A Higgs field, much like an inflaton field, has a curve that records the amount of energy it contains for various values it can a.s.sume. An essential difference from the inflaton field's energy curve, though, is that the Higgs typically settles not at the value 0 (as in Figure 3.1 Figure 3.1), but rather rolls to one of the troughs ill.u.s.trated in Figure 3.6a Figure 3.6a. Picture, then, an early stage in each of two bubble universes, ours and another. In both, the hot, tempestuous frenzy causes the value of the Higgs field to undulate wildly. As each universe expands and cools, the Higgs field calms and its value rolls toward one of the troughs in Figure 3.6a Figure 3.6a. In our universe, the Higgs field's value settles down in, say, the left trough, giving rise to the particle properties familiar from experimental observation. But in the other universe, the Higgs' motion may result in its value settling down in the right trough. If it did, that universe would have properties substantially different from ours. Although the underlying laws in both universes would be the same, the ma.s.ses and various other properties of particles would not.

Even a modest difference in particle properties would have weighty consequences. If the electron ma.s.s in another bubble universe were a few times larger than it is here, electrons and protons would tend to merge, forming neutrons and thus preventing the widespread production of hydrogen. The fundamental forces-the electromagnetic force, the nuclear forces, and (we believe) gravity-are also communicated by particles. Change the particle properties and you drastically change the properties of the forces. The heavier a particle, for example, the more sluggish its motion and so the shorter the distance over which the corresponding force is transmitted. The formation and stability of atoms in our bubble universe rely on the properties of the electromagnetic and nuclear forces. If you substantially modify those forces, atoms will fall apart or, more likely, not coalesce in the first place. An appreciable change to the properties of particles would thus disrupt the very processes that give our universe its familiar features.

Figure 3.6 (a) A potential energy curve for a Higgs field that has two troughs. The familiar features of our universe are a.s.sociated with the field settling down in the left trough; in another universe, however, the field can settle down in the right trough, yielding different physical features A potential energy curve for a Higgs field that has two troughs. The familiar features of our universe are a.s.sociated with the field settling down in the left trough; in another universe, however, the field can settle down in the right trough, yielding different physical features. (b) (b) A sample potential energy curve for a theory with two Higgs fields A sample potential energy curve for a theory with two Higgs fields.

Figure 3.6a ill.u.s.trates only the simplest case, in which there is a single species of Higgs field. But theoretical physicists have explored more complicated scenarios involving multiple Higgs fields (we will shortly see that such possibilities naturally emerge from string theory), which translate into an even richer set of distinct bubble universes. An example with two Higgs fields is ill.u.s.trated in ill.u.s.trates only the simplest case, in which there is a single species of Higgs field. But theoretical physicists have explored more complicated scenarios involving multiple Higgs fields (we will shortly see that such possibilities naturally emerge from string theory), which translate into an even richer set of distinct bubble universes. An example with two Higgs fields is ill.u.s.trated in Figure 3.6b Figure 3.6b. As before, the various troughs represent Higgs field values that one or another bubble universe could settle into.

Permeated by such unfamiliar values of various Higgs fields, these universes would differ from ours considerably, as schematically ill.u.s.trated in Figure 3.7 Figure 3.7. This would make a journey through the Inflationary Multiverse a perilous undertaking. Many of the other universes would not be places you'd want high on your itinerary, because the conditions would be incompatible with the biological processes essential to survival, giving new meaning to the saying that there's no place like home. In the Inflationary Multiverse, our universe could well be an island oasis in a gigantic but largely inhospitable cosmic archipelago.

Figure 3.7 Because fields can settle down to different values in different bubbles, the universes in the Inflationary Multiverse can have different physical features, even though the universes are all governed by the same fundamental physical laws Because fields can settle down to different values in different bubbles, the universes in the Inflationary Multiverse can have different physical features, even though the universes are all governed by the same fundamental physical laws.

Universes in a Nutsh.e.l.l.

Because of their fundamental differences, the Quilted and Inflationary Multiverses might appear unrelated. The quilted variety emerges if the extent of s.p.a.ce is infinite; the inflationary variety emerges from eternal inflationary expansion. Yet, there is a deep and wonderfully satisfying connection between them, one that brings the discussion in the previous two chapters full circle. The parallel universes arising from inflation generate their quilted cousins. The process has to do with time.

Of the many strange things Einstein's work revealed, the fluidity of time is the hardest to grasp. Whereas everyday experience convinces us that there is an objective concept of time's pa.s.sage, relativity shows this to be an artifact of life at slow speeds and weak gravity. Move near light speed, or immerse yourself in a powerful gravitational field, and the familiar, universal conception of time will evaporate. If you're rushing past me, things I insist happened at the same moment will appear to you to have occurred at different moments. If you're hanging out near the edge of a black hole, an hour's pa.s.sage on your watch will be monumentally longer on mine. This isn't evidence of a magician's trickery or a hypnotist's deception. The pa.s.sage of time depends on the particulars-trajectory followed and gravity experienced-of the measurer.12 When applied to the entire universe, or to our bubble in an inflationary setting, this immediately raises a question: How does such malleable, custom-made time comport with the notion of an absolute cosmological time? We freely speak of the "age" of our universe, but given that galaxies are moving rapidly relative to one another, at speeds dictated by their various separations, doesn't the relativity of time's pa.s.sage create a nightmarish accounting problem for any would-be cosmic timekeeper? More pointedly, when we speak of our universe being "14 billion years old," are we using a particular clock to measure that duration?

We are. And a careful consideration of such cosmic time reveals a direct link between parallel universes of the inflationary and quilted varieties.

Every method we use to measure time's pa.s.sage involves an examination of change that occurs to some particular physical system. Using a common wall clock, we examine the change in position of its hands. Using the sun, we examine the change in its position in the sky. Using carbon 14, we examine the percentage of an original sample that's undergone radioactive decay to nitrogen. Historical precedent and general convenience have led us to use the rotation and revolution of the earth as physical referents, giving rise to our standard notions of "day" and "year." But when we're thinking on cosmic scales, there is another, more useful, method for keeping time.

We've seen that inflationary expansion yields vast regions whose properties on average are h.o.m.ogeneous. Measure the temperature, pressure, and average density of matter in two large but separate regions within a bubble universe, and the results will agree. The results can change over time, but the large-scale uniformity ensures that, on average, the change here here is the same as the change is the same as the change there there. As an important case in point, the ma.s.s density in our bubble universe has steadily decreased over our multibillion-year history, thanks to the relentless expansion of s.p.a.ce, but because the change has occurred uniformly, our bubble's large-scale h.o.m.ogeneity has not been disrupted.

This proves important because just as the steadily decreasing amount of carbon 14 in organic matter provides a means of measuring time's pa.s.sage on earth, so the steadily decreasing ma.s.s density provides a means of measuring time's pa.s.sage across s.p.a.ce. And because the change has happened uniformly, ma.s.s density as a marker of time's pa.s.sage provides our bubble universe with a global standard. If everyone diligently calibrates their watches to the average ma.s.s density (and recalibrates after trips to black holes, or periods of travel at near light speed), the synchronicity of our timepieces across our bubble universe will be maintained. When we speak of the age of the universe-the age of our bubble, that is-it is on such cosmically calibrated watches that we imagine time's pa.s.sage being measured; it is only with respect to them that cosmic time is a sensible concept.

In the earliest era of our bubble universe, the same reasoning would have applied with one change of detail. Ordinary matter had yet to form, so we can't speak of the average ma.s.s density in s.p.a.ce. Instead, the inflaton field carried our universe's storehouse of energy-energy that would shortly be converted into familiar particles-so we need to envisage setting our clocks by the density of the inflaton field's energy.

Now, the inflaton's energy is determined by its value, as summarized by its energy curve. To determine what time it is at a given location in our bubble, we therefore need to determine the value of the inflaton at that location. Then, just as two trees are the same age if they have the same number of tree rings, and just as two samples of glacial sediment are the same age if they have the same percentage of radioactive carbon, two locations in s.p.a.ce are pa.s.sing through the same moment in time when they have the same value of the inflaton field two locations in s.p.a.ce are pa.s.sing through the same moment in time when they have the same value of the inflaton field. That's how we set and synchronize clocks in our bubble universe.

The reason I've brought all this up is that when applied to the cosmic Swiss cheese of the Inflationary Multiverse, these observations yield a strikingly counterintuitive implication. Much as Hamlet famously declares, "I could be bounded in a nutsh.e.l.l, and count myself a king of infinite s.p.a.ce," each of the bubble universes appears to have finite finite spatial extent when examined from the outside, but spatial extent when examined from the outside, but infinite infinite spatial extent when examined from the inside. And that's a marvelous realization. Infinite spatial extent is just what we need for quilted parallel universes. So we can meld the Quilted Multiverse into the inflationary story. spatial extent when examined from the inside. And that's a marvelous realization. Infinite spatial extent is just what we need for quilted parallel universes. So we can meld the Quilted Multiverse into the inflationary story.

The extreme disparity between the outsider's and insider's perspectives arises because they have vastly different conceptions of time. Although the point is far from obvious, we'll now see that what appears as endless time to an outsider appears as endless s.p.a.ce, at each moment of time, to an insider. what appears as endless time to an outsider appears as endless s.p.a.ce, at each moment of time, to an insider.13 s.p.a.ce in a Bubble Universe.

To grasp how this comes about, imagine that Trixie, floating within a rapidly expanding inflaton-filled region of s.p.a.ce, is observing the formation of a nearby bubble universe. Focusing her inflaton-meter on the growing bubble, she is able to directly track its changing inflaton field value. Although the region-the hole in the cosmic cheese-is three-dimensional, it's simpler to examine the field along a one-dimensional cross section across its diameter, and as Trixie does so she records the data in Figure 3.8a Figure 3.8a. Each higher row shows the inflaton's value at a successive moment in time, from Trixie's perspective. And as is apparent from the figure, Trixie sees the bubble universe-represented in the figure by the lighter locations where the inflaton's value has dropped-grow ever larger.

Now imagine that Norton is also examining this very same bubble universe, but from the inside; he's hard at work making detailed astronomical observations with his own inflaton-meter. Norton, unlike Trixie, adheres to a notion of time that's calibrated by the value of the inflaton. This is key to the conclusion we're chasing, so I need you to buy into it fully. Imagine, if you will, that everyone in the bubble universe wears a watch that measures and displays the inflaton's value. When Norton throws a dinner party, he instructs the guests to show up at his house when the inflaton's value is 60. Since everyone's watch is calibrated to the same, uniform standard-the inflaton field's value-the party goes off without a hitch. Everyone shows up at the same moment because everyone is attuned to the same concept of synchronicity.

Figure 3.8a Each row chronicles the inflaton's value at one moment of time from an outsider's perspective. Higher rows correspond to later moments. The columns denote positions across s.p.a.ce. A bubble is a region of s.p.a.ce that stops inflating because of a drop in the inflaton's value. The lighter entries denote the value of the inflaton field within the bubble. From the perspective of the outside observer, the bubble grows ever larger Each row chronicles the inflaton's value at one moment of time from an outsider's perspective. Higher rows correspond to later moments. The columns denote positions across s.p.a.ce. A bubble is a region of s.p.a.ce that stops inflating because of a drop in the inflaton's value. The lighter entries denote the value of the inflaton field within the bubble. From the perspective of the outside observer, the bubble grows ever larger.

With this understanding, it's a simple matter for Norton to work out the size of the bubble universe at any given moment of his time. In fact, it's child's play: all Norton has to do is paint by numbers. By connecting all points that have the same numerical value for the inflaton field, Norton can delineate all locations within the bubble at a single moment of time. His time. Insider's time.

Norton's drawing in Figure 3.8b Figure 3.8b says it all. Each curve, connecting points with the same inflaton-field value, represents all of s.p.a.ce at a given moment of time. As the figure makes clear, each curve extends indefinitely far, which means that the size of the bubble universe, according to its inhabitants, is says it all. Each curve, connecting points with the same inflaton-field value, represents all of s.p.a.ce at a given moment of time. As the figure makes clear, each curve extends indefinitely far, which means that the size of the bubble universe, according to its inhabitants, is infinite infinite. This reflects that endless outsider time, experienced by Trixie as the endless number of rows in Figure 3.8 Figure 3.8, appears as endless s.p.a.ce, at each moment of time, according to an insider like Norton.

That's a powerful insight. In Chapter 2 Chapter 2, we found that the Quilted Multiverse was contingent upon s.p.a.ce being infinitely large, something that, as we discussed there, might or might not be the case. Now we see that each bubble within the Inflationary Multiverse is spatially finite from the outside but spatially infinite from the inside. If the Inflationary Multiverse is real, then the inhabitants of a bubble-us-would thus be members not only of the Inflationary Multiverse but of the Quilted Multiverse, too.14 Figure 3.8b The same information as in The same information as in Figure 3.8a Figure 3.8a is organized differently by someone within the bubble. Inflaton values that agree correspond to identical moments, so the curves drawn sweep through all those points in s.p.a.ce that exist at the same moment in time. Smaller inflaton values correspond to later moments. Note that the curves could be extended infinitely far, so from an insider's perspective, s.p.a.ce is infinite is organized differently by someone within the bubble. Inflaton values that agree correspond to identical moments, so the curves drawn sweep through all those points in s.p.a.ce that exist at the same moment in time. Smaller inflaton values correspond to later moments. Note that the curves could be extended infinitely far, so from an insider's perspective, s.p.a.ce is infinite.

When I first learned of the Quilted and Inflationary Multiverses, it was the inflationary variety that struck me as more plausible. Inflationary cosmology resolves a number of long-standing puzzles while yielding predictions that match up well with observations. And by the reasoning we've recounted, inflation is naturally a process that never ends; it produces bubble universes upon bubble universes, of which we inhabit but one. The Quilted Multiverse, on the other hand, by having its full force when s.p.a.ce is not just large but truly infinite (you might have repet.i.tion in a large universe, but you are guaranteed repet.i.tion in an infinite one), seemed avoidable: it might be the case, after all, that the universe has finite size. But we now see that eternal inflation's bubble universes, when properly a.n.a.lyzed from the viewpoint of their inhabitants, are are spatially infinite. Inflationary parallel universes beget quilted ones. spatially infinite. Inflationary parallel universes beget quilted ones.

The best available cosmological theory for explaining the best available cosmological data leads us to think of ourselves as occupying one of a vast inflationary system of parallel universes, each of which harbors its own vast collection of quilted parallel universes. Cutting-edge research yields a cosmos in which there are not only parallel universes but parallel parallel universes. It suggests that reality is not only expansive but abundantly expansive.

*Equivalently, superfast accelerated expansion means that today's distant regions would have been much closer together in the early universe than is suggested by the traditional big bang theory-ensuring that a common temperature could be established before the burst separated them.

*You might think that negative pressure would pull inward and thus be at odds with repulsive-outward-pushing-gravity. Actually, uniform uniform pressure, regardless of its sign, doesn't push or pull at all. Your eardrums pop only when there is nonuniform pressure, lower on one side than the other. The repulsive push I'm describing here is the pressure, regardless of its sign, doesn't push or pull at all. Your eardrums pop only when there is nonuniform pressure, lower on one side than the other. The repulsive push I'm describing here is the gravitational force generated by the presence of the uniform negative pressure gravitational force generated by the presence of the uniform negative pressure. This is a difficult but essential point. Again, whereas the presence of positive ma.s.s or positive pressure generates attractive gravity, the presence of negative pressure generates the less familiar repulsive gravity.

*The rapid expansion of s.p.a.ce is called inflation, but following the historical pattern of invoking names that end in "on" (electron, proton, neutron, muon, etc.), when physicists refer to the field driving inflation, they drop the second "i." Hence, inflaton field.

*Among those who played a leading role in this work were Viatcheslav Mukhanov, Gennady Chibisov, Stephen Hawking, Alexei Starobinsky, Alan Guth, So-Young Pi, James Bardeen, Paul Steinhardt, and Michael Turner.

*I stress fundamental fundamental particles, like electrons and quarks, because for composite particles, like protons and neutrons (each made from 3 quarks), much of the ma.s.s arises from interactions between the const.i.tuents (the energy carried by gluons of the strong nuclear force, which bind the quarks inside protons and neutrons, contributes most of the ma.s.s of these composite particles). particles, like electrons and quarks, because for composite particles, like protons and neutrons (each made from 3 quarks), much of the ma.s.s arises from interactions between the const.i.tuents (the energy carried by gluons of the strong nuclear force, which bind the quarks inside protons and neutrons, contributes most of the ma.s.s of these composite particles).

CHAPTER 4.

Unifying Nature's Laws.

On the Road to String Theory.

From the big bang to inflation, modern cosmology traces its roots to a single scientific nexus: Einstein's general theory of relativity. With his new theory of gravity, Einstein upended the accepted conception of a rigid and immutable s.p.a.ce and time; science now had to embrace a dynamic cosmos. Contributions of