What's Eating the Universe?

by Paul Davies

  5. Where is the Centre of the Universe?

  It’s all very well to say the universe is expanding, but what does that mean? Expanding into what? Coming from where? When I first learned that the galaxies are rushing away from us it seemed to me that Earth must be close to cosmic ground zero – the point in space where the big bang happened. On the other hand, I knew it was crazy to believe we were at the centre of the universe.

  To properly understand the nature of the cosmic expansion, you have to think about it in a totally different way. Forget all those TV animations depicting the big bang as the explosion of a glowing lump, with fragments flung out into the surrounding space. As far as we can tell, the universe has no centre. The impression that we are close to it is an illusion. In reality, every galaxy is moving away from all the others.* The view from any given galaxy would be roughly the same; there is nothing special about our own location.

  A loose analogy is with a dance class where the students form a ring holding hands while the teacher explains the steps. The teacher tells all the students to step back five paces. As a result, the ring expands, and every student is now farther from every other student. But there is no special, privileged student that the others are fleeing from. Of course, the galaxies aren’t arranged in a ring, but the same basic idea is at work.

  Actually, it is better to stop thinking about galaxies moving through space altogether. Instead, imagine the space between the galaxies to be swelling. This image is not just an aid to visualization: space really is growing bigger: every day, a hundred billion billion cubic light years of additional space appears within the observable universe. It takes a while to get used to the idea that empty space – a void – can stretch or swell, but it’s true: space is provably elastic. In fact, not only can space stretch, it can buckle, twist and shudder too. The cosmic expansion is literally ‘putting space’ between the galaxies, and as a result they get farther and farther apart. This new space doesn’t have to ‘come from anywhere’ or expand into anything. It is space.

  Armed with this new insight, we can confront the question ‘Where did the big bang happen?’ The answer is everywhere. The CMB – the afterglow of the big bang – doesn’t have an epicentre; it isn’t a stream of radiation coming from a specific point in space. The entire universe is immersed in microwaves, as if it were entombed in a gigantic oven. Observing the CMB, you are witnessing the birth of the universe at every point in the sky.

  The stretching of space also gives a useful alternative way to think about the red shift. Light waves from a distant galaxy have to travel through expanding space to reach us. As the intervening space stretches, so the transiting light waves get stretched with it, meaning their wavelength increases and their frequency falls: blue becomes red. For the oldest observable galaxies, the wavelengths are eleven times longer when they arrive on Earth than when they set out from the source.

  The farther out into the universe we peer, the bigger the red shift becomes. The stretching factor – the ratio of the wavelength observed on Earth to what it was when emitted by the source – escalates with distance, until it shifts the received light well beyond the visible region of the spectrum into the infrared, microwave and radio regions, longer and longer, without limit. Theoretically, the stretching factor would become infinite at a certain critical distance. In a simple model, that distance corresponds to how far light can have travelled since the big bang. Obviously, we couldn’t see any farther than that, because light hasn’t had time to reach us yet during the age of the universe. There is, therefore, a horizon in space – a light horizon – restricting our view. And because nothing can go faster than light there’s no way to know for sure what lies beyond. No instrument, however powerful, can enlighten us. But just as a terrestrial horizon doesn’t mark the edge of the world – what lies beyond is much the same – so the cosmic horizon isn’t ‘the edge of the universe’. There is no edge. The horizon is simply the boundary of our visible cosmic patch. The universe may very well be infinitely extended in space, but if it is finite in time we can’t look to see. I should mention that, in practice, the horizon is shrouded by the fiery afterbirth of the big bang. The red shift stretching factor of the CMB radiation coming from that epoch is about 1,000.

  One final point. If a galaxy is, say, 12 billion light years away, we are seeing where it was located 12 billion years ago – so one can ask: where is it now? Because of the expansion of the universe it will be many billions of light years farther away today than it was then. For example, the horizon is estimated to now be about 47 billion light years away. It’s important to remember that our telescopes don’t provide a snapshot of the universe as it is today. Instead, the amalgamated images form a compilation, a historical time sequence cascaded together. It’s rather like cutting a movie into frames, stacking the frames into an ordered pile, and looking down through them to see the whole stack at once.

  6. Why the Cosmos is Actually Fairly Simple

  Browsing in the public library as a teenager, I picked up a little book purporting to contain nothing less than the equations of the universe. I confess I was sceptical. Could the great story of the cosmos be captured in just a few lines of mathematics? Was it really that simple? The book, by Dennis Sciama, was called The Unity of the Universe, and it left a deep impression on me. The title says it all: there is a coherence and unity on the largest scale of size that lets one distil the basic story of the universe into three or four equations.

  Why do I say that the universe is simple? Mustn’t it be the most complicated thing we can think about? Well, yes and no. If you tried to describe everything that exists in individual detail, then of course it would be incomprehensibly complex. But just as one can recount the history of, say, the Vietnam War without mentioning the exploits of every given soldier, so we can determine the basic thrust of cosmic history without getting too bogged down in local details. Nevertheless, even this broad-brush narrative wouldn’t be possible without some very specific features.

  The most important of these is the uniformity of nature. As far as we can tell, identical laws of physics apply on the far side of the universe as they do in our cosmic neighbourhood. Why this should be so nobody knows, and it may not even be a proper scientific question anyway, but it is a critical reason why we can even discuss ‘the universe’ as a single entity and construct a narrative for the system as a whole. If the laws differed from place to place or time to time, there would be no common scheme of things.

  But that is not all. Even with universal laws, there are countless ways the universe could be arranged. Imagine an orchestra scheduled to perform a concert featuring, say, Beethoven’s Fifth Symphony. It would make perfect sense to attribute this musical performance to ‘the orchestra’, even though orchestras are made up of many individual musicians. So long as their playing is ‘orchestrated’ the result is harmonious. If each musician played from a different score without regard to the others, the upshot would be a horrible cacophony. It would then be pointless to say the orchestra was performing anything specific; indeed, it would be pointless to even speak of ‘the orchestra’ as opposed to an assemblage of independent musicians.

  The universe is organized in an analogous way. Rather than a lot of complicated parts behaving independently, there is orchestration and harmony and co-ordination. It shows up most obviously in the way the universe is observed to expand at the same rate everywhere. Furthermore, the number of galaxies in a similarly sized patch of sky is the same in every direction, as is the temperature of the CMB. Evidently the big bang went ‘bang’ in a remarkably ordered fashion. Contrast that with explosions on Earth. If a leaking gas main blows up a house, fragments are scattered haphazardly far and wide. It is a mess.

  There is a third crucial simplifying feature. According to folklore, in 48 bce the great library of Alexandria was destroyed by fire, one of the great disasters to befall human civilization. Priceless knowledge was lost as books were transformed into piles of ash in a tragic demonstrati
on of the destructive power of heat. As a rule, the hotter the fire, the more it consumes. Modern incinerators can break down the most stable chemicals. The interior of the sun is so hot that even atoms can’t exist; they are smashed into nuclei and electrons. The big bang was hotter still. At one second after the beginning, it was ten billion degrees; at one microsecond, ten trillion degrees; at a picosecond, ten thousand trillion degrees.

  Hot = simple is one reason cosmologists can talk so confidently about the early universe. It really is true that a handful of simple equations – basic algebra and calculus – accurately captures most of what happened in the primordial phase. A new-born universe in which everything is mushed together and broken down to its basic constituents is far easier to describe theoretically than, say, the Earth.

  7. What is the Speed of Space?

  Many people are fearful of flying, so take-off is a stressful time for them. I suffer from the opposite problem: I’m so stressed about missing the flight that by the time it leaves I’ve usually fallen asleep from nervous exhaustion. I often wake up half an hour later with no idea whether the plane is still stuck on the runway or halfway to Los Angeles.

  The thing is, you can’t tell from within the plane whether it’s moving or not. Of course, if it banks or dives you’d know it right away, but steady motion can’t be felt. It was Galileo who first made this explicit, pointing out that uniform speed can be measured only relative to something else. My car speedometer, for example, measures the speed of the car relative to the road. The plane’s speed is measured relative to the ground, or the air.

  Though physicists long ago accepted that uniform motion is only relative, some of them still wondered how fast Earth moves through space itself as our planet swings around the sun and glides across the galaxy. We don’t feel this motion, but we are travelling through space, which means that from our frame of reference, space is passing through us, all the time, unnoticed. Well, how fast is it going? How many litres of space per second slide through my body with ethereal stealth? After Galileo, it was recognized that no material device could measure the uniform speed of space. We can’t dip a metaphorical finger into the void around us to detect the slipstream as space sweeps by. But what about non-material methods?

  By the late nineteenth century, a new possibility had presented itself. Perhaps light could be used to measure how fast Earth is travelling through space? At that time, physicists imagined space as filled with a sort of ghostly jelly-like substance referred to as ‘the ether’, and they envisaged light waves as vibrations in the ether travelling at a definite, fixed speed – the speed of light. Physicists invented various optical devices to try and measure the speed of the Earth through the ether. After a few years of effort, the result they obtained was, precisely, zero. Apparently, the Earth wasn’t moving through space (i.e. the ether) at all! This made no sense. There was a flat-out contradiction between Galileo’s relativity of uniform motion and the theory of light travelling at a fixed speed. The clash called for a fundamental re-evaluation of the nature of space, time and motion, and the properties of light. The challenge was taken up by Einstein, culminating in the first phase of his famed theory of relativity, published in 1905. Einstein abolished the ether, declared that the speed of light was an absolute fixed number however the observer who measures it is moving, and reaffirmed Galileo’s position that uniform motion is always relative to some other material thing. It is not only impossible, he said, but meaningless to talk about the speed of something relative to space itself.

  All that’s well and good, but what about non-uniform motion, i.e. acceleration? We have no trouble detecting that. If my in-flight coffee spills because the plane hits turbulence I don’t need to look out of the window to tell that our uniform motion through the air has been disturbed. Acceleration is a change in velocity, and we can literally feel it happening. If you slam on the brakes in a car, you get pitched forward. If you yank the steering wheel and take a tight corner, you lurch against the door. Both are examples of acceleration, the first being a change of speed, the second a change of direction.

  A steady rate of rotation represents a constant acceleration, and Newton used a simple example to illustrate its properties. He pointed out that water in a spinning bucket creeps up the sides; we normally attribute this to ‘centrifugal force’. The question, ‘Is the bucket spinning or not?’ is immediately answered by seeing whether the surface of the water is flat. It’s not necessary to look at, say, the ground, to tell, which suggested to Newton that rotation, and accelerated motion generally, need not be gauged relative to other material systems, but is absolute. Sometimes this was expressed by saying that acceleration is relative to absolute space itself.

  Not everybody agreed. Right up to the twentieth century some scientists argued that even rotation must be understood as relative. Relative to what? Physicists used to say relative to the ‘fixed stars’. Of course, the stars aren’t really fixed, they are just so far away that we don’t notice their movement: relative to the distant matter in the universe is a better description. To visualize this proposal, imagine riding with your eyes closed on one of those horrid fairground contraptions where you pay good money to be hurled around in circles in great discomfort at high speed. Open your eyes, look up. What do you see? You see the stars whizzing round. When the stars stop, the ride has stopped, and you can get off.

  This basic observation led some people, most notably the Austrian engineer and philosopher Ernst Mach (the same Mach whose eponymous numbers are used to describe aircraft speed), to attribute the feeling of rotation – and acceleration more generally – to the far-flung stars. When you are pressed against the side of a whirling fairground seat, or feel your ‘stomach left behind’ when a lift suddenly descends, it’s because the stars are pulling on you, said Mach. It was a beguiling theory and became known as Mach’s principle. It had many adherents, including Einstein, who warmed to the idea that all motion was relative, not just uniform motion. He hoped Mach’s principle could be incorporated into a more general version of his theory of relativity, which he worked on in the early part of the twentieth century (see ‘The most beautiful theory of all time’, opposite). His thinking was that the gravity of all the distant stars and galaxies would combine to produce locally detectable effects like centrifugal force.

  The most beautiful theory of all time

  General relativity has been hailed as humanity’s finest intellectual accomplishment, at once a scientific theory and an art form of breath taking elegance. It entwines space, time, matter and force in an ingenious system of equations. Unlike many theories of physics, which are built in stages from the bottom up, general relativity springs whole from a few grand overarching principles, such as requiring that all physical properties must be independent of the co-ordinates used to describe them. Geometrical forms are sculpted from space itself by the arrangement of matter and energy. There is no actual force of gravity as Newton proposed, only restless, evolving geometry through which matter churns and light glides. The blandness of Newton’s immutable void is replaced by a concept of space as a vibrant, dynamic entity, stretching and shrinking, twisting and curving, pulsating and convulsing, and vibrating in energy-transporting undulations that ripple out across the universe at the speed of light (see ‘Gravitational waves’, p. 41). And general relativity works brilliantly. More than a century after Einstein presented his masterpiece, not a single observation has contradicted it.

  Sadly, it didn’t work. Einstein’s general theory of relativity predicts that a planet spinning in otherwise empty space will bulge round the equator (as the Earth does) on account of its rotation, even in the absence of any ‘fixed stars’ with which to gauge its motion. Einstein’s theory is our best current understanding of gravity, so when it comes to rotation, it rather vindicates Newton’s view that it is absolute and not relative. And although we cannot meaningfully speak of the speed of space, the acceleration of an object in empty space still makes sense.

  That
isn’t quite the end of the matter, though. A few physicists and cosmologists have continued to tinker with subtler formulations of Mach’s principle, and the subject is not completely settled. Indeed, many of my colleagues toggle back and forth between thinking of space as some sort of stuff (as did Newton) one day and as ‘nothing there’ the next. The lure of Mach’s principle is that it links everyday human experiences (like the feeling of spinning) with the organization of the universe, a sentiment captured most beautifully in the poet Francis Thompson’s lines:

  All things by immortal power,

  Near or far,

  Hiddenly

  To each other linkèd are,

  That thou canst not stir a flower

  Without troubling of a star.

  8. What is the Shape of Space?

  Hotel rooms sometimes come with those tiny fish-eye lenses in the door so if somebody knocks you can peer through the little hole and see a face with a huge nose and tiny ears. Similar effects are produced with fairground mirrors that are rather unflattering for the figure. Distorting shapes with lenses and mirrors is trickery, but it turns out that gravity does it for real. That is, gravity really does warp space.

  The first spacewarp to be observed was in 1919 when the British astronomer Sir Arthur Eddington led an expedition to West Africa to study an eclipse. To cast their horoscopes, astrologers follow the movement of the sun as it migrates through the constellations of the zodiac, moving against the background of stars. From time to time the sun’s movement brings it into close alignment with a star. When that happens it’s of interest to astronomers too. Eddington wanted to know how the sun’s gravity might affect the light from such a star. The trouble is, you can’t see stars in the daytime except during a solar eclipse. Then the moon blots out the sun’s glare for a few minutes, the sky darkens and the stars become visible. Astronomers know the positions of the stars very accurately when the sun isn’t in the way, and Eddington’s plan was to compare those recorded positions with their positions during the eclipse, to see if they were shifted. And they were. The stars were indeed very slightly askew (see Figure 5). It’s as if there is a gigantic fish-eye lens in the middle of the solar system.