What's Eating the Universe? Page 5
A clue to what dark matter might be comes from particles called neutrinos. Made deep inside the sun as a by-product of the nuclear reactions there, neutrinos are so impervious to matter they fly straight out, unimpeded, from the solar core (unlike photons, which slowly meander their way to the surface). And if neutrinos hit Earth, they go right through that, too. On the face of it, neutrinos look the part for dark matter, but because they have an extremely low mass they can explain no more than a tiny fraction of it.
Maybe dark matter is mostly made of highly penetrating particles like neutrinos, but much more massive? To search for these hypothetical WIMPS (for Weakly Interacting Massive Particles), physicists have built specialized detectors and put them deep underground to shield them from the disrupting effects of cosmic rays, which can trigger false positives in the equipment. So far, however, the researchers have drawn a blank. But, as Mr Micawber liked to say, no doubt ‘Something will turn up’ – sooner or later.
Although dark matter greatly outweighs normal matter, it doesn’t in fact make up most of the mass of the universe. Added together, normal and dark matter represent only about a third of the total. As to what the rest is, the mystery deepens . . .
11. What is Dark Energy?
The ancient structure known as Stonehenge is one of Britain’s foremost cultural sites, and a familiar landmark to tourists from all over the world. Volumes have been written about its astronomical and religious significance, but there has also been considerable puzzlement over how it came to be constructed, around four and a half thousand years ago, by Stone Age people without the benefit of modern technology. How were those massive blocks transported and erected? In particular, how were the huge capstones raised? Levitation would do the job nicely, and it’s a favourite explanation among devotees of the paranormal – not just for Stonehenge but the pyramids of Egypt, the walls of Machu Picchu and UFO propulsion. There is no doubt that the notion of levitation exercises a strange fascination for people. It has been part of our folklore for centuries, from magic carpets to alien abductions, and flying, floating, or levitation dreams are commonplace. There must be something deep in the human psyche that makes us find the idea compelling.
The problem with actual levitation is that it flies in the face of our best understanding of the law of gravity, both in Newton’s original theory and Einstein’s general relativity. But there was always a loophole in the latter. As I explained earlier, Einstein fudged a type of antigravity – levitation, indeed! – to prop up the universe. But this cosmic force wouldn’t have been much use to the Stonehenge work gang, lightening a 10-ton block by a paltry 10−32 grams.
The fudge factor in general relativity has swung in and out of fashion over the decades. When I was a student it was quipped that Einstein’s antigravity was repulsive, in both the physical and psychological sense. All that flipped, however, in the late 1990s when astronomers discovered that the expansion of the universe is in fact speeding up. Previously it was assumed that the expansion rate must slow as the weight of the universe puts a drag on it. Now it seems that antigravity not only exists but actually has the upper hand: a cosmic accelerator that has overwhelmed the brake. Antigravity is dubbed ‘dark energy’, partly because it is invisible, but also because its source is somewhat mysterious. (Don’t confuse dark energy with dark matter or the cosmic Dark Age – they are totally different things. ‘Dark’ seems to be a favourite word among scientists.)
The dark energy antigravity that is gripping the universe today is a faint remnant of inflation – that brief super-fast burst of expansion at the birth of the universe explained in Chapter 9. It may or may not derive from the same basic physical mechanism. If dark energy turns out to be nothing more than Einstein’s antigravity, the source of the force is then simplicity itself – it’s the energy of empty space. Which sounds baffling at first: why should space possess energy if it’s empty? There’s a fundamental reason why, which has to do with that other gem of twentieth-century physics: quantum mechanics.
In the 1930s, it dawned on physicists that there is no such thing as true emptiness. Even in a region of space devoid of all permanent residents (atoms, subatomic particles, photons . . . ) there remains an irrepressible traffic of temporary residents, particles that flit in and out of existence for the briefest duration, bubbling up from the void into half-reality only to dissolve away again almost immediately. But like the grin of the Cheshire cat, these ‘virtual particles’ leave behind subtle physical traces. Experiments confirm that the quantum vacuum really does seethe with energy, and where goes energy, there goes mass (see ‘E = mc2’, above). For those interested in the specific numbers, a billion cubic kilometres of empty space contains about seven micrograms of dark mass-energy. Doesn’t seem like much, but it all adds up – there’s a lot of space out there.
E = mc2
Even non-scientists know this famous equation, which links energy E, mass m and the speed of light c. It follows from the rule that nothing can travel faster than light, a key prediction of Einstein’s theory of relativity. What happens if you try to break the light barrier, for example by using a huge particle accelerator machine, like the famous Large Hadron Collider (LHC) near Geneva – a 27-kilometre ring-shaped underground tube in which counter-rotating high-energy proton beams are brought into head-on collision? Couldn’t one simply make the circulating particles move faster and faster until they exceed the speed of light? Actually, no. When the collider starts up, most of the energy fed to the protons does indeed increase their speed. But as the speed of light is approached, the particles become progressively more massive (i.e. heavier). The machine has to work harder and harder to gain additional speed. It’s a case of diminishing returns. At full power, almost all the input energy goes into making extra mass and very little into extra speed. According to the theory of relativity, no amount of input energy could push the particles through the light barrier. The best the LHC can do is reach 99.9999 per cent of the speed of light. This illustration shows that energy can be manifested as both speed and mass; the formula E = mc2 quantifies how much mass a given amount of energy represents. And it works backwards: mass is a source of energy. Because c (the speed of light) is such a big number, very little mass is worth a lot of energy. For example, just one gram of mass, if fully converted into electricity, will power the average middle-class household for several years.
I have dodged around a potentially confusing aspect of dark energy: why does ‘dark mass’, or vacuum mass, produce antigravity? Surely mass is mass, and it gravitates? For Newton, mass was the sole source of gravity: the greater the mass, the stronger the pull. But in general relativity, mass (= energy) is just one of several physical properties that generate gravity. Another is pressure. This is a bit counterintuitive because we normally think of pressure as a pushing force. And it’s true that the mechanical effect of pressure is just that. But pressure also produces a gravitational pull of attraction, which for everyday gases like air is far, far less than the mechanical push. With heat radiation, though, pressure actually rivals energy in its physical action. We rarely notice the pressure of radiation, but it’s there, all right: the pressure of sunlight helps produce the tails of comets. The extra contribution from pressure gives radiation the edge over ordinary gas when it comes to gravitating power, and for a few hundred thousand years after the big bang, when the heat was so intense, it was radiation that dominated the gravitational pull on the universe – that is, the braking effect on the rate of expansion – outweighing everything else.
When it comes to empty space, you might think that a void should have a pressure of precisely zero atmospheres, but it doesn’t. It’s very slightly less, by about a hundred trillionth of an atmosphere in fact. To sum it up in a phrase: space sucks. If pressure = gravity, then suction = antigravity. It turns out that the antigravity of dark suction beats the gravity of dark energy by a factor of three, so antigravity wins. The upshot is that space repels itself. Even if the universe were nothing but empty s
pace, it would expand, swelling without limit, doubling its own volume every few billion years. This, by the way, is the secret of the free-lunch inflationary universe I discussed in Chapter 9. Self-repelling space is a cosmic perpetual motion machine, yielding as much universe as you like with no need for any input energy. All it needs is a trigger, and away you go.
Although the account I have given may very well be the correct explanation for the accelerating universe, nobody knows how to calculate the actual quantity of quantum energy that is surging through space to produce the minuscule but all-important negative pressure. The numbers I quoted above come from astronomical observations, not theory. A straightforward calculation using quantum physics yields absurd answers. Some calculations estimate dark energy to be 120 powers of 10 too big! In reality, you can cook up any answer you like. At cosmology conferences, scientists place ‘the dark energy problem’ high on the list of cosmic mysteries to be solved, and routinely intone that it is one of the toughest challenges to theoretical physics ever. Personally, I think their hyperbole is somewhat overblown, and physicists will get to the bottom of it soon. But whatever the answer turns out to be, it will, alas, be unlikely to provide us with a magic carpet.
12. Where Does Matter Come From?
If you visit Westminster Abbey in London, and go to Newton’s tomb, you will see a small plaque in the floor nearby with an equation inscribed on it. The plaque commemorates Paul Dirac, one of the twentieth century’s most brilliant scientists. Take a look at Figure 9, and marvel at how simple and compact the formula is, even if the symbols mean nothing to you. This little equation provides an entrée to a wonderland of mathematics and physics, and to a deeper layer of physical reality that you would never suspect existed, even though it is all around us.
Published in 1929, Dirac’s equation merges the two great triumphs of twentieth-century physics – the theory of relativity, and quantum mechanics. It was crafted for the very practical purpose of describing how electrons behave when moving at near the speed of light, as they do, for example, in radioactive emissions or when orbiting heavy atoms like uranium. But the equation is also a work of sublime artistry: these sparse symbols embed novel geometrical structures that lead to an entirely new way of describing space, time and matter (see ‘Spin’, p. 66).
When you invent an equation the first thing you try to do is solve it. In the case of the Dirac equation the solutions correspond to various possible states of the electron, for example in orbit around an atom or just moving freely with a certain energy. When Dirac attempted this, he was puzzled by the fact that for every choice of energy there were two solutions to his equation. Only one was needed to describe the electron. What was the other one for? What was this strikingly elegant equation trying to say? Dirac made a wild guess: his equation was telling us (well, him) of the existence of a type of anti-world, in which for every state of an electron there is a corresponding state of an ‘anti-electron’– a particle with the same mass as the electron but with the opposite electric charge (positive instead of negative). But no such particle was known in 1929. Yet Dirac was right. In 1932, anti-electrons (usually referred to as positrons) were discovered by the American physicist Carl Anderson. Dirac was awarded a Nobel prize the following year.
Figure 9. Plaque in Westminster Abbey commemorating Paul Dirac, featuring his famous equation.
Soon after the discovery of the positron it became clear that other anti-particles must exist: anti-protons, anti-neutrons, anti-neutrinos, etc. Today, the anti-particles of all known particles of matter have been discovered and, in some cases, harvested and used in experiments. Collectively, these anti-particles are called antimatter. The existence of antimatter presents cosmologists with an oddity. Why is the beautiful mathematical symmetry implicit in Dirac’s equation not reflected in the make-up of the physical world? You and I and everything we encounter in daily life are made of matter. Where is all the antimatter? I should say that it’s a good thing there’s so little of it: if matter runs into antimatter then both vanish in a shower of gamma rays.
Dirac wondered whether the universe might actually contain equal amounts of matter and antimatter, but they somehow became sorted into widely separated regions where they wouldn’t often collide. He thought there might be entire anti-stars. As one couldn’t tell by looking whether a star was made of matter or antimatter this wasn’t such a crazy suggestion. However, some mixing of material is always occurring in the galaxy and astronomers don’t see any of the telltale gamma rays that occur when matter and antimatter collide. If there is antimatter out there, it can’t be more than a smidgeon.
The absence of antimatter is part of the wider problem of the presence of matter. All those protons, neutrons and electrons that came out of the big bang – a hundred trillion trillion trillion trillion tons of it within the purview of our instruments – how did it come to exist in the first place? Making matter as such isn’t actually a mystery; it happens all the time. For example, cosmic rays – not really ‘rays’ but high-energy particles, mostly protons – continually pepper the Earth from space. They are hurled our way at nearly the speed of light from powerful sources far across the galaxy. When they slam into the air above us they create whole showers of new particles: one proton may produce dozens of electrons as well as exotica like muons, pions, kaons . . . It was in fact in cosmic ray showers that Anderson first found the positron. Switch on a Geiger counter and much of the background click-click-click will be from freshly minted muons raining down from on high. Controlled matter creation occurs in particle accelerator machines like the LHC. Crucially, however, when processes like these create new particles, they are always accompanied by the right complement of anti-particles.
The essential transformation taking place when new particles appear is the conversion of kinetic energy into matter. All that is needed is enough energy to pay for the masses of the created particles and anti-particles (see ‘E = mc2’, p. 55). Since there was plenty of heat energy in the big bang, the appearance of matter at that time is no surprise. But that still leaves us with the problem of the missing antimatter. It seems that something must have broken the matter–antimatter symmetry in that first fierce flash of cosmic existence, and the study of precisely what has been a nagging problem in physics for two generations. Whatever the mechanism was, it didn’t need to totally favour matter, resulting in zero antimatter. It would be enough for the created particles of matter to very slightly outnumber their antimatter counterparts by, say, one part in a billion. Then, as the universe cooled, all the antimatter and most of the matter would have embarked on an orgy of mutual destruction, leaving only a small residue of unpaired matter particles – a cosmic afterthought, one might say, yet enough to build a universe. If this is correct, then the legions of gamma ray photons resulting from this wholesale annihilation, greatly red-shifted, are what makes up the CMB we detect today.
The (slightly flawed) symmetry between matter and antimatter in the laws of physics is just one of several fundamental symmetries that physicists have focused on in the decades since Dirac’s discovery. Another is mirror symmetry. In a mirror, a right hand reflects into a left hand. You might suppose nature wouldn’t favour left over right, but in 1956 physicists were shocked to learn that it does. The radioactive process known as beta decay, in which a neutron vanishes to be replaced by a proton, an electron and an anti-neutrino, is distinctively lopsided between left and right (see Figure 10). If you sneakily played a movie of beta decay in a mirror, a physicist could spot the deception.
A third symmetry that crops up is time reversal. Just as you can play a movie in a mirror, so you can play a movie backwards. The question then arises of whether, in particle physics, for every process that occurs, the laws of physics permit the reverse process. For example, when an electron meets a positron they annihilate one another and make two gamma ray photons. Can two photons create an electron–positron pair? Yes, they certainly can, but will pair creation always occur with precisely the same li
kelihood as pair annihilation? If nature preferred one direction of time over the other, there might be a small difference in the rate of the forward and reversed processes. In the 1960s it was discovered that, sure enough, there are some examples of particle transformations where this is the case (though not the one just cited). That is, subatomic processes occur that do violate time-reversal symmetry, by a tiny amount. A wily physicist could watch a movie of the process and deduce this without having been told that it was being played backwards.
Figure 10. Nuclear processes called beta decay break the symmetry between left- and right-handedness. The figure shows a spinning atomic nucleus emitting an electron towards the ‘south’. Seen in a mirror, the spin direction is reversed so the electron appears to emanate towards the ‘north’. An experiment performed in 1956 with radioactive cobalt found a marked asymmetry in the direction of emissions between ‘north’ and ‘south’ – on average, the electrons preferred to ‘go south’ – implying a difference between the process and its mirror image.
What about a movie of a particle process played both in a mirror and backwards? Would our clever physicist spot that deception? Well, maybe not, because it turns out that reversing left–right and the direction of time together is equivalent to reversing the electric charge, which occurs in the switch between matter and antimatter (for example, between a negatively charged electron and a positively charged positron). Likewise, switching all the charges has the same effect as reflecting in a mirror and reversing the direction of time. In 1967 the Russian dissident and theoretical physicist Andrei Sakharov pointed out that if there were fundamental particle processes that violated the combined charge reversal and left–right symmetries, then the way would be open to explaining why the big bang coughed out an excess of matter.