Conclusions in science are strange in that they are not for ever. ‘Nothing is static, nothing is final, everything is held provisionally,’ said Jocelyn Bell Burnell, the astronomer who discovered pulsars. Put plainly, no sooner do you get used to one theory than somebody comes up with a better one. It is often forgotten, however, that theories can be useful even when they are not correct: the prevailing theory prevails precisely because it describes reality better than any other. So here are three articles on our latest conclusions about conclusions – the stark consequence of slothful living, the raw nature of reality when it is robbed of energy, and the demise of our collective home, the universe.
It’s an old idea that doing nothing is against the natural order. The message normally comes down from a parent or teacher: you may have been warned that idleness is incompatible with purity of mind or that the devil will commandeer your hands. But in the last century, science showed unambiguously that inactivity is also bad for your body – today we’re finding out just how bad. Andy Coghlan discovers the extraordinary value of a little daily exercise.
It’s 9.00 am in the office – time for my daily medication. As usual, I slink off to the fire escape for my fix. Twenty minutes later, I’m back at my desk, brimming with vitality and raring to go.
I’ve taken this medicine regularly now for years, after developing elevated blood pressure in my mid-40s. I’d heard it could help reduce blood pressure and improve circulation. Sure enough, the high blood pressure vanished long ago.
Amazingly, this drug is freely available to everyone on the planet. It’s completely up to you when you take it, and how much. And as research is now revealing, the more of it you take, the healthier you will be.
What is this wonder drug? It is plain old physical activity of all sorts – from running marathons to simply walking around your sofa while watching television. We’ve all heard that exercise is good for us, but what is becoming increasingly clear is the sheer extent of its benefits and why it works.
A plethora of recent studies show that exercise protects us from heart attacks, strokes, diabetes, obesity, cancer, Alzheimer’s disease and depression. It even boosts memory. And it has the potential to prevent more premature deaths than any other single treatment, with none of the side effects of actual medication. ‘It’s a wonder drug,’ says Erik Richter, a diabetes researcher at the University of Copenhagen. ‘There’s probably not a single organ in the body that’s unaffected by it.’
Throughout evolution, humans have been active. Our ancestors chased prey as hunter-gatherers and fled from predators. More recently, they laboured on farms and in factories. But the decline of agricultural and industrial labour, plus the invention of the car, a multitude of labour-saving devices and – most perniciously – TV, computers and video games, mean we’ve all ground to a sudden and catastrophic standstill.
‘We were built to be active, but the way our environment has changed and the way we live our lives has led us to become inactive,’ says Christopher Hughes, senior lecturer in sport and exercise medicine at Queen Mary, University of London.
Now we’re paying the price. In 2009, Steven Blair, an exercise researcher at the University of South Carolina in Columbia, published a study of more than 50,000 men and women showing that a lack of cardiorespiratory fitness was the most important risk factor for early death.1 It accounted for about 16 per cent of all deaths in men and women over the period of study, more than the combined contributions of obesity, diabetes and high cholesterol, and double the contribution of smoking.
In other words, physical inactivity is killing us. ‘Everyone knows too much booze or tobacco is bad for you, but if physical inactivity was packaged and sold as a product, it would need to carry a health warning label,’ says Hughes.
As we have become inactive, so once-rare diseases have mushroomed. A report from the organisation Diabetes UK reveals that in 1935, when the world’s population was just over 2 billion, an estimated 15 million people globally had type 2 diabetes.2 By 2010 the world’s population had more than trebled and the number with diabetes had shot up to 220 million – more than 14 times the number in 1935. Likewise, results published in 2012 in the Journal of the American Medical Association show that more than a third of men and women in the US are obese, as are about 17 per cent of US children.3
The good news is that we can do something about it. I started running up and down the fire escape for a few minutes each day in the hope of not having to take cholesterol-lowering statins or drugs for high blood pressure. Now I’m eager to know what my daily routine is doing to my body and, more importantly, how it may be protecting me from disease.
The most robust evidence so far comes from the Exercise is Medicine initiative pioneered by the American College of Sports Medicine in Indianapolis, Indiana. Researchers there have collated studies over the past decade or so of people who follow the US government’s advice on physical activity. This prescribes 150 minutes per week of moderate-intensity aerobic activity, such as brisk walking, ballroom dancing or gardening, or 75 minutes of more vigorous activity such as cycling, running or swimming.
What the Exercise is Medicine findings show is that this weekly dose of moderate exercise reduces the risk of premature death through heart disease by 40 per cent, approximately the same as taking statins.
Chi Pang Wen of the National Health Research Institute in Zhunan, Taiwan, offers some insights into precisely how physical activity prevents cardiovascular diseases. ‘Exercise can stimulate circulation, flush out fatty deposits in the walls of blood vessels and dilate small vessels that could otherwise be the cause of a heart attack or stroke,’ he says. In April 2012 he presented results from a study of more than 430,000 Taiwanese men and women, showing that exercise reduced the risk of heart attacks by 30 to 50 per cent.
Exercise also keeps blood vessels clear by helping to destroy the most dangerous fats. Research published earlier in 2012 reveals that it alters the structure of fatty triglyceride particles in the bloodstream, making it easier for enzymes to destroy them before they can gum up the works.4 Many risks to circulatory health come from such fatty particles, in the form of chylomicrons produced in the gut, or very low density lipoproteins (VLDLs) pumped out by the liver. The bigger the VLDL particles are, the easier they are for enzymes to break down, and the findings show that exercise causes the particles to enlarge by about a quarter.
‘A single 2-hour bout of exercise reduced triglyceride concentrations in the circulation by 25 per cent compared with no exercise,’ says Jason Gill, who led the study at the University of Glasgow. His team found a decrease in both types of fat, but the decrease was twice as much for the more insidious VLDL particles.
One of the most startling findings of the Exercise is Medicine initiative is that a modest weekly dose of exercise lowers the chances of developing type 2 diabetes by 58 per cent, twice the preventive power of the most widely prescribed anti-diabetes medication, metformin.5
Type 2 diabetes affects adults when they stop responding efficiently to the hormone insulin, which orders muscle and fat cells to absorb surplus glucose from the bloodstream. When insulin loses its punch, glucose continues circulating and creates the potentially fatal sugar imbalances that are the hallmark of diabetes.
How does exercise reverse this? The story dates back to 1982, when Richter found that insulin activity is enhanced by physical activity – at least in rats.6 Experiments showed that after the rats had run around for a couple of hours, their cells became up to 50 per cent more responsive to insulin compared with the cells of non-exercising rats. ‘We confirmed it later in humans,’ Richter says.
As cells reawaken to insulin, it seems that surplus glucose gets sponged from the circulation. Richter found that the effects lasted for a couple of hours after exercise in rats, and up to two days in humans.7
Recently he and colleagues have unravelled more details about how exercise brings this about. They have discovered that both insulin and muscle contractions during exercise activate a molecule in muscle and fat cells called AS160, which helps them absorb glucose.8 Once activated, AS160 orders the cell to send molecules to the cell’s surface to collect glucose and bring it inside. Without these transporter molecules, glucose cannot get through the fatty cell membrane.
Exercise also helps cells burn off excess sugar. Muscle cells absorb glucose and fatty acids from the bloodstream to replenish adenosine triphosphate (ATP), the molecular fuel found in most living cells. As ATP is used up, it produces waste products that are sensed by another molecule, AMPK. AMPK then orders cells to recharge by absorbing and burning yet more fat and sugar. In the mid-1990s, Grahame Hardie at the University of Dundee found that exercise accelerates this process because muscle contraction activates AMPK.
Hardie says exercise has the potential to reverse obesity and diabetes and prevent cancer. The findings of the Exercise is Medicine initiative show that taking the US government’s recommended weekly dose of exercise halves the risk of breast cancer in women and lowers the risk of bowel cancer by around 60 per cent.9, 10 This is about the same reduction seen with low daily doses of aspirin.
How exercise does this is not yet clear – not least because so many factors are involved in cancer’s appearance and progression, including sex hormone imbalances, the ability of the immune system to clear cancer cells, and damage to genes and DNA generally. However, some clues are beginning to emerge. ‘Exercise reduces body weight, which is a known risk factor for postmenopausal breast cancer,’ says Lauren McCullough of the University of North Carolina at Chapel Hill.
She also thinks that reducing fat deposits in the body results in less exposure to circulating hormones, growth factors and inflammatory substances. ‘All have been shown to raise breast cancer risk,’ she says.
Another clue comes from work by Anne McTiernan of the Fred Hutchinson Cancer Research Center in Seattle, who studies bowel cancer. Biopsies from 200 healthy volunteers showed that, compared with exercisers, non-exercisers had more telltale signs of abnormalities in colonic crypts – recesses in the lining of the colon that absorb water and nutrients.11 Crypts in idle participants had an increased number of dividing cells, and these also climbed higher up the crypt walls, where they had the potential to form pre-cancerous polyps.
Another potential protection against cancer might come back to the ability of exercise to stimulate AMPK. Recent research by Beth Levine of the University of Texas Southwestern Medical Center in Dallas showed that exercise stimulates cells craving extra energy to burn unwanted rubbish, including faulty or mutated DNA that could trigger cancer if it hangs around.12 More recently, Levine has discovered the same processes in brain cells, suggesting that exercise might play a role in staving off dementias and neurodegeneration.
As well as potentially staving off dementia, pounding the stairs might even help boost my brainpower and memory. Back in 1999, Henriette van Praag of the US National Institute on Aging in Baltimore, Maryland, found that mice using a running wheel developed new neurons in the hippocampus, a part of the brain vital for memory.13 ‘We had a doubling or tripling of neurons after they’d been running daily for about a month,’ she says. Subsequently, van Praag and other groups found the most likely reason: a doubling in the level of a substance in the hippocampus called brain-derived neurotrophic factor, or BDNF, which may support growth of new neurons.
More than a decade on, a team led by Art Kramer of the University of Illinois at Urbana-Champaign demonstrated through a brain-imaging study of 120 older adults that exercise increased hippocampus volume by around 2 per cent.14 It also improved their memory, as measured by standard tests. ‘The volume increase we saw can make up for approximately two years of normal age-related decrease,’ says Kramer. ‘We found that even modest increases in fitness can lead to moderate, 15 to 20 per cent improvements in memory.’
The benefits aren’t just restricted to adults. Kramer and his colleagues have also found that pre-adolescent children who exercise develop larger hippocampuses.15
So if exercise is so beneficial, why won’t people take it? At least 56 per cent of US adults don’t meet the government’s exercise guidelines. ‘The most common excuse people give in polls is that they don’t have time,’ says Blair. Perhaps that is not surprising when US citizens spend, on average, almost 8 hours a day watching TV, according to a 2008 study.
For those, like me, who don’t want the fuss of joining a gym, there is plenty people can do at home or the workplace in their own time and at their own pace. Blair cites a study in which researchers asked half of a group of couch potatoes to walk round their sofa during each TV commercial break.16 ‘They burned 65 calories more per hour, and that is 260 calories in 4 hours,’ he says. Over a week, their exertions met the US government recommendations for exercise.
And overweight people can benefit massively from exercise even if they don’t lose weight, Blair points out. One of his studies has shown that for fit fat people, the risk of dying prematurely is half that for unfit lean people.17
Once a marathon runner, Blair now walks for an hour a day, and at the age of 73, he has set himself the goal of walking 5 million steps each year, tracking his progress with a pedometer. He is concerned that not enough doctors recognise that lack of fitness is effectively a disease. He wants them to use fitness as a gauge of health, perhaps making their patients do a treadmill test as a matter of routine, rather than considering it as an afterthought.
Figures published in The Lancet in 2012 back up his assertion that no action, other than abstaining from smoking, is as good for health as being physically active.18 The study also reveals that physical inactivity effectively kills 5 million people a year worldwide, as many as smoking.
As for me, the stair-run does seem to be working, although I don’t have health data from eight years ago to confirm my progress. More recent scans and tests showed my blood pressure and bone density are normal, and I have 6 per cent less body fat than is average for my age. Also, only 20 per cent of my fat is the dangerous sort around organs in the abdomen, compared with 30 per cent in most of my peers. My heart fitness, measured on a treadmill, is above average and I have no chronic diseases that I know of. Now, imagine you were offered a pill that did all that. Wouldn’t you take it?
Cool matter to absolute zero and you rob it of all thermal energy. As it approaches its ultimate state, a new world opens up. It’s a world in which the everyday rules of physics seem to disappear. As Michael Brooks finds out, here you glimpse nature in the raw.
For centuries, con artists have convinced the masses that it is possible to defy gravity or walk through walls. Victorian audiences gasped at tricks of levitation involving crinolined ladies hovering over tables. Even before then, fraudsters and deluded inventors were proudly displaying perpetual-motion machines that could do impossible things, such as make liquids flow uphill without consuming energy. Today, magicians still make solid rings pass through each other and become interlinked – or so it appears. But these are all cheap tricks compared with what the real world has to offer.
Cool a piece of metal or a bucket of helium to near absolute zero and, in the right conditions, you will see the metal levitating above a magnet, liquid helium flowing up the walls of its container or solids passing through each other. ‘We love to observe these phenomena in the lab,’ says Ed Hinds of Imperial College, London.
This weirdness is not mere entertainment, though. From these strange phenomena we can tease out all of chemistry and biology, find deliverance from our energy crisis and perhaps even unveil the ultimate nature of the universe. Welcome to the world of superstuff.
This world is a cold one. It only exists within a few degrees of absolute zero, the lowest temperature possible. Though you might think very little would happen in such a frozen place, nothing could be further from the truth. This is a wild, almost surreal world, worthy of Lewis Carroll.
One way to cross its threshold is to cool liquid helium to just above 2 K. The first thing you might notice is that you can set the helium rotating, and it will just keep on spinning. That’s because it is now a ‘superfluid’, a liquid state with no viscosity.
Another interesting property of a superfluid is that it will flow up the walls of its container. Lift a bucketful of superfluid helium out of a vat of the stuff, and it will flow up the sides of the bucket, over the lip and down the outside, rejoining the fluid it was taken from.
Though fascinating to watch, such gravity-defying antics are perhaps not terribly useful. Of far more practical value are the strange thermal properties of superfluid helium.
Take a normal liquid out of the refrigerator and you find it warms up. With a superfluid, though, the usual rules no longer apply. Heat diffuses so quickly through a superfluid that it doesn’t warm. Researchers working at the Large Hadron Collider (LHC) at CERN, near Geneva in Switzerland, use this property to help accelerate beams of protons. They pipe 120 tonnes of superfluid helium around the accelerator’s 27-kilometre circumference to cool the thousands of magnets that guide the particle beams. Normal liquid helium would warm up considerably if used in this way, but the extraordinary thermal properties of the superfluid version means its temperature rises by less than 0.1 K for every kilometre of the beam ring. Without superfluids, it would have been impossible to build the machine that many physicists hope will reveal the innermost secrets of the universe’s forces and building blocks
The LHC magnets have super-properties themselves. They are made from the superfluid’s solid cousin, the superconductor.
At temperatures approaching 0 K, many metals lose all resistance to electricity. This is not just a gradual reduction in resistance, but a dramatic drop at a specific temperature. It happens at a different temperature for each metal, and it unleashes a powerful phenomenon.
For a start, very little power is needed to make superconductors carry huge currents, which means they can generate intense magnetic fields – hence their presence at the LHC. And just as a superfluid set rotating will keep rotating for ever, so an electric current in a superconducting circuit will never fade away. That makes superconductors ideal for transporting energy, or storing it.
The cables used to transmit electricity from generators to homes lose around 10 per cent of the energy they carry as heat, due to their electrical resistance. Superconducting cables would lose none.
Storing energy in a superconductor could be an even more attractive prospect. Renewable energy sources such as solar, wind or wave power generate energy at unpredictable times. If superconductors could be used to store the excess power these sources happen to produce when demand is low, the world’s energy problems would be vastly reduced.
We are already putting superconductors to work. In China and Japan, experimental trains use another feature of the superconducting world: the Meissner effect.
Release a piece of superconductor above a magnet and it will hover above it rather than fall. That’s because the magnet’s field induces currents in the superconductor that create their own, opposing magnetic field. The mutual repulsion keeps the superconductor in the air. Put a train atop a superconductor and you have the basis of a levitating, friction-free transport system. Such ‘maglev’ trains do not use metal superconductors, because it is too expensive to keep metals cooled to a few K; instead they use ceramics that can superconduct at much higher temperatures, which makes them easier and cheaper to cool using liquid nitrogen.
These are strange behaviours indeed, so what explains them? Superfluidity and superconductivity are products of the quantum world, and to get an idea of what’s going on, here’s a thought experiment. Imagine you have two identical particles, and you swap their positions. The new set-up looks exactly the same, and responds to an experiment exactly as before. However, quantum theory records the swap by a change in the particles’ quantum state, which is multiplied by a ‘phase factor’. Switching the particles again brings in the phase factor a second time, but the particles are in their original positions and so everything returns to its original state. ‘Since switching the particles twice brings you back to where you were, multiplying by this phase twice must do nothing at all,’ says John Baez at the Centre for Quantum Technologies in Singapore. This means that squaring the phase must give 1, which in turn means that the phase itself can be equal to 1 or –1.
This is more than a mathematical trick: it leads nature to divide into two. According to quantum mechanics, a particle can exist in many places at once and move in more than one direction at a time. In the last century, theorists showed that the physical properties of a quantum object depend on summing together all these possibilities to give the probability of finding the object in a certain state.
There are two outcomes of such a sum, one where the phase factor is 1 and one where it is –1. These numbers represent two types of particles, known as bosons and fermions respectively.
The difference between them becomes clear at low temperatures. That is because when you take away all thermal energy, as you do near absolute zero, there aren’t many different energy states available. The only change that can be made is to swap the positions of the particles, with the consequent phase change.
Swapping bosons introduces a phase change of 1. Using the equations to work out the physical properties of bosons, you find that their states add together in a straightforward way, and that this means there is a high probability of finding indistinguishable bosons in the same quantum state. Simply put, bosons like to socialise.
In 1924, Albert Einstein and Satyendra Bose suggested that at low enough temperatures, the body of indistinguishable bosons would effectively coalesce together into what looks and behaves like a single object, now known as a Bose—Einstein condensate, or BEC.
Helium atoms are bosons, and their formation into a BEC is what gives rise to superfluidity. You can think of the helium BEC as a giant atom in its lowest possible quantum energy state. Its strange properties derive from this.
The lack of viscosity, for instance, comes from the fact that there is a huge gap in energy between this lowest state and the next energy state. Viscosity is just the dissipation of energy due to friction, but since the BEC is in its lowest state already, there is no way for it to lose energy – and thus it has no viscosity. Only by adding lots of energy can you break a liquid out of the superfluid state.
If you physically lift a portion of the superatom, it acquires more gravitational potential energy than the rest. This is not a sustainable equilibrium for the superfluid. Instead, the superfluid will flow up and out of its container to pull itself all back to one place.
Superconductors are also BECs. Here, though, there is a complication because electrons, the particles responsible for electrical conduction, are fermions.
Fermions are loners. Swap them around and, as with swapping your left and right hand, things don’t quite look the same. Mathematically, this action introduces a phase change of –1 into the equation that describes their properties. The upshot is that when it comes to summing up all the states, you get zero. There is zero probability of finding them in the same quantum state.
We should be glad of this: it is the reason for our existence. The whole of chemistry stems from this principle that identical fermions cannot be in the same quantum state. It forces an atom’s electrons to occupy positions further and further away from the nucleus. This leaves them with only a weak attraction to the protons at the centre, and thus free to engage in bonding and other chemical activities. Without that minus sign introduced as electrons swap positions, there would be no stars, planets or life.
Electrons are fermions, yet in superconductors they form into BECs. So how does that happen? In 1956, Leon Cooper showed how electrons moving through a metal can bind together in pairs and acquire the characteristics of a boson. If all the electrons in a metal crystal form into such Cooper pairs, these bosons will come together to form, as in superfluid helium, one giant particle – a BEC.
The main consequence of this is a total lack of electrical resistance. In normal metals, resistance arises from electrons bumping into the metal ions bouncing around. But once a metal becomes a superconductor, the electron-pair condensate is in its lowest possible state. That means it cannot dissipate energy and, once the Cooper pairs are made to flow in an electrical current, they simply keep flowing. The only way to disturb superconductivity without raising the temperature is to add energy another way, for example by applying a sufficiently strong magnetic field.
Though superfluids and superconductors are bizarre enough, they are not the limit of the quantum world’s weirdness, it seems. ‘There is yet another level of complexity,’ says Hinds. That complexity comes into play below 1 K and at more than 25 times Earth’s atmospheric pressure, when helium becomes a solid. This form of helium plays havoc with our notions of solidity. Get the conditions right and you can make solids pass through each other like ghosts walking through walls.
Such an effect was first observed in 2004 by Moses Chan and Eunseong Kim at Penn State University in University Park, Pennsylvania. They set up solid helium in a vat that could rapidly rotate back and forth, inducing oscillations in the solid helium. They observed a resonant vibrational frequency which they interpreted as indicating that there were two solids in the vat, which were passing through each other.
Superfluids, superconductors and supersolids owe their bizarre behaviour to the formation of a sort of superatom inside them, known as a Bose–Einstein condensate (BEC).
But might it be possible to create such a state outside a liquid or solid? It took researchers many years, but in 1995 a team at the University of Colorado and the US National Institute of Standards and Technology, both based in Boulder, finally succeeded in coaxing a gas of rubidium into a BEC, its lowest possible quantum state. The breakthrough won team leaders Carl Wieman and Eric Cornell, together with Wolfgang Ketterle at the Massachusetts Institute of Technology, the 2001 Nobel prize for physics.
When Wieman and Cornell made their condensate, their lab briefly became home to the coldest place in the universe, just 20 nanokelvin above absolute zero. It wasn’t the only BEC in the cosmos, though, even discounting superfluid or superconductor experiments that may have been taking place at exactly the same time.
In 2011, the Chandra X-ray telescope showed that the core of a neutron star called Cassiopeia A, which lies 11,000 light years away from Earth, is a superfluid. One teaspoon of neutron star material weighs six billion tonnes and the intense pressure from the outer layers is enough to squeeze the core into a BEC. Yet, despite the name, the core of a neutron star isn’t exclusively made of neutrons; it contains a portion of protons too, which also form a BEC. You can think of this as a superfluid or, because the protons carry electrical charge, a superconductor.
Admittedly the two solids do not fit our usual definitions. One was made up of ‘vacancies’, created when helium atoms shake free of the lattice that forms solid helium. The gaps left behind have all the properties of a real particle – they are so like real particles, in fact, that their quantum states can lock together to form a BEC. The solid helium is also a BEC, and it is these two condensates that pass through each other.
Chan and Kim’s observation is still somewhat controversial; some researchers think there is a more prosaic explanation to do with deformations and defects in the helium lattice. ‘There is a lot of activity, several theory notions and experiments of interest, but no real agreement,’ says Robert Hallock of the University of Massachusetts at Amherst.
Nonetheless, even the fact that it might be possible to create solids that aren’t really solid shows just how odd superstuff can get. And it’s all because the world has a fundamental distinction at its heart. Everything, from human beings to weird low-temperature phenomena like liquids that defy gravity, stems from the fact that there are two kinds of particles: those that like to socialise, and those that don’t. Sound familiar? Perhaps the quantum world isn’t that different from us after all.
Crunch, snap, rip, fade … No, it’s not a new breakfast cereal, but four ways our universe might end its days. What better way to conclude our exploration of nothing than to ask what will ultimately happen to all the matter that appeared in the big bang. Our guide to the finale is Stephen Battersby.
The future ain’t what it used to be. Cosmologists were once confident they knew how the universe would end: it would just fade away. An ever colder, ever dimmer cosmos would slowly wind down until there were only cinders where the stars once shone. But that’s history.
Today’s science suggests many different possible futures. Cosmic cycles of death and rebirth might be on the cards, or a very peculiar end when the vacuum of space suddenly turns into something altogether different. The universe might collapse back in on itself in a big crunch. Or we could be in for an even more violent end called the big rip. Or a weird pixellation – the big snap. Or find our whole universe pouring down a wormhole (the big trip). The slow drift into darkness is still a contender, but fear not: that long night could be a lot more interesting than you might think – imagine the cosmos filled with giant diamonds.
Why this wealth of possibilities? Until recently, the dominant force in the universe seemed to be the gravity of stars and other matter, and that meant there were only two options. Either the universe was dense enough for gravity to halt the expansion from the big bang and pull everything back together in a big crunch or else it wasn’t, in which case the expansion would carry on for ever.
Most cosmologists thought the latter possibility was the more likely. Then, in 1998, astronomers got a shock: they found that the universe’s expansion isn’t slowing down at all, but accelerating. Studies of the light of distant supernovae revealed that something stepped on the gas around 6 billion years ago. The finding seemed to seal our fate, condemning us to a headlong rush to oblivion. Acceleration means a universe that will become cold and boring much faster than we’d thought.
But researchers have now realised this gloomy outlook was premature, because no one knows what is causing the acceleration. Astronomers have named this mysterious force dark energy, but its origin and nature are a mystery. So how can anyone say what it is going to do in the future? ‘People started realising that as long as we have no clue what dark energy is, we can’t be so arrogant,’ says Max Tegmark of the Massachusetts Institute of Technology.
Although the long-term forecast is still open to debate, astronomers do at least agree on what will happen to our neighbourhood in the near future. Things will become rather uncomfortable in about 6 billion years, when the sun swells to become a red giant, boiling the oceans away and possibly even swallowing Earth. Then our star will exhaust its nuclear fuel and shrink into a white dwarf roughly the size of Earth, leaving our old planet (if it still exists) cold enough to be covered in nitrogen ice. At least the view will be lovely: gas blown into space by the red giant will be energised by ultraviolet rays from the white dwarf, so for a while Earth will be surrounded by a glowing multicoloured nebula.
Our galaxy is in for a rough time too. We are heading towards another, larger spiral called Andromeda, and we could collide in as little as 3 billion years. For a while, the merger will create a brilliant, elaborate hybrid galaxy, as streamers of stars are flung outwards, and most of the loose gas in both galaxies is compressed to form bright new stars.
After only a couple of billion years more, those stellar tentacles will subside and the two delicate spirals will have merged into one great blob, an elliptical galaxy. Most of the free gas will have been used up, so relatively few new stars will form, except when small nearby galaxies in our local group are swallowed, each giving up its gas in a little puff of star formation.
Our collision with Andromeda will have a spectacular climax. At the centre of the Milky Way is a giant black hole more than 3 million times the mass of the sun. Another in Andromeda is probably ten times the size. These two black holes will settle towards the centre of the new galaxy, and there they will spiral together and eventually merge. The energy released will be tremendous, sending out a blast of light and X-rays, and a pulse of gravitational waves that will squeeze and stretch every star and planet.
Looking out into the universe we will see other galaxies moving away from our elliptical home, dragged ever faster by the hand of dark energy. But for how long? That depends on the nature of dark energy. For instance, its energy density could decrease with time. Some theorists have even come up with model universes where the dark energy becomes negative. As positive dark energy has repulsive gravity, negative dark energy would have attractive gravity, like ordinary matter.
If that happens in our universe, the consequences will be extreme. First acceleration will slow, and then dark energy will begin to really put the brakes on. Expansion will eventually halt, and then reverse, so that galaxies rush back towards each other and start colliding at ferocious speeds. Eventually, everything will be crushed together in a big crunch, unimaginably dense and hot, like the big bang in reverse.
That won’t happen for a while, though. Those observations of distant supernovae, which trace the expansion of space over time, show that if dark energy is fading, it can’t be doing so very fast. Andrei Linde of Stanford University in California has calculated that we are safe from a big crunch for at least 25 billion years, almost twice the age of the universe today.
But an even more grisly end could be in store. In 2003, Robert Caldwell of Dartmouth College in New Hampshire explored the opposite idea: that dark energy could become stronger. This exotic flavour of dark energy is called phantom energy. The expansion of space makes phantom energy increase, and phantom energy makes space expand even faster, setting up a devastating positive-feedback loop he calls the big rip.
If Caldwell is right, then a crisis could arrive in as little as 40 billion years from now. It would be perhaps the most watchable doomsday that cosmologists have imagined, not entirely unlike the spectacle laid on in Douglas Adams’s The Restaurant at the End of the Universe.
Roughly 60 million years before the end, the phantom repulsion becomes strong enough to tear our galaxy apart. Then, just months before the end, the real show begins. Let’s assume that by this point we have found ourselves a new home in a solar system not unlike this one. We will first see the outer planets fly away one by one. Next our adopted Earth will be torn from its sun. Less than an hour from the end, the sun will explode, and minutes later Earth will be ripped apart too. We might just be able to keep watching until a fraction of a second before the end, but presumably not long enough to see the destruction of molecules and atoms at around t-minus 10–19 seconds, when the phantom overpowers all electromagnetic forces. Neither will we see the subsequent shredding of nuclei, protons and neutrons. Pity.
Phantom energy could have a different outcome if our universe contains even a single wormhole. Wormholes are like tunnels in spacetime, possibly connecting one universe with another, and they would feed on phantom energy, and grow. A phantom-fed wormhole could grow large enough to swallow the whole universe, according to Pedro Gonzalez-Diaz at the Institute of Mathematics and Fundamental Physics, CSIC, Madrid. Gonzalez-Diaz calls this the big trip. It is not clear where the trip would take us.
But there doesn’t have to be anything exotic about dark energy. The most conservative theory – what cosmologists call vanilla flavour – suggests that a given volume of vacuum has an inherent fixed energy, often called the cosmological constant. Many experts would bet that this kind of dark energy is what’s causing the expansion to accelerate, and particle physicists even have a partial explanation for it: according to quantum mechanics, countless ephemeral subatomic particles are constantly popping in and out of existence, even in a vacuum, and their energy might add up to something. The only problem is that physicists struggle to explain the observed value of about 1 nanojoule per cubic metre. They can see how the particles’ energy might cancel to zero or add up to a huge value, but not to next to nothing.
Nevertheless, this remains the most popular flavour of dark energy among cosmologists. If dark energy stays constant, our universe will steer carefully between crunch and rip.
Such a middle-of-the-road future may yet have a radical finale. According to quantum mechanics, the total amount of information in the universe should be constant. If space keeps expanding, that might make things uncomfortable, says Tegmark. The universe might eventually become pixellated, with information spread too thinly to support familar physics. Everything would disintegrate, in an event Tegmark has named the big snap. He has his doubts about this, though, suspecting that it only illustrates how we have no understanding of information at the most fundamental level.
If we avoid the big snap, then vanilla dark energy could lead to a long and lonely future. Acceleration will soon steal most of the universe away, as the increasing expansion of space carries other galaxies beyond our view. Their light will no longer reach us, because it is being dragged back over our cosmological horizon like a tortoise on a treadmill. According to Fred Adams of the University of Michigan in Ann Arbor, every other galaxy will have been pulled out of sight in a couple of hundred billion years.
Then we will be all alone, the observable universe reduced to our one elliptical galaxy, and a dingy one at that. There will be only a trace of free gas left to make new stars. Adams has calculated that even that will be used up after about a hundred trillion years, and all nuclear-powered stars will have gone out. A little faint infrared radiation will come from stars called brown dwarfs, which are too small to ignite fusion in their cores. Other stars will be reduced to dense, dead remnants – black holes, neutron stars and ageing white dwarfs, slowly dimming to black. Our sun will become one of these black dwarfs: a single crystal of carbon, like an ultra-dense diamond, with a surface cool enough to touch.
Occasional flares will lift the gloom, when brown dwarfs collide to form a new star, or a black hole shreds a stellar carcass. Once in a trillion years, two relatively heavy black dwarfs will collide and explode as a supernova.
Every now and again, a star will be thrown out of the galaxy after a close encounter with another star. The whole galaxy will dissipate in about a hundred quintillion (1020) years.
Now our observable universe is reduced to a diaspora of dead stars, loosely centred on a massive black hole surrounded by a cloud of dark matter. If there is a remnant of Earth, then for a while it might trail after the black dwarf that was once our sun. But the system will slowly lose angular momentum by emitting gravitational waves, and Earth’s cinder will eventually spiral in to hit the sun’s.
Meanwhile, dark energy will still be at work. Each star will see all its old companions disappear over the horizon one by one. Our black dwarf will be in a universe of its own.
After that, it gets a lot more speculative, but here’s the best guess. Particle physicists suspect that protons are unstable and probably only last between 1033 and 1045 years. As protons decay into their constituent quarks, all the black dwarfs, neutron stars and planets will crumble away, leaving behind nothing but loose photons, neutrinos, electrons and positrons. Even black holes eventually evaporate, by a process called Hawking radiation, although that takes even longer – more than 1086 years for our central black hole.
And then? Dark energy continues working, even on these ashes. One day, every single particle will find itself alone inside its own horizon.
Aside from crunches, rips, trips, snaps and this lonely death, there is another possibility, a path almost parallel to that of the cosmological constant, but fractionally less bleak. In the cosmologists’ models, some kinds of dark energy gradually fade in strength, but never become negative. Among them are defects in space-time that might be left over from the big bang, and a kind of energy field called quintessence. Just like the cosmological constant, these flavours would give us a chilly future where no stars shine and all solid bodies eventually dissipate into a cloud of fundamental particles. However, the acceleration will eventually tail off, so it won’t isolate every particle within its own horizon. Particles could still interact, albeit at a glacial rate, and some kind of chilly life might just be able to cling to existence. Cold comfort, perhaps.
To find out which of these paths we will take, astronomers are examining the nature of dark energy. If they can pin down how space has expanded in the past, and learn what dark energy is really made of, we should have a clue to the future.
The favoured oracles are distant stellar explosions known as type Ia supernovae. These supernovae are all of about the same power, so measuring both their apparent brightness and their distance tells us how much space has expanded since they went off. Astronomers are gathering more and more observations from the ground, and a proposed space telescope called WFIRST could spot thousands of type Ia supernovae.
To complement these observations, other astronomers are using galaxies to peer into dark energy’s past. Because dark energy counteracts matter’s tendency to clump together, its strength will have affected the number of galaxy clusters that formed at different stages of cosmic history. The best hope of tracing enough ancient clusters to pin down this history is an instrument called the Large Synoptic Survey Telescope, which could be running by 2021.
The results could tell us what the future holds. It may seem like a poor set of options – to be crushed, ripped apart or evaporated away. But in fact none of these scenarios need be the uttermost end. The universe, and perhaps even life, could survive any one of them.
In a big crunch, everything will be squashed into a super-hot, super-dense sea of radiation. It is certainly not going to be healthy for humans, but nobody knows what physics does when stuff gets that hot, so it’s hard to predict what would happen to the universe itself. ‘Reexpansion is a possibility,’ says Linde. Since as far back as the 1930s, physicists have played with this idea, and if the universe can bounce, then maybe our own big bang was preceded by a crunch. It could happen again and again, big crunch leading to big bang and so on. A theory called loop quantum gravity actually predicts that a contracting space-time should bounce back.
It is an enticing idea, but there’s a catch. Oscillating universes are vulnerable to a fatal disease: a plague of black holes. Holes survive the crunch, and in each cycle they grow. ‘They keep getting bigger and bigger till they swallow the whole universe,’ says Katherine Freese of the University of Michigan at Ann Arbor. But she may have a cure. With Matthew Brown, now at Lincoln Laboratory, Kwajalein, in the Marshall Islands, and William Kinney at the University of Buffalo, New York, Freese has concocted a new kind of oscillating universe that is immune to black hole disease.
Oddly, the prescription is a big rip. A dose of phantom dark energy tears everything apart – even black holes, effectively making them boil away. The cure may sound worse than the disease, but in fact this big rip can be repaired. This crazy-sounding idea is based on a respectable speculation, the ‘braneworld’ model, in which our universe of three space and one time dimensions is like a membrane (or ‘brane’) floating in higher-dimensional space.
In Freese’s model, when phantom energy skyrockets to create the rip, it disturbs fields in the higher dimensions outside our ‘brane’. They then transform the phantom energy, turning it negative and making our universe start to recollapse. Although all stars, planets and other structures from before the rip are gone, new objects might form during the collapse. If astronomers are among them, they will look back in time and discover a kind of big mend.
Then, as the universe reaches a big crunch, the energy density of ordinary matter and radiation soars. Fields in the higher dimension react again, making the contraction bounce back to become the expansion of a new big bang. All this may be rather contrived, but at least it shows there is a possibility that neither crunch nor rip need be the end of everything.
Freese suspects that her model universe would eventually run out of steam and stop bouncing. In contrast, the ‘ekpyrotic’ universe (the name derives from the Greek word for conflagration) devised by Paul Steinhardt of Princeton University and Neil Turok, now at the Perimeter Institute in Waterloo, Canada, is supposed to be eternal. It’s another braneworld model, with the twist that our brane is not the only one. Just a fraction of a millimetre away along the fifth dimension there is another universe. ‘They can collide from time to time like a pair of cymbals,’ says Turok. When they do their kinetic energy turns into a blast of radiation that we call a big bang.
Critics point out that when the branes collide, everything becomes infinitely dense, so the equations break down and the theory doesn’t make much sense. Turok and his collaborators have now published a paper using M-theory – string theory’s big brother – to show that this doesn’t happen, but their idea remains controversial.
So what are your chances of surviving the cymbal-crashing big splat? Well, all the particles in any object would briefly become massless and fly apart at the speed of light, so you’d get rather scrambled, but it is possible that life could survive. ‘We would have to figure out how to preserve all our memories and information in the form of radiation,’ says Turok. ‘If you could imagine making a computer out of light, you could transmit it through the big bang and recover it on the other side.’
There might be a similar escape route through Freese’s rips and crunches – at a fundamental level information is preserved, so conceivably there is a way to encode ourselves into the next cycle of creation. Even in the long, slow decline of a constant dark energy there’s a chance the universe could reinvent itself (see box ‘Quantum resurrection’). Who says you can’t live for ever?
It’s time for a confession. All of these forecasts are only local: they apply to the bit of our universe that lies within our cosmological horizon. But it is quite possible that the universe is truly infinite. Far beyond the horizon, conditions may be very different. Even the constants of physics may be different, and perhaps some of those regions may be more durable. In some models an infinite cosmos is constantly spawning new big bangs.
None of this can affect us, or have any bearing on our future – unless, perhaps, we somehow learn to manipulate wormholes in space-time and tunnel to freedom, moving to a fresh region of the cosmos whenever the old one gets tired. But even if we can’t escape our local universe, at least it might be reassuring to think that the cosmos itself is immortal.
Even if we face a future in which the cosmological constant reduces us all to a set of isolated particles, there is some hope. Quantum mechanics tells us that there are always fluctuations in any system. Energy fields waver at random, and particles can appear out of the vacuum. Large fluctuations are very rare, and you’d have to wait an extraordinary length of time for something big to appear – a whole atom or molecule, say.
But if our future is infinite, time is not a problem. Eventually, anything could spontaneously pop into existence. Most of these things will be senseless messes, but a vanishingly small proportion will be people, planets, galaxies, and five-mile-long models of your left arm made from gold. ‘In an infinite amount of time, I will reappear. A crazy thought, but true,’ says Katherine Freese of the University of Michigan at Ann Arbor.
How about a whole new universe? Sean Carroll, now at the California Institute of Technology in Pasadena, thinks that random fluctuations could spark a new big bang. He’s even worked out how long we might have to wait for it, something in the region of 101056 years, or a 1 followed by 1056 zeros.
This dwarfs all the timescales we have met so far – it’s even impossible to write down in conventional longhand notation. It is hard to imagine how any kind of life could survive long enough to take advantage of the new universe. Unless, perhaps, it can find a technology that will trigger the new big bang, restarting the cycle of cosmic life and death.