Erhalten Sie Zugang zu diesem und mehr als 300000 Büchern ab EUR 5,99 monatlich.
- Herausgeber: Icon Books
- Kategorie: Wissenschaft und neue Technologien
- Serie: Hot Science
- Sprache: Englisch
'Clear and compact ... It's hard to fault as a brief, easily digestible introduction to some of the biggest questions in the Universe' Giles Sparrow, BBC Four's The Sky at Night, Best astronomy and space books of 2019: 5/5 All the matter and light we can see in the universe makes up a trivial 5 per cent of everything. The rest is hidden. This could be the biggest puzzle that science has ever faced. Since the 1970s, astronomers have been aware that galaxies have far too little matter in them to account for the way they spin around: they should fly apart, but something concealed holds them together. That 'something' is dark matter - invisible material in five times the quantity of the familiar stuff of stars and planets. By the 1990s we also knew that the expansion of the universe was accelerating. Something, named dark energy, is pushing it to expand faster and faster. Across the universe, this requires enough energy that the equivalent mass would be nearly fourteen times greater than all the visible material in existence. Brian Clegg explains this major conundrum in modern science and looks at how scientists are beginning to find solutions to it.
Sie lesen das E-Book in den Legimi-Apps auf:
Seitenzahl: 222
Veröffentlichungsjahr: 2019
Das E-Book (TTS) können Sie hören im Abo „Legimi Premium” in Legimi-Apps auf:
Ähnliche
DARK MATTER & DARK ENERGY
The Hidden 95% of the Universe
BRIAN CLEGG
For Gillian, Rebecca and Chelsea
ACKNOWLEDGEMENTS
My thanks to the team at Icon Books involved in producing this series, notably Duncan Heath, Robert Sharman and Andrew Furlow. Particular thanks to the late Sir Patrick Moore for being my first inspiration to take an interest in astronomy and my lecturers at the University of Cambridge for making cosmology one of the most inspiring aspects of my degree course.
CONTENTS
1
THINGS AIN’T WHAT THEY SEEM TO BE
Hidden depths
The universe is a big place: phenomenally big by the scale of anything we can directly experience. To be honest, we don’t actually know how big it is, though the part we can see is around 91 billion light years across. Given that a light year (the distance light travels in a year) is around 9.46 trillion kilometres (5.9 trillion miles)*, that’s a fair distance. And as the universe contains many billions of galaxies, the majority of which hold billions of stars, there is a whole lot of stuff out there. Yet in the twentieth century, two challenges to our understanding of the nature of the universe have meant that what we once thought was everything appears to be only around 5 per cent of reality.
Once, our picture of what made up the universe was simple. Ancient Greek philosopher Aristotle made use of an existing theory of four elements – earth, water, air and fire – and added a fifth, the quintessence or aether, which he thought made up the unchanging heavens. As astronomy and science advanced, it became clear that Aristotle’s model was flawed. By the nineteenth century, it was possible to detect the chemical elements that existed in the stars – and they proved to be the same as those that were found on Earth. By the twentieth century, the five elements had been replaced by around 94 natural elements of the periodic table, each made up of a very small number of fundamental particles: protons, neutrons and electrons.
Although later in the twentieth century, those protons and neutrons would be discovered to have smaller components, this broad picture of everything being made of a handful of simple building blocks held. Yet a series of events was to shatter this simplistic picture. If science has one commandment, it’s: ‘Things are more complicated than we thought.’ And the idea that all that existed in the universe could be made up from a few particles of matter, light, and four forces† would not stand the test of time. Gradually, oddities began to be uncovered.
Science is frequently misunderstood as being about the collection of facts. While fact-collecting certainly happens, it’s not really the core of the discipline. As American biologist Stuart Firestein pointed out in his book Ignorance, it’s not what we know that’s important to science: ‘Working scientists don’t get bogged down in the factual swamp because they don’t care all that much for facts. It’s not that they discount or ignore them, but rather that they don’t see them as an end in themselves. They don’t stop at the facts; they begin there, right beyond the facts, where the facts run out.’
And the facts of what the universe was made of had begun to run out by 1933 for a Swiss astronomer named Fritz Zwicky.
Zwicky’s misbehaving galaxies
Zwicky, it is generally agreed, was something of a character. Born in Varna, Bulgaria in 1898, son of an influential businessman and politician of Swiss extraction, he was sent to live with his extended family in Switzerland when he was six. He studied maths and physics at Einstein’s alma mater, the Swiss Federal Polytechnic (Eidgenössische Technische Hochschule) in Zurich. Although he remained a Swiss citizen, he spent most of his working life at the California Institute of Technology, where he was based from 1925.
Like his younger counterpart, English astrophysicist Fred Hoyle, Zwicky was known for the richness of his imagination, producing many ideas in astrophysics and cosmology. Inevitably some of these concepts were little more than speculation: it went with the territory. In fact, it was common in physics circles even as late as the 1970s to comment that ‘There’s speculation, then there’s more speculation, then there’s cosmology.’ But even by cosmological standards, some of Zwicky’s ideas were outlandish.
Also like Hoyle, Zwicky’s outstanding imagination did not stop him having impressive hits. Along with German astronomer Walter Baade, he was the first to give serious consideration to the concept of a neutron star – a star that had collapsed to become an incredibly dense collection of neutrons.‡ He coined the term ‘supernova’ for the explosion resulting in such a star forming, and discovered many supernova§ remains.
Another significant contribution by Zwicky originated in Einstein’s general theory of relativity. This theory describes the interaction between matter and spacetime (see page 92) – matter distorts the spacetime near it, producing the effects we describe as gravity. Inherent in general relativity is the idea that massive objects cause rays of light to bend, as the space the light passes through is warped by the matter. As American physicist John Wheeler put it, ‘Spacetime tells matter how to move; matter tells spacetime how to curve.’ Zwicky realised that this effect was similar to that produced by an ancient optical device – the lens.
Lenses (given the Latin name of a lentil because they are similarly shaped) bend the path of light by different amounts, depending on the thickness of the glass the light hits. The circular shape modifies the light’s path by an increasing amount as we get further from the centre, because the glass is at a more extreme angle to the light, meaning that the lens collects together rays of light hitting it at various points and focuses them.
Thinking about the way a lens worked, Zwicky realised that an extremely massive object such as a galaxy could have a similar effect on passing light. If we imagine light coming from a distant object behind a galaxy, some of the light would attempt to pass around the edge of the galaxy. But the huge mass of the galaxy would bend the light beams inwards from all sides, focusing the light a great distance ahead of the galaxy. If we were positioned appropriately, and the image was cast in such a way that it wasn’t washed out by the light from the galaxy, this ‘gravitational lensing’ would mean that we could see a very distant object by using the intervening galaxy as if it were the lens in a vast telescope.
Light from a distant object is focused by an intervening galaxy acting as a gravitational lens.
Adapted from an image released by ESA/ATG medialab
Gravitational lensing involves something we can see – a galaxy – having a gravitational effect on passing light. But Zwicky’s greatest discovery would involve a gravitational effect that appeared to come from an invisible source. He had been studying a collection of galaxies known as the Coma Cluster. Galaxies are vast bodies – our own Milky Way, for example, a fairly average large galaxy, is over 150,000 light years across. Containing billions of stars each, galaxies have a huge gravitational influence on their surroundings and as a result readily form clusters with other galaxies, held together by gravity.
The Coma Cluster is located about 320 million light years away from us and contains over 1,000 galaxies – as the nearest neighbouring cluster to our local cluster, the one occupied by the Milky Way (the Virgo Supercluster), it has inevitably been of great interest to astronomers. Yet when Zwicky started to analyse the behaviour of the cluster in 1933, he found something odd. It should not have held together.
On the whole, things in the universe spin around. We’re familiar with this being the case in our own solar system. The Earth rotates on its axis once a day and orbits the (rotating) Sun once a year, as do the other planets, each with their own distinct period. Planets, moons, stars, solar systems, galaxies, galactic clusters all spin around. This is a result of the way that they formed. These structures were produced from clouds of gas and dust, pulled together by the force of gravity. If those clouds were perfectly symmetrically dispersed through space, then they could collapse without developing a spin. But in reality, it is far more likely that there will be more matter on one side than the other. As the matter is attracted inwards, the result of this imbalance is that the whole collection of stuff begins to rotate.
It’s no surprise, then, that the Coma Cluster rotates. Zwicky combined the speed of the cluster’s rotation with an approximation of the amount of matter in the cluster – and got a shock. It seemed that the cluster was spinning so quickly that it should fly apart, like a poorly placed chunk of clay on a fast-moving potter’s wheel. Gravity can only keep bodies in orbit at the right speed. If an orbiting body travels too fast, it will exceed the ‘escape velocity’ of the system and fly away. And according to Zwicky’s calculations, the Coma Cluster was rotating not just a little too fast but many times too quickly.
Zwicky estimated that the cluster should have needed 400 times more mass to remain stable. (Since Zwicky’s time, this figure has been reduced, but the cluster still rotates far too quickly for the assumed amount of matter present.) He decided that this could only be caused by large amounts of matter in the cluster that could not be detected. He called this unknown material dunkle Materie in German, which translated as ‘dark matter’.
It might seem odd that such an important result was largely ignored at the time. However, Zwicky’s reputation for inventiveness had the downside that, while his ideas were usually noted, they weren’t always taken further. It was probably assumed that the effect was considerably smaller than Zwicky had calculated. Bear in mind that it required a calculation of the amount of matter in a distant collection of at least a thousand galaxies, each of which contained vast numbers of stars. There was a lot of approximation (scientific language for educated guesswork) going on.
It’s also the case that Zwicky’s idea of dark matter did not sound as exciting as it does today. Any dark matter was just that – perfectly ordinary matter that happened to be dark. It was assumed to be a combination of dust, low-output stars, planets, and more that had not been considered by making use of the observable, light-emitting matter. This wasn’t even a new concept – Scottish physicist William Thomson, Lord Kelvin, had made similar if less dramatic observations on the rotation of the Milky Way in 1904,¶ showing that a considerable amount of the matter in the galaxy was dark, as did other astronomers in the intervening period, particularly the Dutch astronomer Jan Oort in 1932.
Later, though, it would be realised that ordinary matter that did not emit light – even with the addition of the exotic concept of black holes – would simply not provide enough mass to account for this odd behaviour. There was something new and different out there. Far more of it than there was ordinary matter. Dark matter had arrived.
The expansion dilemma
By the 1990s, a second shock echoed through the small world of astrophysicists and cosmologists. It was the culmination of a breakthrough made in 1929. Then, American astronomer Edwin Hubble published data on the red shift of galaxies. We’ll come back to red shift later on, but this is a means to identify the velocity of a light-emitting object. Hubble’s data showed that with a few local exceptions, all galaxies were heading away from our own Milky Way. And the further a galaxy was away, the greater its red shift – the faster it was going. When plotted on a graph, this relationship roughly grew in a straight line, an observation that would be given the name ‘Hubble’s law’. This despite Hubble himself never doing much with the interpretation of his data, being happy simply to collect it.
The data was used to justify the idea that the universe was expanding, a picture we now accept as a commonplace. But there was one thing that wasn’t known: how rapidly that expansion was slowing down. That the rate of expansion should be slowing seemed inevitable, due to the influence of gravity. According to general relativity, the expansion should be countered by the gravitational effects of all the matter in the universe. It seemed unavoidable that there would be a gradual slowing of the expansion of space.
There were two possible outcomes of this braking effect. If the expansion of space was not fast enough, it would eventually be overcome, and space would start to contract, leading to a massive collision nicknamed the big crunch (the opposite of the big bang). If the expansion was too fast for gravity to totally overcome, the rate at which the galaxies moved away from each other would slow down, but would never reverse, leading to a universe that forever thinned out.
Until the 1990s, there was no good way to discover how far away distant galaxies were, to pair up with the red shift information on how fast they were moving. But by then, new techniques based on an understanding of the behaviour of a particular type of supernova were making it possible to combine distance with red shift and get a better picture of how the rate of expansion was slowing. In 1997, two teams raced to achieve sufficient data to quantify this.
Both would reach a conclusion at around the same time, and the outcome was a huge shock. All the evidence was that the rate at which the universe was expanding was not falling with time, but rather it was growing. Something unknown was adding energy to drive the expansion of space, accelerating the rate at which galaxies separated from each other. Without any idea what could be causing this, astrophysicists, taking the term from American cosmologist Michael Turner, termed the phenomenon dark energy. The name tells us nothing about what is involved. It might just as well have been called factor X or unizap.
As more data came in, it became possible to estimate just how much energy was required to cause this acceleration. Locally, the effect is tiny. It requires less than a joule of energy|| for every cubic metre of space to provide such acceleration. But add that up across the whole universe and it is the equivalent of a vast amount of energy. Thanks to Einstein’s familiar equation Emc2 (see page 26), we can equate energy and mass of matter. If we convert the estimate for the required amount of dark energy into mass, there is around fourteen times as much mass/energy in dark energy as there is in all the familiar visible matter in the universe, or around twice as much as there is in ordinary matter and dark matter combined.
Darkness prevails
If the theories of dark matter and dark energy are correct, around 27 per cent of the universe is dark matter and 68 per cent dark energy, leaving only around 5 per cent as everything that we directly observe. This is a big issue. Yet the nature of these phenomena is still under debate. Dark matter may not even exist. The scale of dark energy comes up differently depending on the way that it is measured and is totally at odds with what is otherwise our most accurate physical theory, quantum mechanics. Arguably, this is the most exciting aspect of modern science.
Before we can understand the science behind the study of dark matter and dark energy, we need to fill in some fundamentals about the universe, what makes it up and how it operates. And what better place to start than with one of the oldest disciplines of science – our quest to find out more about the universe without ever leaving the Earth.
* To give a feel for the scale of a light year, to travel a light year you would have to circumnavigate the Earth about 236,500,000 times.
† The four forces are gravity, electromagnetism, plus the strong and weak nuclear forces. The latter two hold together particles in the nucleus and are responsible for particle transmutation, respectively.
‡ Neutrally charged particles found in the nucleus of atoms.
§The supernova features frequently in this book, so I’d like to get one bone of contention out of the way. As ‘nova’ is Latin for ‘new’, and the Latin plural of nova is novae, the plural of supernova is usually given as supernovae. However, supernova is not a Latin word (if it had been, it would have been an adjective, not a noun), and as such I prefer and use throughout the plural ‘supernovas’.
¶ Even the term ‘dark matter’ had already been used by French mathematician Henri Poincaré when referring to Lord Kelvin’s calculation. Poincaré referred to Kelvin’s missing material as matière obscure.
|| Less than the amount of energy required to keep a typical low-power LED light bulb going for ⅕ of a second.
2
EXPLORING THE UNIVERSE
Forget the spaceship
I am a big fan of space exploration – I believe that it is something we need to do as humans, both to satisfy our pioneering spirit and to give ourselves an escape route should the Earth ever become uninhabitable. But to understand the universe that we live in, traditional exploration will never be a practical approach. This book opened with a statement about the size of the universe – but that scale is difficult to grasp. Let’s think for a moment of planning a visit to the nearest star after the Sun, called Proxima Centauri.
That star is around four light years away. This is a tiny distance compared to the Milky Way galaxy’s 150,000-light-year diameter. But it’s still ridiculously far from a human perspective. The fastest a human being has ever travelled with respect to the Earth was on Apollo 10 at 39,896 kilometres per hour. It sounds fast, but that’s only 0.000037 times the speed of light. At that speed it would take over 100,000 years to reach Proxima Centauri.
In some ways, then, it may seem naive to say that the exploration of the universe began on 4 October 1957, when the USSR launched the first artificial satellite, Sputnik 1. Less than 60 centimetres (2 feet) across, this fragile metallic ball sprouting two double antennae was humanity’s first true venture into space. Although primarily launched as a political gesture, the satellite did provide a small amount of scientific data. Sputnik’s 83 kg (183 lb) mass – 51 kg of that was its batteries – made waves totally out of proportion to its capabilities. Over the next 50 years we would see probes reaching the Moon, Mars and the outer solar system, a manned expedition to the Moon and a succession of manned space stations in orbit.
However, despite never venturing out of our near neighbourhood, we shouldn’t underestimate the value of satellites to improving our understanding of the universe. Some of the most impressive space explorers have been direct successors to Sputnik – unmanned satellites carrying instruments that have expanded our knowledge immensely. Innovations such as the Hubble Space Telescope and the COBE and WMAP satellites have been the true explorers of the universe on our behalf. And in doing so, they carry on a tradition of visual exploration that goes back far further than any space flight – back past Galileo’s telescope and Ptolemy’s surveys of the sky to the naked eye observations of the earliest humans.
When it comes to exploring the universe, we can forget spaceships other than those that launch satellites. Light is our primary vehicle of choice. People have explored the universe this way ever since they looked up and wondered at the beauty of the stars. With the naked eye it is possible to see the galaxy M31 in the constellation Andromeda. If the sky is dark enough, the Andromeda galaxy appears as a faint smudgy spot in the sky, on the side of the constellation nearest the W of the adjacent constellation Cassiopeia. Telescopes show that this smudge is, in reality, a massive spiral galaxy, but even the unaided eye enabled early explorers of the skies to see it across 2.5 million light years of space. Compare this with the furthest distance man has travelled at the time of writing – 375,000 kilometres to the Moon. Forget light years – that is 1.25 light seconds.
What’s more, unless we come up with the kind of warp technology familiar from Star Trek, light (or equivalents such as gravitational waves and neutrinos, which move at or near the speed of light) will remain our principal means of exploring the universe. Light is the fastest thing in existence. If we could travel at half of light speed (something inconceivable with current technology), it would still take 5 million years to reach the Andromeda galaxy. But we can see Andromeda because light has already made the journey to us – we don’t need any travel time. We will never explore most of the universe in person, but light allows us to see across immense distances.
We have come a long way since Galileo first used his telescope to discover astronomical bodies that are not visible to the naked eye. We now use the whole spectrum of electromagnetic radiation – radio, microwaves, infra-red, X-rays and more – of which visible light is just a tiny segment. And with these remarkable engines of visual exploration we can venture out to experience the strange cast of characters that populate the universe – black holes and dark matter, supernovas and quasars. This is exploration like no other.
First imaginings
Cosmology, as we’ve seen, is the science of the universe as a unified object, combined with the study of the laws that govern that whole. This definition of cosmology assumes, of course, that we know what ‘the universe’ is. The original Latin from which the word universe is derived means ‘one turn’, which doesn’t help much. In practice, though, what we are dealing with is clear. The universe is all that physically exists, taking in everything from the smallest particle all the way up to the biggest galaxy. It encompasses all matter, all energy, collected together as a whole as if this assemblage were an entity in its own right. That’s an impressive concept, and it is natural to ask questions about it.
From the earliest times, creation myths have been written to explain where that ‘everything’ came from. Humans are born storytellers, and creation myths are storytelling, not science. It’s important to understand, though, that by calling these stories ‘myths’ we aren’t insulting them, or those who consider them to be sacred. A myth is a story with a point. It is a mechanism to give information about a deep question, like ‘Why are we here?’ or ‘Where did everything come from?’. A myth is not history – it is a way to help understand today’s reality, through a story linking us to the past.
For the authors of the early creation myths, the universe was the Earth and the heavens. Land, sea and sky accounted for all space. Yes, there were a few oddities up there like the Sun, Moon and stars – but these were the inhabitants of sky, just as animals and people were the inhabitants of land. To a modern understanding, the logic of creation myths can seem confusing. But the vast majority of early creation myths have one thing in common – a creator. The answer to one of the biggest questions about the universe, how it came into being, was almost universally that someone made it.
This answer was neither irrational nor stupid. Thousands of years after these myths were formulated, a Victorian clergyman called William Paley used the same argument to explain how living creatures were formed. If you came across a watch lying on a beach, Paley said, you would not think it had naturally and randomly occurred. It was too complicated and too functional in its structure. Instead, you would assume that a watchmaker created it. Similarly, faced with the complex vastness of the Earth and heavens, the obvious response was: ‘It could only be like that if someone made it.’
Like every other civilisation that predated them, the ancient Greeks had their creation myths. But they were the first, in the story of our understanding of the universe, to go further. It has been suggested that the ancient Greek approach might have been a reflection of their loose federation of city states with no central authority, leading to a more questioning philosophy than in a society with a rigid hierarchy. Rather than be satisfied by saying that the universe works because the gods make it work, from the sixth century BC, the ancient Greeks began to look for practical mechanisms that such creators could have employed.
A philosopher’s universe
The first of the true Greek philosophers is generally held to be Thales of Miletus, born around 624 BC, who advocated looking for natural, rather than supernatural, causes for what was observed. Probably the first ‘scientific’ cosmology – a self-consistent picture of the universe and its origins that was built on physical forces and structures – came from one of Thales’ pupils, Anaximander. Born in Miletus in Anatolia (now part of Turkey) in the first half of the sixth century BC, Anaximander did not challenge the existence of the gods; his view of the universe, however, was based on simple observation.
Unlike many creation myths that portrayed the universe emerging from water, Anaximander preferred a beginning where the universe arose from chaos in a sea of fire. This had one significant advantage – it allowed him to give a justification for a familiar natural phenomenon. Anaximander wanted to explain the lights in the sky – the Sun, Moon and stars. He reckoned that the primeval sea of fire was still out there, but the universe was protected from the flames by a huge shell (strangely, this was cylindrical rather than spherical). The shell had holes in it, and through these holes firelight escaped to provide the glow of the heavenly bodies and the heat of the Sun.
