Dark Matter and Dark Energy

The Hidden 95% of the Universe

Brian Clegg

• Publisher: Icon Books
• Year: 2019
• ISBN:
  9781785785696
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

'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.

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 ex...