Game of Knowns (Excerpt)

There are Known Knowns, Known Unknowns, and Unknown Unknowns. And then there is Dr Karl Kruszelnicki…



The print edition of Game of Knowns is available in Australia from Pan Macmillan. An ebook edition is currently available worldwide only through iTunes.





For almost a century, astronomers have realised that we have a major problem. It’s a very basic, fundamental and simple problem.

Most of the Universe is “missing”.

We actually know that it’s there. The problem is that we can’t see it. We have many different types of telescopes, covering lots of the Electromagnetic Spectrum—radio, microwave, infra-red, visible light, ultraviolet, X-ray, gamma ray, and so on. But all the “stuff” astronomers find adds up to only about 5 per cent of the mass of the Universe.

What is the Universe Made of?

The latest figures come to us from the Planck Observatory, a Space Telescope launched in 2009.

It tells us that the Universe is about 13.8 billion years old—and that the Universe contains about 4.9 per cent ordinary matter, 26.8 per cent Dark Matter and 68.3 per cent Dark Energy.



So let me give you a sense of where we stand in the Big Picture. Our Universe is dominated by Dark Energy and Dark Matter. You and I are made of stuff that, in percentage terms, is close to a Rounding Error.

Only about 5 per cent of the mass of the Universe is made from “normal” matter. Humans and plants, stars and planets, belly button fluff and peanuts, and the stuff between the stars—all this is made up of regular matter based on atoms, such as protons, neutrons and electrons.

But we are quite confident that Dark Matter is not made from atoms. Furthermore, it’s not made up of Black Holes, nor of stars that have died and no longer shine, nor of planets that have been thrown out of their host solar systems.

Dark Matter is made up of something strange that we currently don’t understand. For example, it doesn’t seem to radiate or interact with any kind of electromagnetic radiation at all. This is very different from stars that emit light, and from humans who both reflect light and absorb it (ask someone with a sunburn). That is why we can’t directly “see” this mysterious Missing Mass of the Universe, Dark Matter.

In a sense, Dark Matter is like the wind. We can’t see the wind directly, but we can see what the wind does. So when we see the leaves on trees fluttering and the branches bending, we know that there’s some wind, even if we can’t see it. In the same way, we can “see” what Dark Matter does.

Dark Matter interacts via gravity, even if it doesn’t interact with electromagnetic radiation.

As a result, Dark Matter has mass, so it “makes” its own gravity. Furthermore, Dark Matter is affected by the gravity from other objects. This attraction goes both ways. Dark Matter pulls on both Regular and Dark Matter. And it can be attracted by the gravity of Regular Matter and other Dark Matter.

Why “Dark”?

Why do we call this mysterious stuff “Dark” Matter?

For the same reason that the early cartographers would inscribe “Here Be Lions” on unexplored areas of the maps they drew. They knew something was there, but had no idea what it was.

In the same way, we are very sure that something is making up a lot of our Universe, but we have no solid evidence for what kind of stuff Dark Matter is.

So the very name “Dark” is a measure of our current ignorance.



The astronomers have suspected the existence of Dark Matter since the 1930s. The clue was the “abnormal” orbiting of galaxies.

There are two types of orbiting. First, there’s the orbiting of galaxies around each other. Second, there’s the orbiting of stars within a galaxy.

Let’s first look at galaxies orbiting around each other. (Actually, a bunch of galaxies in a cluster don’t neatly “orbit” around each other—it’s more buzzing around like angry bees.)

Way back in 1933, the astronomer Fritz Zwicky took a very close look at a cluster (or group) of galaxies called the Coma Cluster. He measured the speeds of the individual galaxies inside this cluster. He found, to his vast surprise, that they were going so fast that based on what his telescopes showed him, they should easily break away from the gravity of this cluster of galaxies.

But the individual galaxies were not breaking away from the others.

Zwicky worked out that to keep the Coma Cluster intact and stable, there had to be at least 10 to 20 times more matter in the Coma Cluster than he could see with his telescopes. This was one of the first hints of the existence of Dark Matter.

Scientific Insult

It was Zwicky who invented the phrase “spherical bastard” to describe people whom he thought were obnoxious. No matter which way you looked at them, they were still a “bastard.”



So what about the second clue, the speeds at which stars orbit inside a galaxy?

In 1973 the astronomer Vera Rubin was one of the first to discover this particular anomaly. In her research, she would pick a galaxy, typically one with about 100 billion stars in it. Then, using a new technology of the time, she measured how fast the stars were orbiting around the centre of that galaxy. She got a real surprise.

It turns out that the orbiting of stars inside a galaxy is nothing like the orbiting of planets inside our Solar System.

In our Solar System, the planets close to the Sun orbit around really rapidly, while the planets further out from the Sun travel much more slowly. So the innermost planet, Mercury, whizzes at around 48 kilometres per second, our Earth travels a little slower at 30 kilometres per second, while Neptune way out on the edge crawls along at around 5.4 kilometres per second.

But that was not what Rubin saw in galaxies. The stars near the bulging core of a galaxy travel around at roughly the same speed as the stars right out on the edge. In our Milky Way, there’s literally and figuratively a whole galaxy of stars—some are close to the central Black Hole, while others are 50,000 light years away from it (way out on the edge). Amazingly, they all travel at roughly the same speed—about 210 to 250 kilometres per second.

There was only one way that the theoretical astronomers could explain how a galaxy could spin like this. The maths told them that the galaxy had to be surrounded by a halo, or spherical ball, of some type of matter. And, because they couldn’t see this mysterious matter, they called it “Dark Matter”.



As a result of its gravity, Dark Matter has another strange property: it seems to be the invisible scaffolding that formed stars and galaxies. That’s right, Dark Matter shaped our current Universe. In fact, it completely controlled the arrangement of our superclusters of galaxies—and the stars and the gas in between just went along for the ride.

After the Big Bang, the Universe was very bright and smooth for about 100 million years. There was lots of Dark Matter back then, probably making up about half of the mass or energy of the Universe.

This Dark Matter had mass, and so it had gravity. It then coalesced under its own gravity, and began to form structures built out of Dark Matter. The gravity of these Dark Matter structures attracted Regular Matter, such as atoms of hydrogen, helium and lithium. These condensed and formed into stars in big clumps of early “proto-galaxies”—which then formed into galaxies.

So Dark Matter set up the original arrangements of matter in the Universe, with the much smaller amounts of Regular Matter just along for the ride.

This pattern persists in today’s Universe.

The Early Days

Back in the Early Days, when the Universe had cooled down enough for atoms and the like to come into existence, the stuff that made up the Universe existed in quite different ratios.

Atoms made up 12 per cent of the early Universe (currently about 5 per cent), Dark Matter made up 63 per cent (currently about 27 per cent), Photons made up 15 per cent and Neutrinos 10 per cent.



The shape of our Milky Way is dominated by the invisible Dark Matter, which makes up about 80 to 90 per cent of the mass of our galaxy (about 10 to 20 times as much as Regular Matter). Dark Matter provides the gravitational “glue” that gives our galaxy its shape.

Dwarf galaxies are dominated by Dark Matter, usually having 100 times as much of it as Regular Matter. Segue 1, a dwarf galaxy that hangs out on the edges of our Milky Way galaxy, is an extreme case—it has about 1000 times as much Dark Matter as Regular Matter. But other structures associated with our Milky Way, such as Globular Clusters of stars, have virtually no Dark Matter.

It seems that Dark Matter is not evenly spread throughout our Milky Way. It also seems that a few galaxies have very little, if any, Dark Matter.

We don’t yet know why.



So what do we know about this Dark Matter?

First, it’s dark because it doesn’t directly interact with visible light, or indeed any electromagnetic radiation. Dark Matter is not burnt-out stars, planets or gas.

Second, it’s definitely not Regular Matter, like the stuff you and I are made of. The astronomers can tell us this from their observations of various galaxies colliding, and of star clusters colliding.

Third, Dark Matter is not antimatter. If it were, we would see very specific and characteristic gamma rays being produced whenever Dark Matter collided with Regular Matter.

Fourth, we know that it’s not Black Holes. Black Holes have a lot of mass crammed into a small volume, so they have a very strong local gravitational field. If they were involved we would expect to see lots of Gravitational Lenses, where gravity bends the light of a distant object. We’re simply not seeing those.

Fifth, astronomers have mapped Dark Matter on a huge scale, analysing the light from 10 million galaxies. These galaxies are typically some six billion light years away. The astronomers analysed this incoming light to see how it was curved or bent—presumably by intervening Dark Matter. After five years of hard work, they mapped an intricate cosmic mesh of intermingled visible galaxies and invisible Dark Matter. This mesh covers many billions of light years.

The Dark Matter seems to be arranged like a giant sponge—with dense and empty regions.

Another way to visualise it is to think of the Universe as a giant web, with long filaments of Dark Matter. And wherever these vast filaments connect with each other, we can usually see giant clusters of galaxies. However, we can’t directly see the long filaments of Dark Matter. But we know that they are there, because the enormous mass of this invisible Dark Matter bends and distorts the light of regular visible galaxies in the background behind them.



There are three main contenders for the title of Dark Matter at the moment, each of them exotic and strange.

The main categories are Hot Dark Matter, Warm Dark Matter and Cold Dark Matter. “Hot” means that the particles that make it up have lots of energy in their velocity, “Warm” that they have less, and “Cold” even less again. At the moment, Cold Dark Matter is very popular, that is, slow-moving exotic particles such as certain kinds of WIMPs or Weakly Interacting Massive Particles (which can also be “warm” or “hot”), but that could change. There are various theoretical reasons why Cold Dark Matter is currently the best choice—and there are very few actual findings that, tantalisingly, can be interpreted as suggesting a candidate.

And if Dark Matter does turn out to be Cold, then this will be a case when the WIMPs won…

Gravitational Lenses

Dark Matter can indirectly interact with light. Anything that has mass automatically has gravity.

This gravity bends light.

So a gravitational field between us and a more distant source of light will bend that light. This is called Gravitational Lensing. Einstein predicted this, but thought that it was purely a theoretical concept. However, it turns out to be an effect that we can see with our telescopes.

This is how Dark Matter can affect light—indirectly. Dark Matter has mass, which means that it has its own gravity. This gravity can then bend the path of any light that happens to pass near the Dark Matter.


Game of Knowns © Dr Karl Kruszelnicki, 2013


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