EIGHT years ago, in Berkeley, California, Ben Moore realised that he didn’t know what the Universe was made of. It was a significant moment.
At the time, the Universe was supposed to be largely composed of a slippery substance called cold dark matter. Although no one had ever detected it, there was enough circumstantial evidence to convince astrophysicists that this invisible gunk made up 90 per cent of the Universe. The ordinary matter that stars, people and magazines are made of was thought to be just a tiny proportion of the Universe’s stuff. But Moore had discovered a flaw in the idea.
If cold dark matter behaves as physicists had thought, its gravity would make it collect in galactic cores. But, Moore noticed, it doesn’t. In an effort to explain this revelation, physicists have had to concoct exotic alternatives to cold dark matter. Others have leaped to its defence, casting doubt on the data that Moore used. This debate is getting heated – and with good reason, for the question could hardly be more fundamental. What is the Universe made of?
Advertisement
There is no doubt that something strange is needed to explain why stars and galaxies are moving around so fast. The Milky Way, for example, rotates once every two hundred million years, and although that may not sound dizzying, it ought be enough to tear the whole thing apart. The only force holding our galaxy together is gravity, and gravity is too weak to rein in that rotation… unless there is a lot more matter out there than we can see, matter that exerts the extra gravity needed to glue the galaxy together.
Similarly, the rapid motion of whole galaxies within clusters makes a lot more sense if they are held together by vast clouds of unseen matter. There are some alternative explanations: that our understanding of gravity is flawed, for example (see “Breaking the law”). But dark matter is still most physicists’ favourite theory.
The reason we need such an exotic explanation is that ordinary dark junk – burned out stars, for example – just won’t do, because we already know how much ordinary matter there is. A few minutes after the big bang, protons and neutrons were packed so close that some of them fused together into heavier nuclei, such as helium. If there had been much more of the stuff, it would have been packed even more tightly and fused into heavier elements still. Stars and interstellar gas would contain much more helium, boron and neon, for example, than astronomers see.
So whatever gives us the missing mass, it needs to be some kind of material that won’t undergo nuclear fusion. The most popular candidate is cold dark matter. CDM is a gas of heavy subatomic particles, individually called WIMPs (standing for “weakly interacting massive particles”), which affect ordinary matter and each other by the weak force and gravity. WIMPs don’t feel electromagnetic forces, or the strong nuclear force that binds proton and neutrons together in nuclei, so they are immune to nuclear fusion and effectively “collisionless”, passing straight through each other.
WIMPs are the simplest kind of particle that astrophysicists think can do the job. What’s more, many tentative new theories of particle physics, developed to try to unify the forces of nature, predict the existence of heavy, weakly-interacting particles. So particle physicists agree that WIMP-like particles might indeed exist.
Most of all, WIMPs are favoured because they make able sculptors of the Universe. Computer simulations show their pull does a good job of clumping matter into galaxy-sized lumps, and that these galaxies then congregate into a web of walls, knots and voids that looks a bit like a half-melted honeycomb, and a lot like the large-scale structure of the real Universe. Quite a triumph for a hypothetical material.
But while the CDM lobby were congratulating themselves on having dreamed up a particle that could solve the dark matter problem, Moore was taking a closer look. He noticed that computer simulations showed cold dark matter piling up in the hearts of galaxies. It would mean that if you were to plot the density of dark matter as you travel from one side of the galaxy to the other, you’d get a curve with a sharp point called a cusp. The curve rises faster and faster as you near the centre of the galaxy until you reach a peak right in the middle – a dark matter Matterhorn (see Diagram).
Moore reasoned that there ought to be evidence of these cusps in data on galactic hydrogen gas gathered from radio telescopes. Hydrogen molecules emit a radio signal at a characteristic wavelength of 21 centimetres. But if the gas is moving relative to us, the wavelength we observe is shifted by the Doppler effect. Measuring the Doppler shift tells you how fast the gas is moving; faster gas usually means more gravity, and therefore more matter in the vicinity. A sharp concentration of dark matter in each galaxy should therefore show up in the form of increasingly frenetic hydrogen near the centre.
However, radio observations of many nearby galactic cores revealed that the hydrogen is pretty sluggish, implying that there is a plateau in the density of dark matter, rather than a cusp – more of a Table Mountain than a Matterhorn. The observations seem to contradict the theory of cold dark matter.
After Moore had published his discovery in Nature (vol 370, p 629), some astronomers pointed out a problem: the spatial resolution of radio telescopes is poor. That is, they might have missed any central cusps because their vision is too blurry to see something so small. But then other astronomers began to gather data with optical telescopes that boast much better spatial resolution.
These new telescopes looked mainly at hydrogen and carbon monoxide gas inside small galaxies called dwarfs, where there is relatively little in the way of ordinary luminous matter such as nebulae and stars. Here astronomers can be pretty sure that the mass they trace is mostly dark matter.
Again, spectral lines emitted by the gas showed that it was moving slowly. Where the theory said there should be heaps of cold dark matter, observations said otherwise. “I find the evidence very convincing”, says Jerry Sellwood of Rutgers University in New Jersey.
This crisis has seeded a whole forest of hypotheses about the nature of dark matter (see “The right stuff?”). There is warm dark matter and strongly interacting dark matter. There is annihilating dark matter, whose particles explode on contact, decaying dark matter, which simply falls apart. And there is even fuzzy dark matter made of particles thousands of light years across.
Each of these crazy flavours of dark matter has what it takes to get around the cusp problem: one way or another they avoid congregating densely in galactic centres. And they all generate the right kind of large-scale structure, too. But they all have drawbacks. Each of the solutions invokes some new property of dark matter particles, governed by a parameter that has to be tuned to fit the observations, such as the mass or lifetime of the particle.
Theorists don’t like having to introduce extra free parameters, and many are disarmingly sceptical about their own speculations. Some of them are so put off by the untidiness of their theories that they are inclined to question the experimental data. “There’s no observation that says we definitely have to go beyond conventional cold dark matter,” says Wayne Hu of the Massachusetts Institute of Technology. “It’s still possible that it’s just WIMPs.” Paul Steinhardt of Princeton University in New Jersey agrees: “I cannot claim that CDM – that is, dark matter consisting of cold, collisionless particles – is ruled out.”
Carlos Frenk, at the University of Durham, doesn’t think that there is a crisis at all. He believes that the observations contain systematic errors, such as telescopes aimed just off a galaxy’s centre, that weaken them fatally. “Any mistake would give you the impression that there is no cusp,” he says. Out of 24 dwarf galaxies that Stacey McGaugh of the University of Maryland in College Park has observed, Frenk claims that only three have good enough data to rule out a cusp. “If 10 per cent of your sample are anomalous, should you throw away your theory, or look more carefully at those examples?” He feels that cold dark matter is backed by such strong evidence that we should hold onto it.
But McGaugh is unconvinced. “This apparent controversy is just a good old fashioned case of pounding the square peg into a round hole,” he says. “The data disagree with theory, so the theorists blame the data.” Sellwood agrees: the dark matter theorists are now clutching at straws, he says. “They’re getting pretty fanciful at this point – wheels within wheels within wheels.”
Sellwood and McGaugh are so disillusioned by these efforts that they prefer an entirely different approach, one first suggested about 20 years ago by Mordehai Milgrom, then at Princeton University’s Institute for Advanced Study. Milgrom’s proposal is a modification of Newton’s 300 year-old law of gravity. In the years since Milgrom formulated his revised gravitational law, it has never been proved wrong. Nevertheless, such an arbitrary upheaval of physics remains an unpopular option.
While Milgrom is playing dark matter’s nemesis, Moore seems just plain dispirited. “There have been about 10,000 papers written on this topic at a cost of more than a billion dollars, and we still know almost nothing about the nature of dark matter,” he says. But physicists are not backing off from the issue. Just this week, for example, they are gathering at London’s Royal Society to discuss ways to search for dark matter.
At the moment, nothing is ruled out, but NASA’s James Webb Space Telescope, the successor to the Hubble Space Telescope, could kill many of the newer theories after its launch in 2010.
So could there be a third way forward – one that doesn’t need funny new particles or a radical rethink of gravity? Jeremiah Ostriker of the University of Cambridge thinks an old, discarded suggestion should be picked up and polished. Maybe most of the matter in the Universe is in the form of huge black holes, each more than a million times the mass of our Sun. Some believe these may have been born in the first split second after the big bang. If so, they would spiral in towards the centres of galaxies, congregating in swarms. In the densest part of the swarm, the black holes would take part in a complicated and violent dance that would eventually fling many of them out of the galaxy altogether – putting a ceiling on the swarms’ density and thus solving the cusp problem.
“Now that massive black holes have been found in all galactic nuclei, the possibility seems even more attractive,” Ostriker says. And, of course, black holes have no problem meeting the criteria for dark matter. They’re big and heavy. What’s more, they fulfil the prime requisite of dark matter. They’re as dark as it gets.
The right stuff?
Strong stuff
WIMP may stand for weakly interacting massive particle, but what if the particles of dark matter aren’t so wimpy? It is possible that dark matter particles interact via the strong force. If so, when a gas of these particles is pulled together by gravity, their strong interactions will resist the squeeze – they will bounce off each other and create shock waves of dark matter. Instead of a spike in the density at the centre of galaxies, you would get a smoother bump, which fits the observations.
This strongly interacting dark matter has its drawbacks, however. Those interactions, and the resulting shock waves, tend to make the clumps of dark matter spherical, whereas galaxies are ellipsoidsin general – and that goes for the dark matter haloes surrounding them too. Last month, David Buote at the University of California, Irvine, and his colleagues announced that the dark matter halo around a galaxy called NGC 720 is indeed elliptical.
This rules out the simplest kind of interactions within dark matter, and forces theorists to postulate a more complex kind of interaction whose strength depends on how fast the particles approach each other. Another possibility is that dark matter interacts weakly with normal matter, and only interacts strongly with itself. The best test will be to look at dark matter distributions in more detail. If future observations reveal that the dark matter cores of galaxies are ellipsoids, then strongly interacting dark matter will be ruled out, once and for all.
Warm stuff
When the specialists talk about cold or hot dark matter, what they really mean is matter whose particles are either slow or fast-moving. The WIMPS of cold dark matter are relatively heavy particles, each with a mass many times that of a proton. The trouble with slow-moving dark matter is that it ought to gather together very densely in the centre of galaxies – which doesn’t fit observations. But if the dark particles were hotter and faster, many of them would escape from the galactic core as the clump forms. This would smooth out the cusp in the dark matter density curve, making it too small for astronomers to detect.
Warm dark matter is not to everyone’s taste, however. Dark matter is supposed to help galaxies grow by clumping together soon after the big bang and dragging in gas. But if it is too warm, the clumps would grow too slowly.
Warm dark matter is still a possibility, but its breathing space is getting very narrow as observations improve. If NASA’s James Webb Space Telescope, the successor to the Hubble Space Telescope, reveals clumping in the very early Universe, warm dark matter will bite the dust.
Crumbly stuff
Renyue Cen at Princeton University has come up with the idea of decaying dark matter: particles that fall apart of their own accord without hitting anything, like radioactive atoms. If dark matter behaves like this, then over billions of years it will gradually turn into photons and neutrinos and other particles that will zip straight out of the galaxy, and any central density spike will dissipate.
This is quite easy to test. If dark matter decays, then there would have been a lot more of it in the early Universe than there is now. And because dark matter’s gravity creates structure in the Universe, there would have been a lot more structure on smaller scales – more miniature galaxies, for example. So astronomers should be able to test the theory if they can look at distant galaxies in great enough detail.
If Cen is right, the future is a strange one: as dark matter disappears, its grip on galaxies will weaken. The Milky Way will gradually spread out and disperse, and eventually every star in it will be alone in the Universe. Fortunately, like most astrophysical apocalypses, this one would not happen for several billion years.
Fuzzy stuff
Perhaps the strangest kind of dark matter is that devised by Wayne Hu of MIT. His idea is that dark matter particles are exceedingly light. They would have a mass of 10−22 electronvolts, which would mean that a proton would weigh as much as a thousand billion billion billion of them.
Common sense would say that these particles must be very tiny if they are so light, but they’re not. Their quantum nature defies common sense: Heisenberg’s uncertainty principle dictates that the lighter a particle is, the less well defined is its position. So Hu’s ultralight dark matter particles would be diffuse clouds of existence, with each vanishingly light particle spread throughout a vast volume several thousand light years across. That rules out any possibility of having a dense concentration of dark matter.
Exploding stuff
What if dark matter particles exploded on contact? That would certainly stop the stuff from getting too dense. If enough of it were to collect in a small area, collisions would happen all the time, and most of the dark matter would be destroyed.
The neat thing about this process is that it only takes place in the centre of galaxies, where the density problem arises. Another plus is that it’s a “self-quenching” process: when the density falls, the particles stop annihilating. That means you get roughly the same final density in every galaxy – a prediction borne out by observations.
It seems like a tidy solution, but its inventor, Manoj Kaplinghat at the University of California Davis, is quick to point out its shortcomings. While warm or strongly interacting particles require only relatively modest tweaks to the theory of particle physics, it is more difficult to find a theoretical basis for annihilating dark matter. “It has the most unpalatable particle physics of all dark matter models,” Kaplinghat says.