Astronomers are embarrassed and perplexed. They study the cosmos with ever more powerful telescopes, but rather than building up a more complete inventory of what the cosmos contains, they confront ever more insistent evidence for material that is invisible â neither emitting light nor absorbing it. Most of the stuff that governs the large-scale universe, and sculpts the structures in it, is very different from the atoms that the shining stars and nebulae are made of.
But astronomers shouldnât have been surprised at this realization. There are many analogies closer to home. For instance, NASAâs spacecraft have given us beautiful images of the Earth at night, reveling the bright lights of the worldâs cities, fires from gas and oil wells in the Middle East, and scattered light from small settlements. But if aliens, viewing Earth from afar, had just these night-time images, theyâd have an incomplete and distorted view of our planet.
Likewise, the ancients, looking up at the sky, would have found it hard to believe that huge cumulus clouds are actually far lighter and less substantial than the invisible air that sustains them.
Astronomers viewing the wider cosmos realize theyâre in the same predicament. It seems that galaxies, and even entire clusters of galaxies, are held together by the gravitational pull of about five times more material than we actually see.
Many lines of evidence support this conclusion. I’ll mention just two:
The first comes from disc galaxies, like our own Milky Way or Andromeda. Stars and gas circle around the central hub of such galaxies, at a speed such that centrifugal forces balance the gravitational pull towards the centre. This is analogous — though on a far larger scale — to the way the Sun’s gravitational pull holds the planets in their orbits. If the stars and gas in the outermost parts were feeling the gravity of the material we see, then the further out they were, the slower they would be moving — for the same reason that Pluto is moving more slowly around the Sun than the Earth is. But that’s not what is found: stars and gas clouds at different distances from the galaxy all orbit at more or less the same speed. If, in our own Solar System, Pluto were moving as fast as the Earth, we’d have to conclude that there was a shell of material outside the Earth’s orbit but within Pluto’s. Likewise, the high speed of this outlying material tells us that there’s more to galaxies than meets the eye. The entire luminous galaxy — an assemblage of stars and glowing gas — must be embedded in a dark halo, several times heavier and more extensive.
And there’s pervasive dark matter on still larger scales. The Swiss-American astronomer Fritz Zwicky argued in the 1930s that the galaxies in a cluster would disperse unless they were restrained by the gravitational pull of dark matter. He proposed that gravitational lensing — the bending by gravity of light rays from objects behind it — could reveal dark matter’s presence. This technique has now, many decades later, borne fruit, and it is sad that Zwicky didn’t live long enough to see images like the one below.
It depicts a big cluster of galaxies about a billion light years away. The image also shows numerous faint streaks and arcs: each is a remote galaxy, several times farther away than the cluster itself, whose image is, as it were, viewed through a distorting lens. Just as a pattern on background wallpaper looks distorted when viewed through a curved sheet of glass, the gravity of the cluster of galaxies deflects the light rays passing through it. The visible galaxies and gas in the cluster contribute far less light-bending than is needed to produce these distorted images — evidence that clusters, as well as in individual galaxies, contain much more mass than we see.
The main case for dark matter rests on applying Newton’s law of gravity on scales millions or even billions of times larger than our Solar System, which is of course the only place where it has been reliably tested. It is proper to be cautious about such a huge extrapolation: indeed, some have suspected that we are indeed being misled, and that gravity grips more strongly at large distances than standard theories predict. In this context the gravitational lensing is important because it corroborates the other evidence, yet it is based on rather different physics — Einstein’s rather than Newton’s.
We shouldnât demur at the prospect that most of the stuff in the universe may be dark: why should everything in the sky shine, any more than everything on Earth does? But what could the dark matter be? The favoured view is that it consists of a swarm of particles, surviving from the ‘big bang,” which have so far escaped detection because they have no electric charge, and because they pass straight through ordinary material with barely any interaction. This hypothesis gains strong support from computer simulations of how galaxies form. Computer simulations based on the so-called “cold dark matter” hypothesis have been very successful in understanding how galaxies evolve and are clustered. Without the dark matter, itâs near-impossible to reconcile the existence of galaxies with what we know about the early universe.
Physicists have theorised about many types of particlea that could have been created in the ultra-hot initial instants after the Big Bang, and could survive until the present. Thousands of these particles could be hitting us every second, but they almost all pass straight through us, and through any laboratory. Sometimes one of them collides with an atom, however, and sensitive experiments might detect the consequent recoil when this happens within (for instance) a lump of silicon. Several groups around the world have taken up this challenge. It requires delicate equipment, cooled down to temperatures near absolute zero, and deployed deep underground to reduce the background signal from cosmic rays and so forth.
One of the experimental difficulties is that other kinds of particles (for instance, decay products from radioactivity in rock) can cause similar signals. But genuine dark matter would have a distinctive signature, which would tell us that it came from our Galaxy and not just from the Earth. The Sun is moving steadily through the swarm of particles that make up our galactic halo. But the Earth is moving around the Sun; in June its speed relative to the galactic halo adds to that of the Sun; six months later it subtracts from it. A genuine signal from dark matter should consequently show this seasonal modulation. There have been claims of such detection. But recent results from a much more sensitive experiment called LUX revealed nothing. This doesnât rule out these particles, but is starting to constrain their properties. Dedicated scientists will have to improve their techniques further, and stay down their tunnels and mineshafts for a few years more before detecting an unambiguous signal. But even then success isn’t assured.
There are other quite different techniques. Dark matter particles in space might occasionally collide with their antiparticles, and produce gamma rays or positrons that could be detected by instruments in space. Here again there is no form evidence. Another possibility is that new types of particles, candidates for dark matter, could be created in accelerators like the LHC in Geneva.
The intellectual stakes are high. Dark matter is the No. 1 problem in astronomy today, and it ranks high as a physics problem too. If we could solve it — and I’m optimistic that we will within the next decade — we would know what our universe was mostly made of; and we would discover, as a bonus, something quite new about the microworld of particles.
Moreover, the dark matter affects the cosmic long-range forecast — how the universe will be expanding tens of billions of years hence. However, even when its gravitational pull is added to that of âluminousâ matter, the total yields only 30 percent of the “critical density”which would be required to overwhelm the kinetic energy and bring the cosmic expansion to an eventual halt. But it would be expected to cause the expansion to gradually slow down.
But in 1998, cosmologists had a big surprise. The expansion of the universe is not slowing down at all, but speeding up. On the cosmic scale gravitational attraction is overwhelmed by a mysterious new force latent in empty space which pushes galaxies away from each other. This force, sometimes called “dark energy,” isnât important within galaxies and clusters â nor for their formation and structure. But this evidence for accelerating cosmic expansion presages a long-range forecast of an even colder and emptier universe. Nearly all the galaxies we can now observe will eventually become so redshifted that they disappear from view â rather like what happens when something falls into a black hole. Only the remnants of our Galaxy, Andromeda and a few of their smaller neighbours will remain in view.
Iâm much more pessimistic at our prospects for tackling the riddle of dark energy. This will have to await new breakthroughs in understanding the bedrock nature of space itself. A generic feature of most theories that aim to unify gravity and the quantum principle is that space has a “grainy” structure and complex texture, but on a scale a trillion trillion times smaller than atoms â far beyond the capacity of any foreseeable experiments to probe directly.
We’re reconciled to the post-Copernican idea that we don’t occupy a central place in our universe. But now our cosmic status must seemingly be demoted still further. Particle chauvinism has to go: we’re not made of the dominant stuff in the universe. We, the stars, and the visible galaxies are just traces of sediment — almost a seeming afterthought — in the cosmos: entities quite different (and still unknown) controlled the emergence of its present structure, and will determine its eventual fate.
1. How much problematic evidence is needed before it’s appropriate to
jettison or modify well-established models or hypotheses?
2. How much of cosmology should be regarded as an “environmental” rather
than “fundamental” science?
3. What are the odds on developing a theory that will clarify the nature
of “dark energy”?
In the article that introduced this discussion, my focus was on phenomena that are no longer controversial – the existence of dark matter, and of a mysterious repulsive force latent in empty space. This progress is owed to powerful telescopes, on the ground and in space. But as always in science, each advance brings into focus some new questions that couldn’t previously have even been posed. In particular, we don’t know why our universe was “set up” with these particular ingredients – even though we can trace cosmic history back to a hot dense “beginning” nearly 14 billion years ago.
But I’d now like to close by raising two speculative issues. First, how close we are to arriving at a theory that explains these (and other) cosmic mysteries? And second, can we, as humans, expect ever to understand our cosmos and the complexities within it?
We’ve known for a long time that, when confronting the overwhelming mystery of what banged in the Big Bang and why it banged, Einstein’s theory isn’t enough. That’s because it treats space and time as smooth and continuous. We know, however, that no material can be chopped into arbitrarily small pieces: eventually, you get down to discrete atoms. Likewise, space itself has a grainy and “quantised” structure – but on a scale a trillion trillion times smaller.
On the largest scale, we may be even further from grasping the full extent of physical reality. The domain that astronomers call “the universe” – the space, extending more than 10 billion light years around us and containing billions of galaxies, each with billions of stars, billions of planets and maybe billions of biospheres – could be an infinitesimal part of the totality. Indeed, the results of our Big Bang could extend so far that somewhere there are assemblages of atoms in all possible configurations and combinations – including replicas of ourselves. Our universe is just one island in a vast cosmic archipelago. On the grandest scale of all, space may have an intricate structure. This scale could be so vast that our purview would be restricted to a tiny fragment: we wouldn’t be directly aware of the big picture, any more than a plankton whose “universe” is a spoonful of water is aware of the entire Earth.
To prove or refute these conjectures – to turn them into firm science – we need to achieve a unified understanding of the very large and the very small – of the cosmos and the quantum. Success, if achieved, would triumphantly conclude an intellectual quest that began with Newton, who unified the force that made the apple fall with the force that held the moon and planets in their orbits.
Such a theory would bring Big Bangs and multiverses within the remit of rigorous science. But it wouldn’t signal the end of discovery. Indeed, it would be irrelevant to the 99 per cent of scientists who are neither particle physicists nor cosmologists, and who are challenged by the baffling complexity of our everyday world. It may seem incongruous that scientists can make confident statements about remote galaxies, or about exotic sub-atomic particles, while being baffled about issues closer to hand – diet and disease, for instance. Yet even the smallest insects embody intricate structures that render them far more mysterious than atoms or stars.
Nearly all scientists are “reductionists” in so far as they think that everything, however complicated, obeys the basic equations of physics. But even if we had a hypercomputer that could solve those equations for (say) breaking waves, migrating birds or human brains, an atomic-level explanation wouldn’t yield the enlightenment we really seek. The brain is an assemblage of cells, and a painting is an assemblage of chemical pigment. But in both cases, what’s important and interesting is the pattern and structure – the emergent complexity.
Will scientists ever fathom all nature’s complexities? Perhaps they will. But we should be open to the possibility that we might, far down the line, encounter limits – hit the buffers – because our brains don’t have enough conceptual grasp. it’s amazing that we can comprehend so much of the counter-intuitive cosmos. But there may be some aspects of reality are intrinsically beyond our ability to grasp them. Just as quantum theory is beyond a monkey’s comprehension, so an understanding of some aspects or reality may have to await the emergence of post-humans.
New Big Questions:
1. How close we are to arriving at a theory that explains these (and other) cosmic mysteries?
2. Can we, as humans, expect ever to understand our cosmos and the complexities within it?