In 1990, the spacecraft Voyager 1 turned its cameras around to gaze back upon its origin. The photograph it took shows a large blank field of empty space, streaked with a few thin beams of light. Hovering delicately in the middle of one beam is a pale blue dot, better known to most of us as the planet Earth. What the image pointedly illustrated for many was our inconsequential obscurity, our minute and fragile insignificance in a universe with other things on its mind. A pale blue dot in a vast black ocean. But Voyager didn’t know the half of it…
Fritz Zwicky was a Swiss astronomer working at Caltech when, in 1933, he observed the Coma galaxy cluster behaving in a way that neither Newton nor Einstein would recognise. Zwicky calculated the mass of the cluster by observing its motion and applying the trusty tool of established gravitational theory. The answer he got was somewhat surprising – there appeared to be several hundred times more mass in the cluster than that which was advertising itself through the twinkle of starlight. Zwicky gave this missing mass the rather blunt title of “Dark Matter” (or would have done, had he been speaking English at the time). This curious problem lurked in the background of astrophysics for thirty years, until Vera Rubin observed a similar phenomenon in the behaviour of stars within a galaxy: far from the galactic core, stars were moving far too fast to be bound to the galaxy, unless some other gravitational agent were hiding in the shadows.
On the face of it, there are two routes out of these curious observations. The predictions of gravitational theory are not compatible with the amount of material we can detect in the night sky, so either
a) Orthodox gravitational theory is wrong (or incomplete), or
b) There exist obscure forms of matter that do not emit light (or other radiation).
The most mundane solution might be to choose option (b) and fill the void with ordinary matter in cold dull forms (brown dwarfs, neutron stars, etc). Collectively known as MACHOS (“Massive Compact Halo Objects”), detailed observations place tight limits on the amount of normal matter around, meaning that these objects can’t fulfill the role demanded of them. If dark matter exists, it must be of a form hitherto unknown to science, for there are no candidates in the roster of normal particles that could hide so effectively.
So what of option (a)? It’s a situation, and a solution, we’ve seen before. In the nineteenth century, the peculiar behaviour of the planet Uranus was brilliantly used by Urbain le Verrier to deduce the existence of Neptune (arguably Newton’s finest moment, albeit coming 119 years after his death). The success of that episode encouraged a similar response from le Verrier when the orbit of Mercury was observed to deviate from its predicted path. A new planet – Vulcan – was pronounced to exist, loitering between Mercury and the Sun, hidden in the bright solar glare. If you’re wondering why your primary school teacher never mentioned Vulcan when outlining the Solar System, that’s because Vulcan does not exist. A figment of Victorian science’s imagination, it was rubbed out of existence by Einstein’s development of the General Theory of Relativity, which precisely predicted Mercury’s errant path, without any need for a new planet. The parallels are obvious and exact. In this example, dark matter was not the answer; instead a modification of theory was required, with much subsequent fruit. It should be no surprise that physicists have eagerly pursued this option again – it is, after all, a chance to topple Einstein!
Alas, this programme (which went under the names Modified Newtonian Dynamics and TeVeS), has ultimately not met with the success enjoyed by its rival, and the need for some new form of matter is accepted pretty much across the board. One needs to invoke dark matter to explain galactic dynamics, galaxy formation, gravitational lensing of galaxy clusters, variations in the cosmic microwave background radiation and, famously, the separation of mass and radiation within the Bullet Cluster. The current favoured model explains the data with “Cold Dark Matter”, positing the existence of new unknown particles called Weakly-Interacting Massive Particles, or WIMPS (one essential attribute of the modern physicist is the willingness to invent new ideas and give them silly names). Much research of the last twenty years has explored a theoretical framework known as supersymmetry to find a candidate for the elusive WIMP, and it is fervently hoped by many in the community that firm experimental evidence to guide the way will be found at the LHC. Supersymmetry has been studied for many years for a variety of reasons unrelated to dark matter, but it also happens to have the attractive feature that so-called supersymmetric particles cannot decay solely into normal particles without producing another supersymmetric particle. The implication of this is that the lightest supersymmetric particle must be absolutely stable (since it cannot decay into anything else lighter) – thus producing a prime candidate for dark matter. Various other pursuits in particle physics have also produced enticing possibilities to solve the dark matter conundrum.
While the observational evidence for mooting the existence of dark matter is compelling, the actual experimental detection of such particles has so far eluded the closest probes. Multiple experiments around the world continue to look for signals that would indicate direct or indirect detection, but none has had indisputable success. The tentative success claimed by certain experiments (most notably DAMA) has not been recreated elsewhere. Since dark matter interacts very weakly at best with normal matter, that is a situation which may continue to frustrate us for quite some time.
What is remarkable is that not only does some wholly new component of the universe exist out there, hidden from view, but the observations indicate that it actually exists in far greater numbers than normal matter. There is in fact more than five times as much dark matter in the universe as there is normal matter. We are very much the side salad to the dark cosmic lunch and while we may have seemed puny through the eyes of Voyager’s distant lens, we are even more diminished when compared to the principal constituent of the material world we inhabit. Everyone you have ever known, everything you have ever seen, is constructed from a small handful of particles, which are but bit players upon the stage, overshadowed by the real stars of the show.
As so often happens, though, it is the supporting cast who have the best lines. The evidence suggests that dark matter is thinly and isotropically spread throughout the galaxy, indicating that it does not interact very strongly with itself or anything else. The relatively strong nuclear and electromagnetic interactions of normal matter allow it to collide, lose energy, condense into planets and stars. The strong force produces nuclear fusion, bringing light and life to the stars and planets. Dark matter particles, on the other hand, are strangers to one another, so weakly do they sense any other presence. They continue to swirl around the galaxies as individuals, with eons left on the clock for their slow spiral into inert gravitational lumps. No dark aliens will ever be born from this frigid matter, none to marvel at this world and congratulate themselves on their cosmic pre-eminence.
If they were to do so, they might find themselves quickly disillusioned, for there is yet another ingredient in the inky cosmos which puts even dark matter in the shade. Permeating every cubic inch of creation is an entity unlike anything else. Once a minor constituent of the universe, it has risen to dominance in the last few billion years and will continue to grow in importance until it is all there is. We know enough about the phenomenon to give it the even more enigmatic moniker Dark Energy, but it is in many respects more of a mystery than Dark Matter – and it’s a subject I’ll save for part 2 of this article…