... Wherever dark stuff accumulates, so the standard story goes, normal matter meekly follows, irresistibly drawn in by its overbearing gravity. This matter forms stars, and then galaxies are born – meagre pricks of light in a domineering dark empire.
But the confidence of such statements now has Frenk worried. "I suddenly realised that young scientists were taking dark matter for granted, and was absolutely scandalised," he says. You can see his point. Experiments that are supposed to conjure up dark matter have so far produced nothing. Searches for its particles streaming through the Earth have thrown up confusing, contradictory results. Models of how the stuff shapes the visible cosmos veer between triumphant confirmation and abysmal contradiction.
As a young theoretical cosmologist myself, I am among dark matter's disciples. To my mind there is just too much in the universe we can't explain without the stuff. But there is perhaps a way out of its worst dilemmas. Dark matter really does exist; we just need to rethink the idea that it holds all the power in our star-spangled cosmos.
It was about a decade ago that my undergraduate physics lecturer casually introduced me to the idea that five-sixths of the matter in the universe is invisible. Dark matter was originally invoked to explain the observation in the 1930s that clusters of galaxies whirl around too fast for the amount of ordinary matter in them. In the 1970s it was also used to explain why galaxies themselves are spinning too fast, as if subject to an extra gravitational tug. Even so, I recall thinking that you might as well base explanations of the cosmos on magic fairy dust.
But experience made me a true believer. The way galaxies and other massive objects bend light vindicates the idea that there is more to the cosmos than meets the eye. Patterns in the cosmic microwave background, the big bang's afterglow, reveal matter in the early universe caught in a finely balanced competition between gravitational contraction and expansive pressures in a way that agrees with dark matter theory in stunning detail. In my own research on how galaxies form, to reproduce anything like the web of galaxies spun across the cosmos we need dark matter just as Frenk and others ordered it: a cold soup of stuff that barely moves at all.
Pleasingly, particle physics supplies a ready recipe for this soup. The theory of supersymmetry is a favoured step beyond our current "standard model" of particles and their interactions. It holds up a mathematical mirror to reality by asserting that every particle so far discovered has a generally heavier partner. Some of these super-partners are weakly interacting massive particles, or WIMPs. These have mass (and so produce and respond to gravity) but do not interact with light (and so can't be seen). The number of WIMPs that should have been created in the big bang coincides tidily with the density of dark matter inferred from cosmological observations – a happy conjunction sometimes known as the WIMP miracle.
But do miracles really happen? No experiment that might have produced supersymmetric particles, not even the Large Hadron Collider at CERN near Geneva, Switzerland, has seen a hint of them so far. The simplest supersymmetric theories have already been ruled out, and more complex versions await their fate when the LHC restarts at a higher energy, probably in 2015. "After that, if they don't find supersymmetric particles within about a year, I think it'll be dead," says Ben Allanach, a particle theorist at the University of Cambridge. "I'll start to work on something else, and I think a lot of other people feel the same way."
That's not the only difficulty. Fiddly experiments looking for the fingerprint of cosmic WIMPs as they stream from space are producing highly confusing results. The DAMA experiment at the Gran Sasso National Laboratory in central Italy has seen a signal that changes on a yearly cycle. That is what we would expect if Earth is moving relative to a placid cold dark sea as it trundles round the sun – but other experiments flatly contradict the finding. Space-based missions such as the PAMELA satellite and the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station have measured excesses of antimatter particles that might be produced when two WIMPs collide – but these don't really fit our expectations. Overall, "there's huge scepticism about the claims of dark matter detections because other experiments rule that out," says Frenk.
Perhaps the most damaging blow, however, is that when we look at the details WIMP-based cold dark matter doesn't seem to be the consummate galaxy sculptor we thought. Last year Michael Boylan-Kolchin, a cosmologist at the University of California, Irvine, was running simulations of standard cold dark matter's effect on the formation of dwarf spheroidals, mini galaxy-ettes that swarm around the Milky Way. Boylan-Kolchin could infer the dark matter content of these dwarf galaxies by watching how stars move around inside them (Monthly Notices of the Royal Astronomical Society, vol 415, p L40). "It didn't seem to make sense: things were more massive and dense in the simulation than the things we see in the real universe," he says.Pontzen goes on to note several other theories of dark matter--hot and luke-warm--which solved some issues, but not all of them.
... So we are at an impasse. Cold dark matter does not quite do all the jobs we ask of it – but then again, nor does anything else.
My own hunch is that, oddly, cold dark matter might be the right stuff after all. The price we must pay is to stop assuming that it is the totalitarian force in the governance of galaxies. Stars generate huge amounts of energy in their lifetimes. When their time is up, they explode in supernovae. Gas spiralling into black holes generates vast amounts of heat. The energy from either of these sources is enough to send enormous quantities of gas swirling violently around inside a galaxy. Dark matter is not immune to these huge gravitational ructions: it begins to move in concert. Simulations I and a number of colleagues have been performing over the past few years suggest that, if the normal gas is shaken enough, it sends dark matter into a real funk, swirling it around like snowflakes in a snow globe (Monthly Notices of the Royal Astronomical Society, vol 421, p 3464).
Dark matter particles could then be cold and supersymmetric again, and simply get hot under the collar when bullied by exuberant normal matter. The increased energy of this protogalactic soup stops it coalescing too densely, so the structure of the Milky Way's dwarf satellites makes sense again. The only remaining puzzle is why direct searches for dark matter have produced such ambiguous results so far. Perhaps cutting-edge particle physics experiments are just hard.Pontzen then describes some experimental and modeling data that suggest that cold dark matter is still correct. However, he ends by noting:
So, all is well with standard cold dark matter, as long as you factor in the effects of normal matter. Not so fast, says Frenk. If supersymmetric particles annihilating each other were the source of the gamma rays, they would be producing them not with one standard energy, but with a spread: the annihilation mechanism generates electrons and positrons, which gradually give up their energy in unpredictable fits and bursts. "The case is absolutely fascinating, but I don't think we've found anything yet," he says.
Things might change in a moment if the many experiments looking for dark matter were to start producing consistent results. But that will take years at best. In the meantime, the discord is music to Frenk's ears. "We don't know whether cold dark matter's right," he says. "If everyone just buys into an idea, things don't progress."Although Pontzen describes the problem with dark matter research and theories--that too much is accepted on faith rather than challenged or supported with empirical data--he ultimately doesn't accept it. He believes that it is just a question of time. Thus, the statement above, that "[p]erhaps cutting-edge particle physics experiments are just hard." However, the issue reminds me of that raised in Lee Smolin's book, The Trouble With Physics. Smolin's book is about how "string theory" has become almost a religion, stifling exploration of possible alternatives. Pontzen's description seems to describe similar thinking as to dark matter--it is the only answer, but have faith and, in time, there will be evidence. There are other theories. It would be a pity if they were abandoned or left unexplored because it didn't fit the consensus.