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End of darkness: The stuff that really rules the cosmos
26 March 2013 by Andrew Pontzen
Dark matter is the Goliath that supposedly dominates our galaxy. But it might already have met its David
YOU’D think Carlos Frenk would be pleased that no one calls him a crackpot any more. He wasn’t always so lucky.
“I would stand up at conferences and have people almost throwing rotten tomatoes at me,” he says.
His offence was to be an ardent advocate of a then controversial idea – that most of the universe’s matter comes as a cold, heavy soup of invisible “dark matter”. Today that is the orthodoxy. 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.
Is it time to change our dark matter order?
A warmer broth
There are alternatives on the menu. If, rather than a cool gazpacho, dark matter were a hot broth of zippier particles, it would lump less readily and so form more diffuse galaxies. In the 1980s, measurements of neutrinos convinced some researchers that the collective mass of these oddball particles, which race around at close to the speed of light, would be enough to explain dark matter. But this mass turned out to be a huge overestimate, and neutrino-based hot dark matter had the opposite problem to cold dark matter: it moved around too fast to settle down into relatively compact structures like galaxies at all.
There is a third way. A couple of years ago, Frenk set his team to work on a “Goldilocks” solution: dark matter that is not too hot, not too cold, but just right (see diagram). To their surprise, they could make a lukewarm variant of dark matter produce the right, wafty dwarf spheroidal galaxies – without ruining the rest of cosmology (Monthly Notices of the Royal Astronomical Society, vol 420, p 2318).
That still has consequences. The leading candidate for a warm dark matter particle is a heavier, more elusive sibling of the neutrino known as a sterile neutrino. The LHC might manufacture sterile neutrinos indirectly in its particle collisions, but their signature is so subtle it wouldn’t necessarily know it had. Our best hope of spotting sterile neutrinos is when normal neutrinos spontaneously turn into them and disappear off a detector’s radar.
So if warm dark matter is the solution, experiments such as DAMA have been looking in the wrong place. “From an experimental point of view it would be very tragic because there’s been a huge investment, particularly in the direct detection of WIMPs,” says Frenk. The idea has consequences for theorists, too: warm dark matter is divorced from the sort of particles predicted by supersymmetry, leaving both ideas considerably weaker.
In the meantime, Jorge Peñarrubia of the University of Edinburgh, UK, and his colleagues have been looking at how dark matter is distributed within nearby dwarf spheroidal galaxies. It seems to be evenly spread across their diameter (The Astrophysical Journal, vol 742, p 20). “This constant density was something we did not expect,” Peñarrubia says. Simulations with any temperature of dark matter – cold, hot or warm – produce dwarf galaxies that are more densely packed towards their centres. This discrepancy between theory and observation repeats itself in slightly larger, more distant galaxies.
Some exotic flavours of dark matter might help out, such as “self-interacting” dark matter, feisty particles that constantly ricochet around and so resist being sculpted into tight central cusps. But we are at a loss as to what hypothetical particle would bounce around just the right amount in dwarf galaxies and not too much in galaxy clusters. To me, this particular soup smells a little fishy.
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.
Peñarrubia thinks it a credible enough picture. “The redistribution of dark matter requires vast amounts of energy, and supernova explosions are the only plausible source,” he says. But he cautions against thinking that supernovae are enough on their own, especially to produce dwarf spheroidal galaxies of the right density. “The small number of stars in these galaxies limits the supernova energy to values that are only just sufficient,” he says. Boylan-Kolchin is also sceptical that jumbling everything up with cosmic explosions solves all the problems. “My personal feeling is that it’s unlikely that these effects can save cold dark matter theory,” he says.
Of course I think he’s being pessimistic – and would point to other, albeit highly tentative, evidence for the idea of rambunctious normal matter kicking dark matter around. It comes from NASA’s gamma-ray satellite telescope Fermi, which has been on the lookout for visible signals given off by dark matter since 2008. That might seem futile; the whole point is that you can’t see dark matter. But even cold dark matter theory suggests you can see indirect signs of the stuff if, for instance, its particles collide and annihilate in a puff of energy and a very visible flash of gamma-ray light.
The Milky Way’s dwarf satellites are again a good place to look. They are close by, and naturally very dim, meaning any gamma-ray photons from them are likely to come from dark-matter annihilations. Officially, we haven’t seen any yet. “The bottom line is that so far we have not detected any signal,” says Fermi team member Andrea Albert of Ohio State University in Columbus.
Last April, however, a researcher from outside the Fermi collaboration, Christoph Weniger of the Max Planck Institute for Physics in Munich, Germany, set astrophysics afizz with the suggestion that Fermi had indeed seen a dark matter signal – not from dwarf galaxies, but from near the heart of the Milky Way itself. His work indicated that a significant excess of gamma rays from that general direction, all with the same energy of around 130 gigaelectronvolts, was buried in publicly released data from the Fermi satellite (Journal of Cosmology and Astroparticle Physics, DOI: 10.1088/1475-7516/2012/08/007).
The signal is weak, and where it is strongest is not at the Milky Way’s centre, where you would expect dark matter to accumulate, but around 1 degree off-centre. This misalignment nourishes a suspicion that it is the product of some obscure miscalibration of the telescope.
Alternatively, normal matter might be buffeting the dark stuff around. That at least is the implication of simulations by Mike Kuhlen from the University of California, Berkeley, last year (arxiv.org/abs/1208.4844). “It surprised me, but you almost immediately see the dark matter gets offset in the simulation when gas and stars are included,” he says.
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.”
This article appeared in print under the headline “Shaken and stirred”
Andrew Pontzen is a theoretical cosmologist at the University of Oxford
Opdatering fra Cern: 5. april 2015:
The Large Hadron Collider (LHC) is entering its second season of operation. Thanks to the work done in the last two years, it will operate at unprecedented energy – almost double that of season 1 – at 6.5 TeV per beam. With 13 TeV proton-proton collisions expected before summer, the LHC experiments will soon be exploring uncharted territory.