Delicate might not be the first word that springs to mind when you think of the Milky Way. But when started tinkering with the recipe for our galaxy, she found it surprisingly fragile.
Lisanti, a particle physicist at Princeton University, was simulating what would happen if dark matter – the mysterious stuff thought to account for over 80 per cent of all the matter in the universe – was more exotic than researchers typically assume. She swapped a small fraction of standard dark matter with something more complex. “We thought, we’re only adding 5 per cent, everything will be fine,” she says. “And then we just broke the galaxy.”
There is good reason for such meddling. Since the 1980s, astronomical signs have pointed towards dark matter being a single type of slow-moving particle that doesn’t interact with itself. Particle physicists have gone to great lengths to search for that particle. But decades later, it remains a no-show – perhaps because dark matter isn’t how we have tended to imagine it.
Advertisement
Recently, a series of galactic anomalies has sparked a scramble to explore alternatives. This “complex” dark matter might be as simple as sub-atomic particles that bounce off each other, or as complicated as families of dark particles that form dark atoms, stars and even galaxies. There is a daunting variety of possibilities.
But now, observations of anomalies in our galaxy finally promise to help us narrow down the options. And with new telescopes on the horizon, we could be closing in on the true nature of this most mysterious substance.
Everything we know about the cosmos and how it became what it is today relies on dark matter, an idea that first captured astronomers’ attention in the 1930s. Back then, astronomer Fritz Zwicky showed that there must be more matter than the eye could see holding clusters of galaxies together. Things really firmed up in the 1960s, when Vera Rubin showed individual galaxies were rotating so quickly that they should be flying apart unless there was some form of invisible matter within them acting as gravitational glue. Then, in the 1980s, cosmologist suggested that perhaps dark matter was a sluggish collection of massive, as-yet-undiscovered particles.
Peebles’s version of dark matter has proved remarkably robust – so much so that it is now essential to our understanding of the entire universe. Our best cosmological model of how the universe took the shape we see today, called lamda-CDM, postulates that the cosmos is composed of three components: ordinary matter, a still-mysterious energy called dark energy and cold dark matter. The “cold” in cold dark matter means that it is slow moving and doesn’t interact through any forces other than gravity.
This take on dark matter agrees with observations of the universe on the largest scales, which is why it has stuck around for so long. “With cosmology, [cold dark matter has] held up amazingly well,” says Lisanti. “On those scales, it’s beautiful.”
The problems with cold dark matter
But there are a few niggling problems. The most obvious one is that no signs of particles that fit this description have shown up, even after decades of searching. For a long time, a leading candidate was thought to be a kind of supersymmetric particle, which are hypothetical partners of the particles that make up regular matter. The photino, the supersymmetric partner of the ordinary photon, just so happened to be the perfect cold dark matter candidate. But the Large Hadron Collider, the particle accelerator situated at CERN near Geneva, Switzerland, which was expected to produce supersymmetric particles by the bucket load, came up empty. Not a photino in sight.
More recently, careful examination of the structure of the universe has thrown up fresh reasons to doubt cold dark matter. Models of the cosmos based on this form of dark matter may work well at the largest scales, but on the scale of individual galaxies, something seems awry: strange features are popping up to suggest that cold dark matter isn’t the whole story.
Consider the missing satellites problem. Large clouds of dark matter are thought to be what attracted sufficient normal matter to coalesce and spark star and galaxy formation. As a result, every galaxy is thought to be surrounded and permeated by a sphere of dark matter, known as a halo. But theories of cold dark matter predict that instead of being uniform, every halo should have broken into many smaller sub-halos, each of which would also attract matter to form dwarf galaxies that orbit the central galaxy like satellites.
When it comes to our own galaxy, these sub-halos would contain the 60 or so dwarf galaxies going around the Milky Way. But there should be hundreds of sub-halos, and therefore dwarf galaxies – and we don’t see them.
It is possible that only the largest dark matter halos drew in enough matter to form a satellite galaxy. “That gives you a very reasonable explanation of why you don’t see lots of [satellite] galaxies,” says , a galaxy formation theorist at the University of Texas at Austin.
If this were the case, though, there would still be lots of these sub-halos in our backyard. “We expect them to be really dark because you just can’t form stars in them,” says Boylan-Kolchin. But again, we don’t see them.
There is another way to detect these clumps, and to rescue cold dark matter, and that is through their influence on nearby objects. In light of this, astronomers have been turning their attention to star streams. These were once clusters of stars or small galaxies that have been cannibalised by our own galaxy, leaving them ripped apart and stretched into ribbons. Data from the European Space Agency’s Gaia spacecraft has shown that the . If something were to fly through them, like a dark sub-halo, it would mess them up and we should be able to detect this. “That’s definitely a place where there’s a concrete prediction from cold dark matter,” says Lisanti.
According to the cold dark matter model, dark matter sub-halos of all sizes, right down to Earth-scale masses, should exist. That is a lot of invisible cannonballs to be floating around the Milky Way and messing with star streams. But wait for it… hardly any evidence for such interactions has been found. In 2018, at Princeton and at the Harvard-Smithsonian Center for Astrophysics found that one particular star stream, known as GD-1, has gaps along its length, “It’s super exciting and intriguing, but it’s also still only one,” says Lisanti.
The upshot is that questions about the cold dark matter model aren’t going away. What’s more, just recently, an even more perplexing halo-related issue, known as the core-cusp problem, has cast further doubts.
Based on our understanding of cold dark matter, simulations show halos should get denser towards the centre of the galaxy. “As you get to the centre, there should be more dark matter per unit volume,” says Boylan-Kolchin. Yet, when astronomers look at the way galaxies move, which is influenced by the gravity of dark matter, this isn’t what they see. Instead, the dark matter appears evenly distributed across the halo of our galaxy, especially in the core regions. This is a hint that something more complex is going on.
As a result, some astronomers have turned towards more complicated dark matter models, which contain multiple particles and new forces of nature – under a suite of theories broadly known as complex dark matter.
What is complex dark matter?
We aren’t short of possibilities. Some models include a mixture of cold and warm dark matter components – warm indicating faster-moving particles. Others involve self-interacting, decaying or annihilating dark matter. Weeding out this huge array of complex dark matter models is the hard part.
In cold dark matter scenarios, the particles only interact through gravity, but in many of these new ideas, other interactions are possible. Fast-moving dark matter particles from other parts of the halo could collide with the sluggish particles accumulating in the core, giving them more energy. This could allow them to distribute more freely, creating the kind of constant-density cores that we see.
To this end, Lisanti and her collaborators have been tweaking the recipe for galaxy formation, running computer simulations using our knowledge of how the universe evolved but modifying the behaviour of dark matter to see what might play out. And they aren’t the only ones. at the Massachusetts Institute of Technology and at Rutgers University in New Jersey are doing similar work, testing the many possible combinations of dark matter particles and forces to find the “Goldilocks” recipe that produces galaxies like those we observe.
The trouble is that there are already so many theoretical suggestions for complex dark matter that – even with multiple teams working on it – completing simulations on them all would be impractical. “Each one of these simulations takes three or four months to run,” says Lisanti. Instead, what she and her colleagues do is take characteristics or properties – such as whether the dark matter interacts with itself, decays or if its constituent particles can inhabit the same physical space as one another – and run models of particles with the special attribute they want to test, just to see if it changes dark matter’s behaviour.
It was when Lisanti and her colleagues were experimenting in this way that they found transforming just 5 per cent of cold dark matter into more complicated varieties made it impossible to form the Milky Way. “I think that was an interesting lesson,” she says. “We need to be very careful when we think about new models. You don’t have to add very much of [other forms of dark matter] to your models to really mess up the astrophysics.”
In this case, the rich new array of behaviours that come from different complex dark matter models is both a blessing and a curse. When Lisanti and her colleagues allowed their particles to lose energy, they found a whole new world of possibilities. “[The dark matter] would collapse down and then we don’t really know yet what all the structures are that it can form when it collapses,” says Lisanti.
This is where particle physicists come in. David Curtin, a theorist at the University of Toronto, Canada, has been developing another form of supersymmetry that posits switching all matter into a twin set of particles, each with their own set of forces, rather than keeping the forces the same. This would mean twin particles can’t interact with their conventional cousins through forces other than gravity – making them dark matter – but could interact with each other. “You’ve just predicted a complex dark sector,” says Curtin.
Atomic dark matter
What this means is a hypothetical complex realm in which various dark matter particles are perhaps capable of accumulating into dark atoms, and much more. Curtin calls it atomic dark matter. “Atomic dark matter [is] much like our matter; it cools, it collapses, it forms discs, it forms stars – dark stars,” he says. Since this brand of dark matter would behave in roughly the same way as normal matter, other than the fact we can’t see it, that also means dark stars formed by dark atoms could end up becoming a dark galaxy invisibly superimposed on our own Milky Way.
It is a strange concept, but an invisible galaxy would have a unique fingerprint that we could see with future telescopes. If Curtin is right, dark stars will have formed a disc-like galaxy structure that will gravitationally bend light in a process called microlensing. The effect of this is to cause stars in the background to become momentarily brighter. “If you find a microlensing signal and it’s concentrated in the Milky Way disc, that is a very strong indicator of atomic dark matter,” he says.
In 2022, Curtin and his colleagues showed that the Vera C. Rubin Observatory in Chile when it begins working next year, should they exist. But he hopes to narrow down which signals would be definitive.
Many astronomers, like Boylan-Kolchin, think it is still a little premature to write cold dark matter’s epitaph. “There are lots of little indications that things are at least not as simple as one might think,” he says, “[but] it’s not clear there’s something more complicated happening in dark matter.”
One alternative is that normal, non-dark matter is behaving in ways we find difficult to understand. Astronomers focus on gravity, but if we could better model thermodynamics, perhaps we would see more complex behaviour. For example, this could come from a better understanding of what happens when supernovae explode and energise surrounding gases, redistributing them through thermodynamic processes rather than gravitational ones.
In the meantime, Lisanti and her colleagues will keep running simulations to see if there could be complex dark matter in our universe. For the moment, it seems like it won’t replace cold dark matter entirely. The fact that straying from current models can so easily break the galaxy shows that our existing recipe is, on the whole, working. Perhaps, then, the key to perfecting it isn’t to replace a whole ingredient, but simply to add a little seasoning.
Stuart Clark is a 91av consultant. His latest book is Beneath the Night
