Katherine Taylor for Quanta Magazine

Katherine Taylor for Quanta Magazine

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Make it. Shake it. Break it. That’s the three-part refrain of dark-matter detectives, including Tracy Slatyer, a theoretical physicist at the Massachusetts Institute of Technology.

We don’t know what kinds of particles are responsible for dark matter, the missing mass that outweighs the universe’s normal matter by a factor of five. We don’t know how big they are or how they behave. But there are three possible paths to finding out: We can hope to make them in accelerators like the Large Hadron Collider (LHC). We can try to sense them as they collide with and shake Standard Model particles in sensitive direct-detection experiments. Or — the method Slatyer focuses on — we could check them as they wreck themselves, watching through telescopes as they crash together or decay out in space, producing a faint, luminous signal.

So far, researchers have drawn a blank. The LHC has failed to make new particles beyond the Higgs boson. Direct-detection experiments also haven’t picked up a conclusive signal. And astronomical searches for dark matter haven’t returned any hard evidence of its identity. Yet many scientists, including Slatyer, argue that the sheer number of telescopes observing the cosmos makes it more feasible for astrophysicists to do broad searches for many different types of dark-matter particles. “Because we do astrophysics, we already have these telescopes that cover a huge range of energies,” Slatyer said.

Slatyer’s career thus far demonstrates how the new era of open-source astrophysics data allows early-career researchers who wouldn’t ordinarily be able to secure observing time on a big telescope to make important discoveries. In 2009, NASA’s gamma-ray-sensitive Fermi telescope released its data to the public. Shortly thereafter, observers including Slatyer pointed out two locations where extra gamma rays were being produced: one at the very core of the Milky Way, and another just around it.

This is exactly the kind of signal that dark matter is thought to be able to produce. Colliding dark-matter particles could give off electrons and positrons; these could accelerate nearby photons to gamma-ray energies. The gamma rays would appear to emanate from a thick dark-matter cloud that astronomers could infer separately from gravitational evidence. The connection would be clear.

In 2010, Slatyer, then a graduate student at Harvard University, along with her fellow graduate student Meng Su and their adviser, Douglas Finkbeiner, showed that the gamma-ray haze around the center of the Milky Way was not a diffuse cloud but two massive bubbles of plasma linked to the black hole at the center of the galaxy, structures now named the “Fermi bubbles.” Then, in 2015, Slatyer was part of a team that argued that the gamma-ray excess in the galactic center itself may have been caused not by dark matter, but by a previously unknown population of faint, mysterious astrophysical objects — probably pulsars.

NASA's Goddard Space Flight Center

An artist’s impression of the Fermi bubbles, luminous lobes of plasma that extend above and below the plane of the Milky Way galaxy.

Slatyer is now working to determine exactly what is creating all those extra gamma rays in the galactic center. She’s also considering how dark-matter annihilation could have changed the history of the cosmos. Quanta caught up with her recently to chat about these projects. An edited and condensed version of the conversation follows.

QUANTA MAGAZINE: When the Fermi data first appeared, you and others suspected that the strong gamma-ray signal at the center of the galaxy might be coming from dark-matter annihilation. But you don’t think so anymore, do you?

TRACY SLATYER: My personal guess at the moment is that this is some new population of gamma-ray point sources, probably some new population of pulsars. But it’s a really weird population! I mean, it doesn’t look like the disk of the galaxy at all. It looks like a new population that’s systematically fainter than the pulsars that Fermi has already seen. We don’t really have a good explanation yet for where these pulsars would have come from, and we haven’t yet confirmed that they are pulsars. It’s quite a puzzle.

So even if it’s not dark matter, it’s unclear what astronomical objects could so comprehensively mimic what dark matter should look like?

I think it would be very good for the field if we could understand the astrophysics that can give rise to signals like this. Because this looks a lot like the dark-matter signal. If it’s not dark matter, it’s a very effective fake. It has pretty much the right spectrum. It has a spatial distribution that is very much what we would expect from dark-matter annihilation. If we’re going to do searches for dark matter, we need to understand signals like this.

Of course I’d like to discover dark matter, but part of the reason I did science in the first place is that it’s so cool to find something that nobody has ever seen before. With these signals, I want to know what they are; I want to know what the answer is. If the answer is dark matter, that would be great, but I just want to know what the answer is.

How did you get into this niche between particle physics and astronomy?

I went to Harvard for grad school thinking: OK, I’m going to work on extra-dimensional models of the universe with Lisa Randall and Nima Arkani-Hamed. At the end of my first year, Nima announced that he was moving to [the Institute for Advanced Study at] Princeton, and Lisa announced that she was going on sabbatical. And I went: Hmm, a plan B is required. Very much to my good fortune, I was introduced to a new young professor in the astrophysics department named Doug Finkbeiner. And Doug at that time was looking for a student with some particle physics training. I had been talking to a lot of particle physicists, and at that stage the whole field was really just waiting with bated breath for the LHC to switch on and finally start giving us data. But when I talked to Doug, I found out that in astrophysics there were all these data around, and the number of people seriously digging into that information was much smaller than the number of people planning to work on LHC data, especially if you focused in on people looking for the signals of new fundamental physics.

I was like: Well, there are all these data, there are all these unanswered questions, there all these signals hiding in these data that might be telling us something very interesting, and there’s a shortage of people to work on them. I can do that.

Dark matter is, by definition, invisible. It’s also undiscovered. How do you know what to look out for?

Suppose you say: All right, I’m going to hypothesize that my dark-matter particles collide, and that when they collide, they annihilate and produce Standard Model particles. Now, this has presumably been happening for the whole lifetime of the universe. If the annihilation rate is too fast, you wouldn’t have any dark matter left today. If it’s too slow, you would have too much dark matter left today. Just from measuring the amount of dark matter in the universe — which has been done — we could predict how rapidly dark-matter particles should be colliding and converting into Standard Model particles. Then that gives us an estimate for how large a signal we should see from various objects.

According to this estimate, should we be able to observe dark-matter annihilations today?

Katherine Taylor for Quanta Magazine

Video: Tracy Slatyer explains why she’s not disappointed when a mysterious cosmic signal turns out to be something other than dark matter.

We are genuinely at a pretty interesting time. If the dark matter is less than about 100 gigaelectron volts — about 100 times the mass of a proton — then we should already be seeing annihilation signals from dwarf galaxies. Just with our current telescopes. We should only have started to see it in the past couple of years. So we’re right at a special moment when our telescopes are probing this very interesting region.

You’re also looking back in time, for evidence of dark-matter annihilation in the early universe. How might that have worked?

So, there are potentially quite a few interesting effects. There was a period in our universe’s history before the stars turned on, or the galaxies turned on, when the universe was a pretty low-energy place, as Donald Trump might say. It was basically just a mass of neutral hydrogen and neutral helium and dark matter, with clumps of dark matter here and there.

If dark-matter annihilation in the early universe acted as this continuous, ubiquitous pump of high-energy particles into the visible universe, it could have had pretty striking effects on the evolution of that dark universe. That’s not something that can be easily mimicked by astrophysical processes.

We have observational handles on that period. We have the photons of the cosmic microwave background, which are emitted at the start of this period, and they travel to us through that period — through this dark, neutral universe. Any changes to it leave imprints on the cosmic microwave background that we can measure very sensitively.

Once you get into the period where stars are starting to turn on, where galaxies are starting to form, the dark matter is also clumping up, and so dark-matter collisions can become much more frequent. Dark matter again at this period could provide this steady stream of high-energy particles. Those particles can ionize the hydrogen; they can heat it up.

In a paper from this April, though, it seems like you argue that dark matter couldn’t have changed the early universe much. Does that mean these effects would be subtle, at best?

We were looking at one very specific question, which was: Could dark matter have played a significant role in reionization, the period in which the universe went from being almost completely neutral to almost completely ionized quite abruptly? And what we found is that it’s hard for dark-matter annihilation or decay to contribute a lot to that particular process.

So that paper was just a stab at that one first question, but the broader program is to understand more generally what are the potential observable impacts of dark-matter annihilation or decay from the cosmic dark ages through the period close to reionization.

Looking forward, are there any other ideas you’re working on for new dark-matter signatures?

There’s a cute problem I’ve been thinking about for a while. I suspect that for this particular model the interesting signatures are not observable in the immediate future, so this is kind of a further-future story.

The idea of the dark sector is that maybe the dark matter isn’t the only new particle. Maybe the dark matter will interact with itself through a force that it feels and that ordinary matter doesn’t, in the same way that ordinary matter feels electromagnetism and dark matter doesn’t.

If I have two particles of opposite charge, the fact that there is electromagnetic attraction between them means they are much more likely to collide with each other. So, in the same way, if the dark matter has its own force, that makes it much more likely to interact with itself. If you think about the other things that pairs of charged particles could do under the electromagnetic force, one thing that happens is that they form atoms. And in these dark sectors you could have similar effects. Dark matter could form atom-like bound states.

Eventually this bound state will probably annihilate, so it would destroy itself in the same way that dark-matter annihilation usually works, providing a cascade of Standard Model particles. But the formation of the bound state, and transitions between different bound states, could also produce lower-energy photons. In some distant, far-future scenario, if you did see the dark-matter annihilation signal, then searching for this little forest of spectral lines from the bound-state transitions could potentially give you a lot of information on the particle physics of the dark matter — even if, for example, it was too heavy to ever make in the LHC.

Just this question, of how do you properly compute the formation of bound states and figure out their impact on observations, is something I’ve been thinking about for a long while.

So we might learn about dark matter from its spectral lines?

You’d see the high-energy event, which is like a positron-electron annihilation, where all the energy of the dark matter gets converted into Standard Model products, but you would also get the little forest of transition lines at much lower masses, probably, and they would be telling you about the potential the dark matter experiences, how it interacts with force carriers.

You could do dark-matter spectroscopy. But we’d have to actually find the dark-matter signal first before you thought about doing dark-matter spectroscopy!

This article was reprinted on Wired.com.

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  • If one accepts the (I know, I know) sweeping statement that the least well understood of all forces, gravity, is likely to be at the bottom of many physical mysteries, I am struck by Ms Slatyer's remark that she had originally intended to work on multi-dimensional models of the universe. It'll be interesting to follow her work and see if her serendipitous "switch" to dark matter astrophysics doesn't lead her right back to what she was planning to study in the first place.

    Anyway: fascinating stuff, good article, thanks.

  • The hype machine is working overtime here and elsewhere. These ideas are completely non-starters, and moderately light (or moderately heavy) axions are the last man standing. Light as in not ultra heavy, and heavy as in not ultra light. meV scale or less.

    On top of finding weakly coupled light axions, and outside of astronomical detection, myself and others have posited that axion electrodynamics in topological physics, simulated and tested by condensed matter physics experiments and verified by cold atom optical trapping methodsm is the only way for experiment to inform theory such that the horizon, inflation, baryogenesis, hierarchy, fine tuning problems, etc,, in other words, the energy scale problem – from Fermi to Planck scale gravitational collapse. Using only well motivated examples from the standard model, that by definition will involve axion – Higgs – graviton coupling physics or some variation on that meme.

    It's wonderful that vast amounts of effort and money are being invested in this search, but in order to make connection to the real world and produce useful results, this effort is better invested in something that has ample motivation in condensed matter physics and the standard model.

    Axions are that motivation. Everything else is merely flailing around in the darkness.

  • I give credit to any Astro-physics discussion that invites additional reading and analysis. Slatyer’s is one of these. Hat’s off to you. Comment #1: “…the interesting signatures are not observable in the immediate future…”. I’m not convinced that this field of signatures has been thoroughly examined nor that the signatures already under the lens have had adequate filters/reasoning applied in a way that would assist more heavily in separating the wheat from the chaff. The universe is wild with ‘noise’ (energies) in shapes and forms we have yet to examine. Comment #2: We tend to live and think in a physics world many of whose limits are categorized at the outset as binary. The immediate cases in point are those of attraction/deflection, up/down, positive/negative. This suggests that the arity function, ternary, is universally ignored. I suggest that there are situations where the causes and effects should not be limited to binary explanations only because they ‘always have been’. Additionally, as to the understanding of our x-y-z world (excluding time). I remain unconvinced that this ‘real life’ 3D limit remains supportable. It certainly is not mathematically.
    My apologies if these remarks appear as bizarre products created by some wild-eyed idiot.

  • I like to think of matter as imperfectly analogous to clouds: heat acts as a "dimension" to the atmosphere (in terms of relative molecular value count at some particular scale for various regions), thus concentrating the moisture in the atmosphere into clouds.

    The Universe is analogously composed of some kind of stuff (matter, dark and otherwise) which is analogous to moisture in the atmosphere, with matter that we see being the analog of clouds, but instead of following the dimension of heat matter is following the "dimension" of gravity (which following General Relativity is again arrived at by relative value count at some particular scale for various regions) and the dark energy is analogous to the moisture in the atmosphere that is there but not seen in the form of a cloud. Definitely an imperfect analogy, but it is useful for conveying a different way of looking at something familiar. It also explains the absence of the graviton: gravity IS the shape of spacetime, not an attractive force between celestial bodies. I like to refer to this as my visualization of LQG 🙂

    Additionally, I would like to add that I think that the description of spatial characteristics logically/mathematically requires this kind of relationship to be present, i.e. any statement regarding differing characterizations of space, such as General Relativity or just heat expanding matter, logically appeals to the concept of a grid imposing itself onto some kind of substratum.

  • Questions.
    Is the amount of dark matter in our galaxy constant over time?
    Has it undergone some form of evolutionary process?
    Has the black hole at the center of our galaxy consumed some of that dark matter?
    Has it consumed a significant portion of it, so that most of the accumulated mass of the galaxy's dark matter mass resides within the black hole?
    Is the process of dark matter 'ingestion' of dark matter ongoing?
    Do these processes leave a detectable pattern of interaction in the disk of milky ways normal matter?
    These question speak to the nature of dark matter as compared to normal matter in a gravitational field.
    Wish I'd been a scientist.

  • As dark matter particles experienced conversion the amount of dark matter would be steadily declining. Wouldn't that show up in the evolutionary pattern of both host galaxies and the galactic web of galaxies?

  • It doesn't make sense. "Does not experience the electromagnetic force." That means no photons, ever. Not even one.

    Given the amount of dark matter supposedly out there if it were capable of emitting light through collisions we would have surely seen it. If it was capable of reacting with photons it would have absorption lines. If it was capable of having inelastic collisions even with itself it would be congealing and orbiting massive objects, not floating around without constraint the way it apparently does. What they are looking for does not match the known characteristics of dark matter. An axion experiment using the magnetic field of a pulsar or similar collapsing star is what the astrophysicists should be doing.

  • When the visible (light) matter collisions always ensue (and are completely accounted for) in terms of the visible (known, light) daughter matter and/or detectable radiation, (then) how justified is the hypothesis on the dark matter collisions to offspring a set of (visible, light) standard model particles and photons etc? Existence of any such dark-to-light (or vice-versa) interactions would be an even bigger mystery than the dark matter itself. Therefore, light-to-dark matter-transformation being established as non-existent, by symmetry, any such interactions' (un)feasibility would definitely forbid the back-conversions just as well.

    It's way too unreal to fancy the detectable "Photonic Spectroscopy" to be applicable to any contrived dark matter atoms, the transition in them are sure to involve only some unbeknown "dark photons", just as undetectable as the dark matter itself.

    Disclaimer: May be I am missing something in between the lines, and need to be en'light'ened about what is yet a 'dark' knowledge to me!

  • As I understand it, dark matter is supposed to form a spherical 'halo' around a galaxy. Is this article implying that these two huge Fermi bubbles are an equivalent structure?

  • Keeping in mind that science, in particular physics, have been developing, as we know, not so far in past, these is a kind of interesting research – dark matter.
    Many people, scientists for sure, deeply believed, in the past, that a kind of fluid should be around everything. They do not seem them, explain them but, for a sort of experiences and observations "it must be there". So, they call it the "Eter or Ether".
    For a strange and weird coincidence, today we speculate about a dark matter. We cannot see them, explain them! But, again, "it must be here"! Surrounding us all the way!!
    This is astonishing and I'll do follow this kind of research.
    Thanks for the great article and keep them coming!
    Eduardo

  • Dark Matter, Or How Inquiry Proceeds
    How to find really new science? Not by knowing the result: this is what we don’t have yet. Any really new science will not be deduced from pre-existing science.
    The case of Dark Matter is telling: it has been in evidence for 80 years. As it did not fit existing science, it was long religiously ignored as a subject not worthy of serious inquiry. Now Dark Matter, five times more massive than Standard Model matter, is clearly massively sitting outside of the Standard Model, and obscuring the pretense of known physics to explain everything.

    Physicists are presently looking for Dark Matter, knowing what they know, namely that nature has offered them a vast zoo of particles, many of them without rhyme or reason (some have rhyme, a symmetry, a mathematical group acting upon them, and symmetries revealed new particles).
    However, remember: a truly completely new piece of science cannot be deduced from pre-existing paradigm. Thus, if Dark Matter was really about finding a new particle type, it would be interesting, but not as interesting as it would be, if it were not, after all, a new particle type, but from a completely new law in physics.

    This is the quandary about finding truly completely new science. It can never be deduced from ruling paradigms, and may actually overthrow them. What should then be the method to use? Can Descartes and Sherlock Holmes help? The paradigm presented by Quantum Physics helps. The Quantum looks everywhere in space to find solutions: this is where its (“weird”) nonlocality comes in. Nonlocality is crucial for interference patterns and for finding lowest energy solutions, as in the chlorophyll molecule. This suggests that our minds should go nonlocal too, and we should look outside of a more extensive particle zoo to find what Dark Matter is.

    In general, searching for new science should be by looking everywhere, not hesitating to possibly contradict what is more traditional than well established.

    An obvious possibility is, precisely, that Quantum Physics is itself incomplete, and generating Dark Matter in places where said incompleteness would be most blatant. More precisely, Quantum processes, stretched over cosmic distances, instead of being perfectly efficient and nonlocal over gigantically cosmic locales, could leave a Quantum mass-energy residue, precisely in the places where extravagant cosmic stretching of Quanta occurs (before “collapse”, aka “decoherence”).

    The more one does find a conventional explanation (namely a new type of particle) for Dark Matter, the more likely my style of explanation is likely. How could one demonstrate it? Not by looking for new particles, but by conducting new and more refined experiments in the foundations of Quantum Physics.

    If this guess is correct, whatever is found could actually help future Quantum Computer technology (because the latter works with Quantum foundations directly, whereas conventional high energy physics tend to eschew the wave aspects, due to the high frequencies involved).

  • It seems to me that the path forward has always involved looking at the existing body of knowledge and picking out the wheat from the chaff, so to speak. The example I like to use is when humanity went from believing the earth was flat to believing the earth was round. In this process, no "knowledge" which was mutually verified through observation in conjunction with mathematics was overturned, rather, areas of "knowledge" which were NOT mutually verified through both observation as well as mathematics (at least not logically consistent i.e. non-contradictory mathematics) were re-worked or outright overturned. The most obvious example is explaining why ships disappeared over a horizon line while sailing away: whatever previously described this phenomena got overturned.

    In this spirit the path forward has seemed obvious to me for nearly a half dozen or so years now. We have two lovely theories, neither of which disagree with any experiments, but which supposedly disagree with one another about the nature of reality, specifically with reference to infinity. And infinity certainly fits the above description of something in our current body of knowledge which is NOT mutually verified through both observation as well as mathematics, and is in fact logically self-contradictory and logically inconsistent. Infinity is also a concept which I argue inherently cannot be verified through observation, at least not directly. That being the case, the least we could do is to come up with a logically coherent/consistent definition of infinity (the current one has major internal inconsistencies), and see if this does anything to help us. As a result, this is the approach I have taken, and it leads to the answer that the size of space is arrived at via relative value count, at infinity, s.t. the higher the count, the smaller the space, and the greater the mass/gravity. Gravity then becomes an increase in the probability of some event occurring at some time and some particular location, as expressed by that particular place and time being representative of a larger volume of spacetime, which is in turn geometrically expressed as a condensation of spacetime, as opposed to some attractive force between distant objects.

    Thus effectively what I suggest is to re-write what Minkowski spacetime is in the same sense that Minkowski spacetime was a re-write of what came before it. Yes, I have done some work on this, namely in reference to something called "The Continuum Hypothesis" and another thing called "0#".

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