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Alex Millar | University of Melbourne | Australia

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Why Dark Matter Exists: Believing Without Seeing

Saturday, July 4th, 2015
The Milky Way rises over the Cerro Tololo Inter-American Observatory in northern Chile. The Dark Energy Survey operates from the largest telescope at the observatory, the 4-meter Victor M. Blanco Telescope (left). Photo courtesy of Andreas Papadopoulos

The Milky Way rises over the Cerro Tololo Inter-American Observatory in northern Chile. The Dark Energy Survey operates from the largest telescope at the observatory, the 4-meter Victor M. Blanco Telescope (left). Photo courtesy of Andreas Papadopoulos

For decades physicists have been convinced that most of our universe is invisible, but how do we know that if we can’t see it? I want to explain the thought process that leads one to believe in a theory via indirect evidence. For those who want to see a nice summary of the evidence, check this out. So this post isn’t 3000 words, I will simply say that either our theories of gravity are wrong, or the vast majority of the matter in our universe is invisible. That most of the matter in the universe is invisible, or “dark”, is actually well supported. Dark matter as a theory fits the data much better than modifications to gravity (with a couple of possible exceptions like mimetic dark matter). This isn’t necessarily surprising; frankly it would be a bit arrogant to assume that only matter similar to us exists. Particle physicists have known for a long time that not all particles are affected by all the fundamental forces. For example, the neutrino is invisible as it doesn’t interact with the electromagnetic force (or strong force, for that matter). So the neutrino is actually a form of dark matter, though it is much too quick and light to make up most of what we see.

The standard cosmological model, the ΛCDM model, has had tremendous success explaining the evolution of our universe. This is what most people refer to when they think of dark matter: the CDM stands for “cold dark matter”, and it is this consistency that allows us to explain observations from almost every cosmological epoch that is so compelling about dark matter. We see the effect of dark matter across the sky in the CMB, in the helium formed in primordial nucleosynthesis, in the very structure of the galaxies. We see dark matter a minute after the big bang, a million years, a billion years, and even today. Simply put, when you add in dark matter (and dark energy) almost the entirety of cosmological history makes sense.  While there some elements that seem to be lacking in the ΛCDM model (small scale structure formation, core vs cusp, etc), these are all relatively small details that seem to have solutions in either simulating normal matter more accurately, or small changes to the exact nature of dark matter.

Dark matter is essentially like a bank robber: the money is gone, but no-one saw the theft. Not knowing exactly who stole the money doesn’t mean that someone isn’t living it up in the Bahamas right now. The ΛCDM model doesn’t really care about the fine details of dark matter: things like its mass, exact interactions and formation are mostly irrelevant. To the astrophysicist, there are really two features that they require: dark matter cannot have strong interactions with normal matter (electromagnetic or strong forces), and dark matter must be moving relatively slowly (or “cold”). Anything that has these properties is called a dark matter “candidate” as it could potentially be the main constituent of dark matter. Particle physicists try to come up with these candidates, and hopefully find ways to test them. Ruling out a candidate is not the same as ruling out the idea of dark matter itself, it is just removing one of a hundred suspects.

Being hard to find is a crucial property of dark matter. We know dark matter must be a slippery bastard, as it doesn’t interact via the electromagnetic or strong forces. In one sense, assuming we can discover dark matter in our lifetime is presumptuous: we are assuming that it has interactions beyond gravity. This is one of a cosmologist’s fondest hopes as without additional interactions we are screwed. This is because gravity is by far the weakest force. You can test this yourself – go to the fridge, and get a magnet. With a simple fridge magnet, weighing only a few grams, you can pick up a paperclip, overpowering the 6*10^24 kg of gravitational mass the earth possesses. Trying to get a single particle, weighing about the same as an atom, to show an appreciable effect only through gravity is ludicrous. That being said, the vast quantities of dark matter strewn throughout our universe have had a huge and very detectable gravitational impact. This gravitational impact has led to very successful and accurate predictions. As there are so many possibilities for dark matter, we try to focus on the theories that link into other unsolved problems in physics to kill two birds with one stone. While this would be great, and is well motivated, nature doesn’t have to take pity on us.

So what do we look for in indirect evidence? Essentially, you want an observation that is predicted by your theory, but is very hard to explain without it. If you see an elephant shaped hole in your wall, and elephant shaped foot prints leading outside, and all your peanuts gone, you are pretty well justified in thinking that an elephant ate your peanuts. A great example of this is the acoustic oscillations in the CMB. These are huge sound waves, the echo of theCMB big bang in the primordial plasma. The exact frequency of this is related to the amount of matter in the universe, and how this matter interacts. Dark matter makes very specific predictions about these frequencies, which have been confirmed by measurements of the CMB. This is a key observation that modified gravity theories tend to have trouble explaining.

The combination of the strong indirect evidence for dark matter, the relative simplicity of the theory and the lack of serious alternatives means that research into dark matter theories is the most logical path. That is not to say that alternatives should not be looked into, but to disregard the successes of dark matter is simply foolish. Any alternative must match the predictive power and observational success of dark matter, and preferably have a compelling reason for being ‘simpler’ or philosophically nicer then dark matter. While I spoke about dark matter, this is actually something that occurs all the time in science: natural selection, atomic theory and the quark model are all theories that have all been in the same position at one time or another. A direct discovery of dark matter would be fantastic, but is not necessary to form a serious scientific consensus. Dark matter is certainly mysterious, but ultimately not a particularly strange idea.

Disclaimer: In writing this for a general audience, of course I have to make sacrifices. Technical details like the model dependent nature of cosmological observations are important, but really require an entire blog post to themselves to answer fully.




Where Do I Come From?

Wednesday, February 4th, 2015

It’s the oldest question in the world and it occurs to every child, sooner or later: where do I come from? Mum and Dad of course, but where did they come from? Genetics only takes us so far; our line of ancestors actually stretches back beyond our first single celled forebears. Chemistry proceeds biology, and before that the world was made only of protons, neutrons and electrons. Now this takes us pretty far back, to the first second of the universe. In many ways, our fate was decided in this instant. The protons and neutrons we are made of formed a millionth of a second after the proverbial lights went on, condensing out of quarks. But where did the quarks come from?

Photo courtesy of NASA

Photo courtesy of NASA

Baryogenesis as a concept is not too difficult to follow. Every molecule you see around you is a survivor of a vast catastrophe that struck the early universe, when 30,000,000 of every 30,000,001 quarks in the universe were destroyed. The culprit of this disaster is antimatter – the bizarro version of matter. The crux of the matter is that matter and antimatter have a love-hate relationship; they annihilate each other, but also prefer to be created together. In the present day our universe is just too cold to create matter out of thin air (actually, through interactions with particles like photons), but this was not always so. When we go far enough back, at temperatures of about 10^13 degrees Celsius pair creation kicks off and the universe is filled with massive amounts of matter and antimatter. While this is lukewarm for a particle physicist there are more orders of magnitude between this temperature and the sun’s core than the sun’s core and you. From what I have said, the origin of matter doesn’t seem like much of a mystery; pair creation made matter. The problem is that it also made antimatter, and (according to the Standard Model) in equal amounts. When the universe cooled, matter could no longer be created, only destroyed, and so both matter and antimatter dwindled into nothing.

Creation (or destruction) of an electron-positron pair. Canny readers will notice that I have used this little diagram before.

Creation (or destruction) of an electron-positron pair. Canny readers will notice that I have used this little diagram before.

Clearly this is not the case – as any child can see, our universe is a populated and interesting one, filled with stars and planets and puppies. Above all, our universe is made of matter – no antimatter allowed. So there must have been a kind of discrimination against antimatter for some matter to survive this rampant destruction. Either this asymmetry between matter and antimatter existed from the start, as some sort of initial condition, or it somehow has dynamically evolved since then. Inflation dilutes any primordial asymmetry even more than a homeopathic remedy, so there must have been some matter creating process – baryogenesis. In any case, simply citing “initial conditions” is almost like saying “just because”, which never really seems to work with children.

When you need to explain something, it is usually best to start by what broad features your theory must have. For baryogenesis, Sahkarov did this back in 1967. For any theory (that doesn’t violate CPT) to create an asymmetry between matter and antimatter, three conditions must be satisfied:

  1. Baryon number must be violated. If you wish to count the number of protons and neutrons, it turns out that assigning them a quantity called “baryon number” is useful, a proton and neutron each have a baryon number of 1, and a quark has a baryon number of 1/3. Antimatter versions have a negative baryon number. The process that leads to the predominance of matter over antimatter, and so baryons over anti-baryons, is referred to as “baryogenesis”. It turns out that the total baryon number of the universe is conserved under perturbative effects in the Standard Model, what is known as an “accidental symmetry”. If we want more protons than antiprotons this number cannot be inviolate. There is a similar counting of electrons and neutrinos called lepton number, which is also believed to be broken. Unfortunately as neutrinos are extremely difficult to observe there is no direct evidence of a total lepton asymmetry.
  2. Matter and antimatter must be treated differently. This means that charge conjugation (where you swap particles with antiparticles) and charge-parity conjugation (swap particles with antiparticles and also reflect them like a mirror image) cannot leave the physics unchanged. More succinctly, C and CP must be broken. While C violation is trivial (the weak force violates C maximally), CP is almost entirely preserved in the Standard Model. This is both a major failing of the Standard Model and a fantastic prediction – we know that CP violation is absolutely fundamental to our universe, and that there must be more of it than we have seen so far. You have probably seen CP violation mentioned many times, both on this site and through news reports. The necessity for CP violation to explain our own existence is the real reason why CP violation deserves our attention.
  3. The universe must go out of thermal equilibrium. In thermal equilibrium any process creating a baryon asymmetry would be balanced by its reverse. Fortunately for us, the fact that the universe expands creates periods of thermal non-equilibrium, such as phase changes (like when the Higgs mechanism breaks the electroweak symmetry of the Standard Model).


While the Standard Model does technically satisfy all three of these, it does so in a trivial way. The amount of CP violation is far too low, and a universe in which the Standard Model is entirely correct never gets far enough out of equilibrium to allow a large difference in matter and antimatter to form even if it did violate CP more. The only really useful element that the Standard Model has is baryon number violation; a non-perturbative process called sphalerons occurs above the electroweak phase transitions which violates baryon and lepton number. More importantly, it preserves a linear combination of the two, so if you manage to make a baron asymmetry or a lepton asymmetry, you automatically get both. Theories like leptogenesis use this to turn a lepton asymmetry into a baryon asymmetry. While there are many possible scenarios that could have lead to the present day world (my own work is in one of these, asymmetric dark matter), the truth is that we simply don’t know which of these, if any, is correct.

Despite this being a question of the most fundamental kind, baryogenesis does not get nearly the same kind of media attention as dark matter or dark energy. Partly this is because we have little chance of experimentally finding an answer – baryogenesis could have occurred at almost any energy scale, which includes a good many far out of the reach of our colliders. But it is still important to push for an answer. Nothing is a better mark of our progress in understanding our origins than seeing how the question we ask about our origin evolves.


How to build a universe

Thursday, January 8th, 2015

How do you make a world? This is the purview of theologists, science fiction authors and cosmologists. Broadly speaking, explaining how the universe evolves is no different from any other problem in science: we need to come up with an underlying theory, and calculate the predictions of this theory to see if they match with the real world. The tricky part is that we have no observations of the universe earlier than about 300,000 years after the big bang. Particle colliders give us a glimpse of conditions far earlier than that, but to a cosmologist even the tiniest fraction of a second after the big bang is vitally important. Any theorist who tries his or her hand at this is left with a trail of refuse models before reaching a plausible vision of the universe. Of course, how and why one does this is deeply personal, but I would like to share my own small experience with trying to make a universe.


For me, the end was very clear; I wanted to design a universe that explained the existence of dark and visible matter in a particular way. Asymmetric dark matter is a class of theories that try to link the origins of dark and visible matter, and my goal was to explore a new way of creating matter in the universe. So what do you start with? As a particle physicist, the most obvious (but not the only) building blocks at our disposal are particles themselves. Starting with the Standard Model, the easiest way to build a new theory is to just start adding particles. While adding a new particle every time you want to explain a new phenomenon seems indulgent (and some people take this to excess), historically this is a very successful tactic. The neutrino, W and Z bosons, the charm, bottom and top quarks, and Higgs boson were all introduced before they were discovered to explain various theoretical or experimental problems. While back in 1930 Pauli apologised for the bad grace of introducing a particle no one had ever seen, theoretical physicists have well and truly overcome this reticence.

So what ingredients does a dark matter model need? Clearly there must be a viable candidate for dark matter, so at least one new particle must be introduced. While the simplest case is for dark matter to be made of one particle, there is no reason for a substance that makes up 85% of matter in the universe to be any simpler than the matter that we are made of. But, for the sake of simplicity, let us say just one particle for now. For my work to explain the creation of visible matter as well as dark matter, there must also be some interaction between the two. To do this there must be a “mediator”, something that communicates between the visible and the dark. So at least two particles are necessary. Now, two particles doesn’t sound so bad, not when we already know of 17.

The model I was originally going to study (one that already existed) was like this, with dark matter interacting with neutrons. Unfortunately, this is also when the realities of model building sank in; it is rare for any model to be this simple and still work as advertised. Under closer scrutiny it turned out that there was no satisfactory way to make the dark matter stick around for the lifetime of the universe – it quickly decayed away unless you made some theoretical sacrifices I wasn’t comfortable making. Thus began my first foray into model building.

The first hurdle to overcome, for a first-time model builder, is simply the vast size of the literature itself. I was constantly worried that I had missed some paper that had beaten me to it, or had already considered some aspect of my work. Even though this was not the case, even the simplest of possible universes has a lot of complicated physics going on in a variety of areas – and any single aspect of the model failing could mean scrapping the whole thing. Most of these failing points are already known to those experienced in these matters, but a first timer has to find them out the hard way.

In the weeks I spent trying to come up with a model worth studying in detail, I had almost a dozen “Eureka” moments, which were almost always followed by me finding a flaw in the next few days. When you have no strict time limits, this is simply disheartening, but occasionally you can find flaws, or potential flaws, when you are already significantly invested and close to a deadline (such as thesis submission). Unfortunately the only real way to avoid this is to develop a level of care bordering on paranoia, to try and think of all the possible ways a theory might implode before getting bogged down in calculations. Of course, some things are inherently unforeseeable (otherwise why is it research) but many can be divined beforehand with enough experience and thought. This was driven home to me after spending a month working out the predictions of a theory, only to discover that I had drastically underestimated the consequences of a minor change to the model. Fortunately in research little is wasted; even though no part of that work appeared in the final version of my thesis, the methods I learnt certainly did.


My pride and joy, a model of ADM via a lepton portal. Leptons (like electrons and neutrinos) interact with scalar dark matter (the phi) to create the matter we see today.

Trying to come up with a theory yourself also forces you to confront your theoretical biases – naturalness, simplicity, renormalisability, testability and fine tuning are all considered by theorists to be important considerations, but it is almost impossible to satisfy all of these at once. Even worse, there are often many different competing interpretations of all of these. So, almost inevitably, sacrifices must be made. Perhaps your theory has to give up on technical naturalness, or has a hell of a hierarchy problem (which mine definitely did). That being said, this is not always an issue;  many models are made to explore a particular avenue, or to provide a working example. The fact that some of these traits cannot be satisfied is important information. You have to pick and choose what you care about, because if the history of physics has shown us anything, it is that theoretical biases, even very well grounded ones, can simply be wrong. The discovery of CP (and consequently time reversal) violation and the non-deterministic (or the apparently non-deterministic, depending on whether you prefer a many worlds interpretation) nature of quantum mechanics are just a couple of examples where “essential” elements of a proper theory turned out to simply not apply.

While this seems like a frustrating experience, I actually greatly enjoyed model building. Too much of university coursework is rushed – you have to learn all of a subject in 12 weeks, and are tested in an exam that only lasts four hours, sometimes in quite shallow ways. This kind of research emphasises patience and care, and allows (or requires) you to deeply understand the physics involved. Calculations are irrelevant for a large part of the process. You simply don’t have time to try and brute force your way through dozens of theories, so you must devise more elegant ways to discriminate and choose those worth the time. I very much doubt that the model I worked on is the underlying truth of our world, but it was very fun to try.





Dark Matters: Creation from Annihilation

Thursday, November 13th, 2014

Hanging around a pool table might seem like an odd place to learn physics, but a couple of hours on our department’s slanted table could teach you a few things about asymmetry. The third time a pool ball flew off the table and hit the far wall I knew something was broken. The pool table’s refusal to obey the laws of physics gives aspiring physicists a healthy distrust of the simplified mechanics they learnt in undergrad. Whether in explaining why pool balls bounce sideways off lumpy cushions or why galaxies exist, asymmetries are vital to understanding the world around us. Looking at dark matter theories that interact asymmetrically with visible matter can give us new clues as to why matter exists.

Alternatives to the classic WIMP (weakly interacting massive particles) dark matter scenario are becoming increasingly important. Natural supersymmetry is looking less and less likely, and could be ruled out in 2015 by the Large Hadron Collider. Asymmetric dark matter theories provide new avenues to search for dark matter and help explain where the material in our universe comes from -baryogenesis. Baryogenesis is in some ways a more important cosmological problem than dark matter. The Standard Model of particle physics describes all the matter that you are familiar with, from trees to stars, but fails to explain how this matter came to be. In fact, the Standard Model predicts a sparsely populated universe, where most of the matter and antimatter has long since annihilated each another. In particle colliders, whenever a particle of matter is created, an opposing particle of antimatter is also created. Antimatter is matter with all its charges reversed, like a photo negative. While it is often said that opposites attract, in the particle physics world opposites annihilate. But when we look at the universe around us, all we see is matter. There are no antistars and antiplanets, no antihumans living on some distant world. So if matter and antimatter are always created together, how did this happen? If there were equal amounts of matter and antimatter, each would annihilate the other in the first fractions of a second and our universe would be stillborn. The creation of this asymmetry between matter and antimatter is known as baryogenesis, and is one of the strongest cosmological confirmations of physics beyond the Standard Model. The exact amount of asymmetry determines how much matter, and consequently how many stars and galaxies, exists now.

And what about the other 85% of matter in the universe? This dark matter has only shown itself through gravitational interactions, but it has shaped the evolution of the universe. Dark matter keeps galaxies from tearing themselves apart, and outnumbers visible matter five to one. Five to one is a curious ratio. If dark and visible matter were entirely different substances with a completely independent history, you would not expect almost the same amount of dark and normal matter. This is like counting the number of trees in the world and finding that it’s the same as the number of pebbles. While we know that dark and visible matter are not the same substance (the Standard Model does not include any dark matter candidates), this similarity cannot be ignored. The similarity in abundances between dark and visible matter implies that they were caused by the same mechanism, created in the same way. As the abundance of matter is determined by the asymmetry between antimatter and matter, this leads us to a relationship between baryogenesis and dark matter.

Asymmetric dark matter theories have attracted significant attention in the last few years, and are now studied by physicists across the world. This has give us a cornucopia of asymmetric dark matter theories. Despite this, there are several common threads and predictions that allow us to test many of them at once. In asymmetric dark matter theories baryogenesis is caused by interactions between dark and normal matter. By having dark matter interact differently with matter and antimatter, we can get marginally more matter in the universe then antimatter. After the matter and antimatter annihilate each other, there is some minuscule amount of matter left standing. These leftovers go on to become the universe you know. Typically, a similar asymmetry in dark matter and its antiparticle is also made, so there is a similar amount of dark matter left over as well. This promotes dark matter from being a necessary, yet boring spectator in the cosmic tango to an active participant, saving our universe from desolation. Asymmetric dark matter also provides new ways to search for dark matter, such as neutrinos generated from dark matter in the sun. As asymmetric dark matter interacts with normal matter, large bodies like the sun and the earth can capture a reservoir of dark matter, sitting at their core. This can generate ghostlike neutrinos, or provide an obstacle for dark matter in direct detection experiments. Asymmetric dark matter theories can also tell us where we do not expect to see dark matter. A large effort has been made to see tell-tale signs of dark matter annihilating with its antiparticle throughout the universe, but it is yet to meet with success. While experiments like the Fermi space telescope have found potential signals (such as a 130 GeV line in 2012), these signals are ambiguous or fail to survive the test of time. The majority of asymmetric dark matter theories predict that there is no signal, as all the anti dark matter has long since been destroyed.

As on the pool table, even little asymmetries can have a profound effect on what we see. While much progress is made from finding new symmetries, we can’t forget the importance of imperfections in science. Asymmetric dark matter can explain where the matter in our universe came from, and gives dark and normal matter a common origin. Dark matter is no longer a passive observer in the evolution of our universe; it plays a pivotal role in the world around us.