What We Don't Know
What We Don't Know
Dark matter
What is dark matter? Why do we need it, and how can we find it? Dark matter is notorious for how it evades detection, and its presence is one of the biggest mysteries in cosmology. Yet we think it must exist. Not only that, but we think it must make up 80% of matter in the universe.
This episode I will explore how physicists discovered dark matter, what we know so far, and the particles which attempt to solve its secrets. Then I’ll consider what dark matter fails to explain and the possible alternative theories that complement dark matter’s weaknesses.
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Hello everyone, welcome to the eighth episode of ‘What We Don’t Know’, a podcast that explores the boundaries of human knowledge, investigating the unanswered questions and theories that unravel them at the frontiers of science. During this podcast I hope to get you interested in new areas of science, maths and technology, teaching you about existing concepts and igniting a curiosity for the things we have yet to know.
What is dark matter? Why do we need it, and how can we find it? Dark matter is notorious for how it evades detection, and its presence is one of the biggest mysteries in cosmology. Yet we think it must exist. Not only that, but we think it must make up 80% of matter in the universe.
This episode I will explore how physicists discovered dark matter, what we know so far, and the particles which attempt to solve its secrets. Then I’ll consider what dark matter fails to explain and the possible alternative theories that complement dark matter’s weaknesses.
First, let’s clarify between normal matter, dark matter, and dark energy. Of the observable universe, interactive matter - things like quarks, electrons, neutrinos, nearly all of the Standard Model of Particle Physics - makes up 5%. This means that everything we have ever interacted with, observed or measured, all people, planets, solar systems and galaxies, everything we hold dear in this universe, is only 5% of what exists.
Dark energy makes up 70%. This is responsible for the otherwise unexplainable expansion speed of the universe, but I’ll leave dark energy for another episode.
Around 25% is dark matter. Dark matter seems to interact only via the gravitational force. This makes it particularly difficult to research. We can’t see it, since it doesn’t absorb, reflect or emit light using the electromagnetic force. We can’t analyse dark matter using the strong force, like we can for the binding of quarks into protons and neutrons. We can’t even repeat our exploitation of neutrinos’ weak interactions, because dark matter doesn’t cause detectable boson exchanges.
But some cosmological observations cannot be explained using only the mass of detectable matter. Get out your forensic kits, because we’re investigating the biggest crime scene yet: the universe.
One such observation concerns the velocities of stars orbiting around the centre of galaxies. Newton’s law of universal gravitation says that every particle attracts every other particle with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres. Einstein’s equations of general relativity also suggest that the force bodies experience due to gravity decreases as distance increases. Similarly, the speed with which an object orbits another - the orbital velocity - decreases as distance between the objects increases.
This effect is measured in our solar system. Mercury orbits the sun at 47.9 km/s, Earth at 29.8 km/s, and Neptune at 5.4 km/s. And yet, when one measures the orbital velocities of stars around a galaxy’s centre of mass, they do not drop with distance, as expected, but stay at a high constant. Instead of a nice sloping rotation curve, the rotation curve is flat. This was first noticed by Vera Ruben in 1968 about the Andromeda Galaxy, but has since been observed about many other galaxies.
The number of massive objects in a galaxy clearly decreases with distance from its centre, so for the outer objects to orbit so fast, there must be another source of mass which interacts with the gravitational force, and it must be embedded in a halo around the galaxy in order to produce the observed rotation graphs. This source of mass is dark matter.
It is, however, Fritz Zwicky who is best known for discovering dark matter in 1933. It should be noted that he was not the first astronomer to detect gravitationally affective, undetectable matter, nor the first to use the term ‘dark matter’, but his work was some of the most influential in the field.
While studying the movement of galaxies within galaxy clusters - structures of up to thousands of galaxies held together by gravity - Zwicky noticed that some galaxies moved faster than their mass would indicate. Higher mass means higher velocity. He applied the virial theorem, a set of mathematical calculations, to galaxies within the Coma Cluster. Their predicted velocities were significantly lower than their observed velocities. He then stated in a paper that ‘if this would be confirmed, we would get the surprising result that dark matter is present in much greater amount than luminous matter’.
Another convincing piece of evidence for dark matter comes from gravitational lensing.
Space-time is like a four dimensional fabric composed of three spatial dimensions and the one dimension of time. This unification of space and time was integral to Albert Einstein’s theories of relativity. Matter, as mass-energy, curves space-time. Almost like how a 3D object deforms a 2D bedsheet, a 3D object deforms the 4D curvature of space-time.
A large, massive star would curve space-time more than a small nearby planet. The planet thus follows the curvature towards the star. We observe this as gravity. This space-time curvature in general relativity is represented by its gravitational fields, and the relativistic fields are very similar to those of classical mechanics, except in how the path of light also follows space-time curvature.
After that crash course in relativity, you’re equipped to understand gravitational lensing. A gravitational lens may occur when light from distant galaxies is distorted and magnified by the gravitational field of matter in front. The matter must be sufficiently massive to produce distortion, for example by a galaxy cluster, and the amount of lensing observed can be used to calculate the mass of that cluster.
The bullet cluster, 3.8 billion light years from Earth, is one of the most important case studies for gravitational lensing. It was formed when two galaxy clusters collided. Individual galaxies passed through each other but intergalactic gas collided, heating up and emitting observable x-rays which told us the normal matter distribution in the Bullet Cluster. But when we used gravitational lensing data to reconstruct where most of the mass lies, it wasn’t with the central matter, but alongside the galaxies. Thus, there must be dark matter. This dark matter would pass through each other during the collision, unlike the regular gas clouds which interacted. And this evidence for dark matter has been observed in the lensing of other galaxy clusters too, like CL 0024+17.
There’s plenty of data which seems to require dark matter to explain. The final puzzle pieces we’ll consider examine the cosmic microwave background (CMB) and primordial nucleosynthesis.
The CMB is residue radiation from the heat of the Big Bang. Small temperature fluctuations (called anisotropies) in the CMB can be organised into peaks and troughs, and some of the peaks imply that 5% of the total energy density of the universe is ordinary matter, while 26% is dark matter.
Lastly, primordial nucleosynthesis refers to the production of non-ordinary-hydrogen nuclei at the beginning of the universe. The predictions of helium-4, deuterium and Lithium-7 abundances only match recorded data if ordinary atoms make up only 5% of the universe.
Like a detective sleuthing across the cosmos, picking apart clues in galaxies and galaxy clusters, analysing the way light bends and the speeds that stars spin, we know of a culprit: dark matter. The next step is to put a face to the name. Rather, a particle to an elusive idea. Ultimately, ‘dark matter’ is a placeholder for the unknown. Something is out there, and here, and everywhere, in huge quantities, but it’s not in the Standard Model of Particle Physics.
Let’s investigate the candidates for dark matter.
Astronomers have steadily eliminated most of the ordinary matter options, like unusually dim stars, leaving strange theoretical particles that quantum physics predicted. Most dark matter is thought to be composed of these subatomic particles.
Firstly, weakly interacting massive particles, aka WIMPs. They’re weakly interacting because they interact only with gravity and the weak force. ‘Massive’ means they have high relative mass. They’re also called cold, not because they appear during snow, but because they move at speeds significantly below the speed of light. Mathematical modelling shows that if WIMPs exist, there must be around five times more WIMPs than normal matter, which is the same abundance of dark matter we need. Moreover, we should be able to detect them through their collisions, which would produce observable light through the recoil of charged particles on Earth. We haven’t detected anything so far.
Axions are similarly promising. Their properties match dark matter: they have low mass, do not emit much light, rarely interact with normal matter, and have no charge. Another hypothetical elementary particle, axions were postulated in 1977 to resolve a problem in quantum chromodynamics, but if they exist with a low mass within a specific range, they could also solve the identity crisis of dark matter. If axions do exist, they could decay into a photon pair, providing an opportunity for detection, but no such detection has been made.
Neutrinos are part of the Standard Model and have been detected. They are fermions which interact only via gravity and the weak force, and they have no charge, so they used to be the best candidates for dark matter. Unfortunately, neutrinos are ‘hot’ in that they move with speeds close to that of light, so neutrinos cannot explain all of dark matter. They are too fast to allow halo formations around galaxies. We need cold dark matter to explain the gravitational formation of (relatively) small structures. Sterile neutrinos, hypothetical versions of neutrino that interact only with gravity, are heavier than standard neutrinos, could be responsible for warm dark matter, but are still more reactive than would be ideal.
Next we have massive compact halo objects, known as MACHOs. These are not particles, and are composed of ordinary matter, but emit little light. The classification includes neutron stars, brown and white dwarfs, planets like Jupiter and primordial black holes. However, analysis of a star cluster in the Eridanus II satellite galaxy by Timothy Brandt suggested that dark matter cannot be made of MACHOs with over five solar masses. MACHOs are probably not the dominant form of dark matter.
Finally, there is the Kaluza-Klein particle, predicted by a theory of a fifth-dimension, which could interact via gravity and electromagnetism, and very light forms of the hypothetical gravitino, a supersymmetric superpartner of the hypothetical graviton.
So many options: WIMPs, axions, neutrinos, MACHOs, Kaluza-Klein particles and gravitinos. This is not an exhaustive list. But no candidate has enough evidence to be accepted as the primary form of dark matter. In fact, dark matter is likely made from multiple sources, each specific to the form of dark matter and its cosmological effects.
On the other hand, there are some cosmological observations which particle dark matter cannot explain.
For example, we know our universe to have uniform density throughout, with slight fluctuations at small scales (like an individual galaxy) but reliable consistency, with a precision of 0.01%, on the largest scales. Simulations of dark matter halos predict density maximums in the centre incompatible with universal density uniformity and with our observations, especially for low-mass galaxies. This problem is known as the core-cusp problem, and it’s built on a process called violent relaxation.
It has been a pressing issue for dark matter for a while. Until 2018, when Anton Baushev and Sergey Pilopenko published a paper asserting that the density cusps are numerical artifacts - aka technical problems - of the simulation, rather than a result of actual violent relaxation.
Perhaps a more convincing counter-argument to the existence of dark matter comes with Renzo’s Rule. Renzo’s Rule says that every feature in the curve for the visible emission of a galaxy, where features include wiggles or bumps, corresponds with a feature in its rotation curve. This is an observational correlation. However, the relation does not make sense if most of the universe’s matter is dark, because dark matter would remove any correlation between luminosity and rotation curves.
Scientists have proposed other problems with dark matter and named phenomena that it does not explain. Despite all the evidence for dark matter, there is discourse over whether particulate, undiscovered matter is actually responsible in the way many believe. Indeed, no predicted particle has been detected. This is disappointing, especially given the scale of experimental effort to understand dark matter.
If not dark matter, then what? What causes all the strange cosmological observations?
Perhaps it is simply that our version of gravity is incorrect.
In science, when unexpected observations are made, the scientific community creates a variety of theories which battle for acceptance. One may be highly popular - like dark matter - but that does not mean it is the absolute truth.
Einstein’s equations of general relativity describe gravity over relatively small distances, of up to several light years, with an amazing degree of accuracy, but galaxies can have radii of 50,000 light years, and galaxy clusters can be millions of light years wide. Perhaps there’s not another source of gravity, but just that gravity works differently over very large distances.
This is what Mordehai Milgrom proposed in 1983 with Modified Newtonian Dynamics (MOND). He noticed there was a critical value of acceleration for all galaxies, above which there was no need for dark matter, and so introduced an interpolating function and new fundamental constant. This meant that at high acceleration, gravity decreased by 1/r^2, but at low accelerations, gravity became greater than that.
MOND fits observations of velocities within and of galaxies very successfully. Unfortunately, it does not explain the motion of galaxy clusters, like the Bullet Cluster. Nor does it explain the cosmic microwave background fluctuations and events of the early universe.
It does not necessarily have to be one or the other: particle dark matter or modified gravity. Both can be true. The theories work better in different situations, and so can be used when studying different cosmological phenomena. If this is the case, however, scientists need to find reasons why there exists two sets of equations, and mathematics describing how the equations transition when the situation changes.
This episode we became detectives, carefully analysing the evidence for dark matter. This evidence is remarkably strong. It includes the orbital velocities of stars within galaxies, the velocities of galaxies within galaxy clusters, and gravitational lensing by galaxy clusters like the Bullet Cluster. There are challengers to particulate dark matter theories, such as Modified Newtonian Mechanics, but none are truly satisfying. Neither are the proposed identities of dark matter. WIMPs, axions, sterile neutrinos, MACHOs - a variety of theoretical particles and known objects, each with its triumphs, flaws and connections to qualities of dark matter.
I believe that dark matter is one of the most beautiful ideas in modern cosmology. It is so fundamental, so important to the construction of our universe. It enabled the production of the most basic elements. It makes the galaxies and their stars follow their dances across the cosmos. And it exists in much greater quantities than the ordinary matter we are so accustomed to.
On the other hand, it remains so mysterious. Almost unknowable. There yawns a disconnect between dark matter’s importance and our comprehension of it.
Every question we ask leads to more mysteries, more unknowns, more questions. And if, incrementally, we edge closer to an answer? Maybe that is reason enough to keep asking.
Thanks for listening.
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