A not so dark tale on dark matter

Anwesha Sahu debunks the dark matter myths that are all too often portrayed by Hollywood.

Let’s take a little trip down memory lane to the good old days, before our worlds were rocked with COVID-19. It’s 31st October, 2019—Dark Matter Day. We are sitting in a coffee shop, not two metres apart, and the coffee shop TV shows a trailer for a movie the channel will broadcast in an hour. The name of the movie? Dark Matter.

For the sake of this speculative story infused with physics, you and I are physics students. We have just overheard a couple, Alice and Bob, sitting at the table next to us, say something which sparks our intellects. What they have said has irresistibly drawn us into their conversation.

“Bob, want to watch Dark Matter later tonight?” asks Alice.

“Of course! It will surely be a thrilling sci-fi movie about the extremely dense, exotic, heavy and incredibly powerful, elixir-like black chemical/force that can destroy the world! I’m sure it’ll be full of drama and action—count me in.”

Blaring sirens go haywire in our physicist minds, and the rest of their conversation fades into the surrounding noise. Why did Bob say what he did? Was it sarcasm, or an innocent call for knowledge? My mind has already wandered off into a daze of debunking dark matter myths and before I know it, I’m lost in thoughts of dark matter experiments. I want to give Bob evidence that dark matter is not quite what he thinks it is.

The matter that we can explicitly observe, such as atoms, this magazine, and entire galaxy clusters, are made of ordinary matter. This observable universe makes up a feeble 5% of the total universe. Approximately 27% of the universe is dark matter. The remaining 68% is dark energy—this deserves an article all of its own! Unlike ordinary matter, dark matter is not directly observable. Given the scarcity of information that the physics community has about this elusive type of matter, Hollywood has opportunistically seized this enigmatic physics concept to spice up fiction.

How do we know that dark matter exists? We exist. Let’s take a step back for a moment: this statement implies that the Milky Way is the way it is because galaxy clusters like ours exist in stable rotations without flinging galaxies apart. This allows stars like our sun to form and orbit the centre of the galaxy. Consequently, advanced life has developed on our planet.

The presence of dark matter was inferred in 1933 by Swiss-American astronomer Fritz Zwicky. Upon observing the mass of the stars in the Coma Cluster—a galaxy cluster about 323 million light years away from Earth—he discovered that their combined mass provides a mere 1% of the mass required to prevent the galaxies from escaping into the void. Yet, the Coma Cluster exists. This conundrum lay on questionable grounds until a similar observation was made by astronomers Vera Rubin and W. Kent Ford forty years later. The observed mass of stars in typical galaxies is approximately 10% of the total mass physically required to keep these stars orbiting the galactic core.

Rubin and Ford observed that the orbital velocity of stars is independent of their distance from the core. At first, this sounds counterintuitive; we would expect orbital velocity to drop off with distance. Instead, these velocities are either constant or increase as the distance increases. This oddity can be explained if the mass of the galaxy in the region within the orbit increases linearly along the orbit radius. This mass is ‘invisible’—hence the name ‘dark matter’.

The key to demystifying dark matter lies in studying its interactions with ordinary matter. This is where experimental particle physics comes into play. There is a plethora of classifications for dark matter: hot, cold, heavy, light, self-coupling and intermediary, among others. Combine these with associated theories and you have a mesh of countless possibilities for dark matter interactions. For the sake of this article, we will focus on two key categories: heavy dark matter, and light dark matter.

Heavy Dark Matter Candidates

As suggested by its name, this category’s candidates are heavier than other proposed candidates. This category consists of a class of particles called ‘weakly interacting massive particles’, otherwise known as WIMPs (particle physicists have an impeccable sense of creativity when it comes to acronyms!). Another class of objects which can partially account for the proposed quantity of dark matter in the universe are MACHOs: massive (astrophysical) compact halo objects. This class comprises objects such as neutron stars, primordial black holes, and brown and white dwarfs.

While these constitute ordinary matter, they have the potential to escape detection. If bodies as heavy as Jupiter filled galactic halos, they would remain undetected via light emission and absorption. Likewise, brown dwarfs and black hole remnants from early generations of stars would also slip observation. MACHOs are further classified as typically being baryonic, implying they are composed of the same kind of matter that build up atomic nuclei. WIMPs, on the other hand, are not baryonic.

Light Dark Matter Candidates

These proposed particles are significantly lighter than their heavier counterparts. The key contender here is a particle called the axion. Axions are incredibly light; with a mass between 10-12 eV and 106 eV, they barely interact with matter as we know it.

Given the elusive nature of dark matter and how little we know about its interactions with matter, detecting it requires ingenious, innovative experiments beyond standard detection methods. In a nutshell, particle physics dictates that to study elementary particles and beyond, you smash them together. Similarly, to observe dark matter, the dark matter particle is collided against something. By studying the traces of resulting particle sprays and their energies, one may be able to detect a possible dark matter candidate.

Researchers at the University of Birmingham’s Particle Physics Department in the School of Physics and Astronomy are actively involved in the search for dark matter particles, particularly WIMPs and axions, with the NEWS-G collaboration. NEWS-G (New Experiments with Spheres – Gas) is a novel experimental concept consisting of a spherical proportional counter (SPC). The NEWS-G collaboration consists of researchers all across the globe and the primary experimental set-up is to be installed at the Sudbury Neutrino Observatory (SNOLab), 2 km underground in Sudbury, Ontario, Canada.

Simply put, the SPC is a large copper sphere filled with a noble gas. At the centre of the sphere lies a small, charged anode at a high voltage. The electric field within the sphere is inversely proportional to the square of the distance from the anode. This allows for a split in the region within the vessel—a large drift region and small amplification region a couple of millimetres close to the anode. If a dark matter particle interacts with the gas, the interaction liberates electrons via primary ionisation. The electrons subsequently drift to the positively charged anode in a span of about 100 microseconds. Once they enter the amplification region, the strong electric field boosts the electrons’ kinetic energy, giving them the punch to trigger what is known as a Townsend avalanche. Each primary electron generates thousands of secondary electron-ion pairs. These secondary electrons then drift back to the grounded spherical shell to induce the signal detected.

It goes without saying that successfully detecting dark matter will be no less than a revolution in our understanding of the cosmos. Detecting dark matter means breaking the barrier of conventional particle physics observations. It will be a testament to human ingenuity as well as a glowing reminder of how, through innovation, we can decode the intricate workings of the universe from the comfort of our home planet.

As the friendly waiter serves the hazelnut praline hot chocolates at our table, I step out of my mid-day physics daze. Alice and Bob are no longer sitting next to our table. The trailer for Dark Matter is long gone on the TV. Taking a sip from my mug, we begin chatting about the insights of my daze. Meanwhile, a dark matter particle unsuspectingly sweeps through the four walls around us.

From Issue 21