With the discovery of protons, neutrons and electrons, we thought that we had sussed out the basic building blocks of our universe. We thought it was a done deal. Then came a zoo of other particles from anyons and bosons to tachyons and quarks. Then came the shattering knowledge that this zoo of, mind-bogglingly small, building blocks of our world, only made up 5% of our universe.
Five per cent
5%. We only understand 5% of what the universe is made from – that includes everything that we know and are able to measure on Earth and beyond. That leaves us with just shy of a complete mystery – so what do we know about this missing 95%?
Scientists have proposed 2 parts to this missing matter:
Dark matter – A mysterious substance that makes up 27% of our universe.
Dark energy – An undetermined type of energy that makes up the other 68% .
And there’s our unknown quantum.
Why is this dark matter so elusive?
Dark matter cannot absorb or reflect light or carry an electric charge, hence making it invisible, or at least very imperceptible. This scuppers most standard methods for observation and discovery since we cannot physically see it. By deduction, this also means that dark matter cannot be made from the conventional atomic composition that makes up everything in our known world.
It was initially proposed that those faint, fleeting particles known as neutrinos perhaps contributed to dark matter. Neutrinos do not carry an electrical charge and move at close to the speed of light. Plus, we already have evidence of their existence. Bonus. However, particles moving at such high speeds would not be able to stick around long enough to create a galaxy; and neutrinos are so tiny that they simply don’t have enough mass for the unaccounted 27%. So dark matter is not the evasive neutrinos. Darn.
The four fundamental forces
However, sterile neutrinos are a candidate for dark matter. These hypothetical particles are very much like standard neutrinos but much heavier. They only interact with 1 of the 4 fundamental forces under the standard model of particle physics:
- The Strong Force (the strongest of all forces, works at limited range, holds together atoms)
- The Weak Force (ironically not the weakest force, works at limited range, responsible for atomic decay)
- The Electromagnetic Force (infinite range, attacts and repels charges plus gives substances/ objects their strength and shape)
- The Gravitational Force (infinite range, the weakest force)
Sterile neutrinos in the Antarctic
Sterile neutrinos are proposed to only interact with the gravitational force (since they have mass). They do not interact electromagnetically so are not visible. Scientists have been encouraged that sterile neutrinos are a viable possibility with various evidence hinting towards their existence.
The IceCube Neutrino Observatory in Antarctica comprises of a particle detector which monitors the interactions of neutrinos from ‘violent astrophysical sources: events like exploding stars, gamma-ray bursts, and cataclysmic phenomena involving black holes and neutron stars.’
IceCube detects a neutrino approximately every 6 minutes over a wide range of energies. When neutrinos reach the earth, they interact with matter and crash into the nuclei of atoms causing an emission of light. A secondary reaction produces high speed particles that give off a blue light, called Cherenkov radiation. This is picked up by IceCube’s detectors and gives a golden opportunity to fork out sterile neutrinos.
The theory is simple. A neutrino is more likely to morph into a sterile neutrino if it travels through a very dense region of matter – such as the Earth’s core. To find these hypothetical particles, researchers at IceCube are looking for normal neutrinos that are on the trajectory towards the detectors via the Earth’s core. If a neutrino morphs into a sterile neutrino, it will simply disappear from the detector’s view. It will not reflect or radiate the light needed to be picked up by the detectors. But, IceCube has not picked up any trace of sterile neutrinos – so particle physicists are going to have to rethink the model for sterile neutrinos to fit.
WIMPs being cryogenically cooled
So, that is sterile neutrinos ruled out, for the time being at least. Weakly Interacting Massive Particles (WIMPs) are next on the wanted list for dark matter particles. Like all candidates, their lack of interaction with the ‘normal’ methods of detection (i.e. visibility through electromagnetic forces) makes determining their existence rather tricky.
The theorised WIMPs’ large mass and slow speeds would cause gravitational clumping of masses which could explain some of the stunning structures visible in the universe today.
Only gravity will affect these mass[ive] particles and this could be used to explain the inexplicable way that our galaxies rotate.
Most detectors are set up on the basic assumption that every now and then, a gravitationally-affected particle will bump into an atom of normal matter – making it vibrate and produce a signal. However, how heavy the dark matter particle is and how fast it moves will affect how it interacts with the visible matter.
Scientists involved with Super Cryogenic Dark Matter Search (SuperCDMS) have reported a breakthrough with a WIMP like signal using ‘cryogenically cooled (-273°C) germanium and silicon targets to search for the rare recoil of dark matter particles’.
The results are looking good but they are only 99.8% sure. They need to be 99.9999% to confirm the discovery.
Macros mean known matter. But bigger…
In the meantime, some scientists are pondering whether dark matter is simply composed of ‘large’ chunks of normal matter; an assemblage of quarks, known as macros. The theory is that these macros would have a very dense, high percentage composition of strange quarks; the highly unstable, extremely light variety of quark that is observed in high-energy collision experiments.
These clumps of strange quarks must be larger than 0.05 kg, otherwise they would have been picked up by particle detectors (bear in mind that on a universal scale, 0.05 kg is pretty small!). They must also be smaller than 1 billion trillion kg because otherwise the gravitational field would warp starlight – which has not been observed. Unfortunately, experiments needed to scour out macros, such as placing seismometers on the moon, have not managed to secure funding.
Gravitinos: a supersymmetric solution
Moving from a more tangible dark matter theory to one of the most speculative: the gravitino theory. The gravitino is the proposed ‘super symmetric partner’ of the hypothetical particle, the graviton. Gravitons are a speculated ‘force carrier particle’ mediating gravity for the fundamental gravitational force. Just like photons mediate light for the electromagnetic force.
Now, gravitons are expected to be massless and, like photons, should travel at the speed of light. Supersymmetry describes a partner particle for each of the carrier particles in the standard model:
- Strong force is carried by ‘gluons’.
- Electromagnetic force is carried by ‘photons.
- Weak force is carried by ‘W and Z bosons’
- Gravitational force is carried by (yet to be discovered) ‘gravitons’
For each of these carrier particles, there is predicted to be a super partner particle.
Supersymmetry – a physicist’s dream
Gravitino theory encompasses the supersymmetry principle. Currently, comparing equations for force and equations for matter is like comparing apples and oranges.
Supersymmetric principle dictates that there must be a matter particle (fermion) that equals a force particle (boson), and vice versa, for each of the known bosons and fermions.
Supersymmetric theories can explain many of the unanswered questions in physics including what dark matter is made from and why gravity is so weak. But, it relies on the existance of matter particles (fermions) that are equivalent to force particles (bosons) and vice versa.
Supersymmetry has incredible popularity amongst physicists with over 10,000 research papers devoted to it. Mathematically, supersymmetry provides a single unified explanation for the fundamental forces of the universe – something that many physicists yearn for. The properties of a gravitino would fit the dark matter model beautifully. The only problem is, we have yet to find one!
Everything we know is wrong? Modifying the classic laws of physics
The theory that makes many physicists shift around uncomfortably – Modified Newtonian dynamics (MOND). Quite simply, MOND is an adjustment to the classic Newtonian Laws of Gravity to take into account the discrepancies seen at large scales, such as in galaxies. These discrepancies inspired theories about dark matter in the first place. Essentially, MOND poses that it is not, in fact, dark matter that is causing mass discrepancies, but an ‘incomplete understanding of gravity on these scales’.
Essentially, the velocities of stars within galaxies are larger than they should be if using classic Newtonian equations to calculate velocities and mass. Newton’s laws have been extensively tested and confirmed within the environments of the Earth and solar system, but further afield, our Newtonian laws don’t give the correct answers. Using MOND, many of the inconsistencies seen within our universe can be explained.
Yet, there is a reason that the case is not closed. The current problem for MOND is that it still doesn’t completely eliminate the need for dark matter. Galaxy clusters still show mass discrepancies and exhibit gravitational lensing which indicates unseen mass bending the light. So far, this cannot be reconciled with MOND without the addition of dark matter. Although MOND may simplify some of the questions, it has not yet eradicated the need for dark matter.
Do we know anything at all? An era for discovery.
We are on the cusp of an historical period for particle physics and discovery. We have explored the Earth many times over but now we are looking in places impenetrable to the eye and that – that is very exciting.
If the composition of dark matter is determined, the discovery doesn’t stop there. If you think of how much we know about the mere 5% of ‘conventional matter’ – think how much there is to understand about the remaining 95%.
This unknown is inspiring – we don’t know the answers so who knows what these discoveries could reveal, what doors they could open. Perhaps dark matter will explain the big unsolved mysteries on Earth – unexplained sightings, other dimensions, supernatural occurrences and maybe even divine entities.
Nothing is ruled out. This is just the beginning.