Hang on, didn’t we read this article last time? No, last edition we learnt about the latest on anti-matter. Today, we’ll be exploring the intricacies of dark matter.
To comprehend the concept of dark matter, one must first understand what matter is. Matter is anything that occupies space and has mass. Generally, to us, matter is made up of atoms. So what about dark matter then?
In the early 1930s, Fitz Zwicky, an astronomer from the California Institute of Technology, discovered the gravitational effects of dark matter while studying the movement of galaxies in the Coma Cluster, a cluster of about 1000 galaxies. Using Newton’s law of universal gravitation (a law that states that an object with greater mass will have greater gravity) and the virial theorem (an equation that relates the velocity of orbiting objects to the amount of gravitational force acting on them), Zwicky was able to calculate the total mass of the Coma Cluster by measuring galactic velocities.
Zwicky also calculated the mass of visible matter in the Coma Cluster by measuring the amount of light emitted by the stars in its galaxies. He found that the mass derived from visible matter was significantly less than the previously calculated total mass. Considering the fact that gravity bends light, he further observed that light would ‘bend’ in places where there was no visible matter (hence no gravity) to ‘bend’ the light in the first place. These observations led Zwicky to coin the term ‘dark matter’ to describe the missing (invisible) matter.
Later measurements of individual galaxies also showed that there just wasn’t enough ‘visible mass (stuff)’ in galaxies to produce the gravitational force required to hold them together. This again pointed to the existence of dark matter.
Currently, the best evidence of dark matter comes from the measurement of cosmic microwave background. This is the radiation left over from the time of recombination in the Big Bang Theory, when the first hydrogen atoms started forming from electrons and protons. Simulations of the Big Bang and galaxy formation showed that without the extra mass of dark matter, galaxies and their constituents would have drift apart due to the lack of gravity (which we have learnt is proportional to mass).
So what is dark matter composed of?
The truth is, we don’t know. But the principle of supersymmetry (SUSY for short) might be able to help.
To start, supersymmetry is a conjectured symmetry of space and time that relates two basic classes of particles which were previously thought to be unrelated. These particles are fermions (particles that make-up matter such as electrons and quarks) and bosons (particles that carry force such as photons and gluons).
For a long time, fermions and bosons were considered to be two separate entities, each described by separate equations, the Fermi-Dirac statistics and Bose-Einstein statistics respectively. As such, we could only explain the relationship between particles of the same class, electrons and quarks, and photons and gluons, but not electrons and photons or quarks and gluons.
But what if we had a single equation that could describe the behaviour of both fermions and boson? And what if there was a single equation that could relate their individual behaviours? What then?
There are two classes of particles: fermions and bosons. Within the fermions there are electrons, (sub-atomic particles with a negative charge), and quarks, (sub-atomic particles with a fractional charge and act as the building blocks for protons and neutrons). Within the bosons there are photons (particles that represent a quantum (portion) of light) and gluons (hypothetical massless sub-atomic particle believed to bind quarks together).
The black arrows describe the relationship between the constituents of each class (fermions and bosons) only. The addition of red arrows (supersymmetry) creates a new relationship that links electrons with photons and quarks with gluons, creating a relationship that binds matter to force.
It was proven mathematically in the 1960s that supersymmetry was the only symmetry that could be added to the existing symmetries of Einstein’s theory without resulting in equations that may be inconsistent with our current world.
So when supersymmetry is implemented into current equations a whole range of new particles will be predicted – particles which cannot be seen normally and will either be very heavy (hence degrade rapidly) or interact with matter abnormally. However, new particles like axions will have to materialise in the Large Hadron Collider (LHC) first. Once they do, we’ll be able to see if dark matter is composed of any of these unseen particles.
Are fireworks going off in your head yet? How about one more dose?
During the early 2000s scientists discovered that the expansion of the universe was neither constant nor decelerating but accelerating. Adam Reiss along with Saul Perlmutter and Brian P Schimdt were awarded the 2011 Nobel Prize in Physics for providing evidence of the accelerating expansion of the universe. This advancement was priceless to other scientists studying the outer reaches of space.
Since the expansion of the universe is accelerating, scientists logically came to two possible conclusions: either gravity behaves differently in outer space or some kind of unknown energy is propelling this acceleration. Most scientists found the latter to be more plausible. They called this energy ‘dark energy’.
According to NASA, 68% of the entire universe is dark energy, 27% is dark matter and everything we perceive as normal matter, including everything observed by our scientific instruments, accounts for a measly 5%.
So for the record: we more or less know what matter is, and once supersymmetry is put to use we can understand dark matter. That only leaves us in the dark for the other 68% of the universe occupied by dark energy.