On Thursday 19 January, the Feynman Society had the pleasure of welcoming Dr Veronica Sanz (University of Sussex): Review by Matteo Bonsignore (Year 13):
Almost a century ago the known universe was believed to be made up mostly of atomic matter, particles that often bond together to form larger objects; yet, observations of stars’ orbits suggested that this matter only accounts for a very small proportion of the universe’s energy. Where was the rest coming from? Charterhouse pupils have recently had the privilege of hosting Dr Veronica Sanz from the University of Sussex to receive an answer to this question and to better understand what really lies in the ‘Dark Side of the Universe’.
Dark matter, which makes up about a quarter of the universe’s mass-energy, has been described by many as one of the biggest mysteries of physics. Contrary to luminous matter it is invisible and today it can only be measured through its gravitational effects. Newton’s laws of gravitation suggest that celestial bodies move slower when farther away from the object they are orbiting; however, physicists observed that celestial bodies in the Andromeda galaxy all moved at approximately the same velocity. This was the first suggestion that there was something other than luminous matter in the universe.
To understand this better, Dr Sanz directs our attention to the history of the universe. It is believed that the universe started with an explosion, the Big Bang, which caused it to expand quickly; as the expansion slowed down, the dark matter particles moved slowly enough for their gravitational attraction to join them into clusters, creating a gravitational well into which luminous matter then fell into. Galaxies and clusters formed, allowing the existence of planets that sustained life. The idea of invisible matter may seem abstract in the context of our daily lives, but without it we would not even exist. It even surrounds us wherever we go. So where do we see it? When dark matter forms large enough clusters, light passing near these clusters is ‘bent’, just like light passing through a lens – this is what we call gravitational lensing.
Dark matter can also hopefully be formed in CERN’s Large Hadron Collider, where protons are accelerated to high velocities, gaining kinetic energy, and annihilate with each other to hopefully form dark matter. It is almost as if kinetic energy has been ‘transformed’ into mass. In fact,E = mc 2 (where E is energy, m is mass and c is the speed of light): as Einstein tells us, mass and energy are actually equivalent to one another.
The remaining three quarters of the missing energy of the universe can be found in an even more ‘obscure’ concept: dark energy. Einstein defined it as “the biggest blunder of my career”, and in fact physicists still do not fully understand it today. According to Dr Sanz, dark energy can be visualised as the ripples caused by a disturbance in water: particles are continuously created and destroyed in a vacuum, according to quantum physics, and the vacuum energy generated from this process causes disturbances that accelerate the expansion of the universe, just like in water. Still, predictions from quantum field theories do not agree with observations on dark energy.
“Why do we care?” Dr Sanz finally asks her audience. History has the answer: trying to explain the unknown has revolutionised physics various times and led to progress for humanity – quantum field theories are the most pertinent example of this. Maybe, understanding what lies in the dark could help physicists do the same.