How to Design a Universe, by Professor Catherine Heymans
Members' Presentation: Under the Shadow
Frederik opened the evening by welcoming members and guests, and thanked the refreshment team and the AV team for their work in making the meeting run so smoothly.
Our members' presentation came from Yvonne Jacobs, who had the room in fits of laughter with her account of a lifetime spent chasing eclipses. Her first attempt was in 1999, when she travelled to Penzance for the eclipse, only to find the sky completely covered in cloud.
Yvonne went on to describe a clouded-out annular eclipse in Scotland in 2003. Her persistence finally paid off in the Faroe Islands in 2015, when a gap in the clouds allowed her to see totality. Most recently, she travelled to northern Spain for the total solar eclipse on 12 August 2026, enjoying clear skies.
Frederik hosting the lecture, and Yvonne gives her short talk on eclipses. Pictures by Mike Meynell and Paul May.
How to Design a Universe?
Our main lecture was given by Professor Catherine Heymans, Professor of Astrophysics at the University of Edinburgh and Astronomer Royal for Scotland. Catherine is the first woman to hold the title in its 200-year history, and her research investigates the invisible dark matter and dark energy across the Universe.
Catherine began by describing her “Ask the Astronomer Royal Anything” sessions with schoolchildren across Scotland, where the most common question from younger pupils was about the protective mask on her face. Her own children, keen to delay bedtime, ask trickier questions, such as why the sky is blue. The question behind this lecture came as she was setting off on a two-week observing trip to ESO’s Paranal Observatory in Chile, when her son asked whether we could be living inside a computer game.
The idea that our reality might be an artificial simulation is known as the simulation hypothesis. It is most closely associated with the philosopher Nick Bostrom, who argued in 2003 that if advanced civilisations can and do run large numbers of simulated worlds, simulated beings would vastly outnumber unsimulated ones. Catherine outlined Elon Musk’s claim that, on this reasoning, there is only a one-in-billions chance that we live in “base reality”.
Professor Catherine Heymans speaks to the Flamsteed. Pictures by Mike Meynell and Paul May.
Catherine’s approach to the question was mathematical. She described mathematics as a language: like English it has rules, but it is the language we use to describe our observations of nature and, by following its rules, to predict what will happen next. Astronomers rely on it because we cannot poke and prod the Universe; we can only observe it and model it. Anything that can be described mathematically can, in principle, be programmed into a computer.
Scientists already do exactly this, and Catherine showed a number of examples:
Galaxy formation: computer simulations that grow realistic populations of galaxies from the early Universe to today.
Dark matter and dark energy: Catherine’s own simulations of how the invisible components of the Universe shape its structure.
The early Solar System: a simulation of what our Solar System may have looked like 4.5 billion years ago.
Particle physics: simulations of quarks, the building blocks of protons and neutrons.
Catherine described these simulations as running on grids at very different scales. If the Universe is truly mathematical, could we code all of it?
Catherine then asked how we would know if our own reality ran on a grid. She compared our vision to a digital camera, which builds an image from pixels: our eyes do the same with millions of light-sensitive rod and cone cells. Our sense of time is similarly limited. A television refreshing around 60 times a second looks smooth to us, but a dog, which perceives faster flicker, may notice flicker that we miss. Information also has a speed limit. Just as thunder reaches us after the lightning, light takes time to reach our eyes, and Einstein’s theory of relativity tells us information cannot travel faster than light. Catherine suggested this limit on how quickly information crosses the “grid” makes our experience surprisingly similar to virtual reality, although this is not evidence that we are living in a simulation.
To code a universe we would need a grid and quantum fields. The fields and their interactions are summarised in the equation of the Standard Model of particle physics, our best description of the fundamental particles and the forces between them (except gravity), which Catherine showed printed on a T-shirt. As she pointed out, astronomers and particle physicists are not always friends, because the T-shirt is not quite right.
The Sun is powered by nuclear fusion, which produces enormous numbers of neutrinos: tiny, almost massless particles that pass through matter almost unhindered. Catherine described the Kamioka Observatory, run by the University of Tokyo, where the Super-Kamiokande detector sits deep underground in a vast tank of ultra-pure water, catching the faint flashes of light produced on the rare occasions a neutrino interacts. There are three types of neutrino (electron, muon and tau), and the original Standard Model assumes they are massless and cannot change from one type to another. Experiments have shown that they do, oscillating between types as they travel, which means they must have mass. The discovery was recognised with the 2015 Nobel Prize in Physics, and the T-shirt equation is missing it.
The bigger gap is the dark Universe. Our current understanding is that the Universe is made up of roughly:
5% ordinary matter: everything made of atoms, including stars, planets and us.
27% dark matter: matter we detect only through its gravity.
68% dark energy: the unknown cause of the accelerating expansion of the Universe.
Catherine maps dark matter by measuring how its gravity bends the light from distant galaxies, and colours it pink on her maps because it is her favourite colour. After 20 years of research she can tell us where dark matter is and where it clumps, but not what it is, and nobody yet knows what dark energy is either. Without the full rules for 95% of the cosmos, we cannot yet code it completely.
Catherine then gave reasons to doubt that we are inside a game:
Why watch? As atheist scientists have asked of a creator, if someone built a universe, why would they sit back and watch the laws of physics play out rather than get involved?
The energy cost: simulating everything from the beginning of the Universe through evolution to the present day would take so much energy that a future civilisation might decide it was not worth it.
A cheaper option would be to simulate just a “bubble” around one person. Catherine recalled René Descartes’ “I think, therefore I am”: our own thoughts are the one thing we can be sure of. Even a bubble would need a great deal of energy, although an AI might be able to generate convincing surroundings for someone. As for why anyone would do it, neuroscientists working in computational neuroscience might, as a way of answering the biggest question in their field: what is consciousness? Catherine concluded that she cannot prove whether the Universe is real or a simulation, although she considers the latter highly unlikely.
As a parting gift, Catherine described straying into the realm of philosophy. She wrote to Professor David Chalmers, the philosopher known for framing the “hard problem” of consciousness, with her questions. He replied “Dear Your Celestial Eminence”, and the two went on to discuss over Zoom what it means to be real. His answer was that if you feel real, then you are real.
A further thought, discussed during the questions afterwards: if only a bubble needs to be simulated, nothing outside it needs to be generated until it is encountered, a cosmic version of the old question of whether a tree falling in an empty forest makes a sound.
Q&A at the end of the lecture in front of a packed lecture theatre. Pictures by Manish Patel, Poly Philippou and Simon Hurst.
This was a thought-provoking and hugely entertaining lecture, delivered with warmth and humour, and we really recommend you view it online. Catherine’s book, How to Design a Universe, explores these ideas in more depth, and all net royalties go to the Royal Observatory Trust to support astronomy youth projects across Scotland.
Frederik closed the evening with a look ahead to the Society’s upcoming events, details of which can be found on the FAS website.
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