Our Universe, the Movie! – Fellows’ seminar by Romeel Davé

10 April 2024

“This is a fast-moving field,” said Romeel Davé of the Institute for Astronomy at the University of Edinburgh and Extraordinary Professor of Physics and Astronomy at the University of the Western Cape. “We are trying to answer the big question humans have asked since the dawn of time: where did it all come from?”

It’s a question that spans religion, culture and mythology and as “we are in the ancestral lands of the San”, Davé started with one example – the story of an angry young girl, in her first menses, scooping up the ashes of a fire and flinging them into the sky to create the Milky Way. “It’s one example of many such stories,” he said. “But not terribly scientific. We are aiming to put together a scientific verification by putting an entire universe into a computer.”

“The question of our cosmic origins is as old as humanity,” he continued. “Today, numerical cosmologists are trying to answer it by simulating universes on a computer, allowing us to run numerical experiments to test the input physics, interpret observations from our latest telescopes, and develop a holistic framework for understanding the evolution of our universe from the Big Bang until today. Creating scientifically motivated movies of our universe’s evolution has reached new levels of maturity within the past decade, where we can now simulate the beautiful diversity of objects seen from the earliest cosmic epochs to today.”

STIAS Fellow Romeel Davé

He described the field of modern cosmology as having been born in the 1920s with new technologies utilised by US Astronomer Edwin Hubble who first verified that the universe extended beyond the Milky Way to incorporate many distant galaxies. Using Doppler shifts, he was able to show that these were far away and fast moving. This work, alongside that of Belgium Jesuit Priest Georges Lemaître, formulated the idea of a continuously expanding universe where galaxies are moving at different speeds. The Hubble-Lemaître law (originally the Hubble Law) provided the first evidence that at the beginning of the universe everything was initially in one place and that dynamic change (or a Big Bang) caused the universe to keep expanding and moving further apart. This law is considered the first observational basis for cosmic expansion.

“Think of raisin bread,” said Davé. “Each raisin is a galaxy, and as the bread rises they move further apart but no individual raisin thinks it’s moving. All galaxies are doing this and ours is not at the centre. This made us realise you could study the universe by studying the objects across the sky.”

This work alongside many others, including James Peebles who won the 2019 Nobel Prize in Physics, made it clear that to understand the evolution of the universe you had to understand galaxies.

“Galaxy formation is the largest field in astrophysics and stands at the crossroads of many areas,” explained Davé. “But you must be a bit of an ‘astro-dilletante’ – understanding, among other things, cosmology, star formation, stellar evolution, heavy element dark matter, black holes and feedback.”

The current estimate is that about 70% of the universe is made of dark energy, a quarter is dark matter, and only about 5% is visible or normal matter. This includes everything made up of elements in the periodic table including galaxies, stars, planets and us.

“We are the 5% normal matter,” said Davé, “anything we can see that is made of atoms comprises the 5%. The rest of the universe is dark matter with 70% of this dark energy – no one really knows what is going on there but it causes the universe’s expansion to be speeding up.” (The 2011 Nobel Prize in Physics was awarded to Saul Perlmutter, Brian Schmidt and Adam Riess for their work confirming the accelerating expansion of the universe through observations of distant supernovae.)

Simulating models – the cosmos in a computer

Of course, this work is not as simple as doing an experiment in a laboratory on earth so it’s reliant on computer-based models – “in the cosmos you can’t control the experiments”.  The field has grown hugely resulting in several such simulations which Davé describes as being based on “best guesses” and “putting everything relevant we currently know into a computer”. Different types of telescopes ranging from radio to gamma ray (with the most well-known ones being the James Webb and Hubble telescopes) see different objects at different times and a computer-simulation model aims to bring all this data together into a cohesive story.

Davé group’s simulation called Simba is a state-of-the-art suite of galaxy formation simulations for exploring the co-evolution of galaxies, black holes and intergalactic gas (Simba Simulation Repository (roe.ac.uk)).

Most of this work uses as starting point the Baby Universe – a heat image of the universe released in 2013 drawing on data from the European Space Agency’s Planck satellite. The map, a constellation of red and blue speckles, captures the imprint of the first light that shone through the cosmos 380 000 years after the Big Bang or 13.8 billion years ago and shows the likely points from which stars and galaxies grew.

Using stunning movie clips from Simba, Davé showed some of the findings of ongoing simulation work including the fact that there are two types of galaxies in the universe – elliptical and spiral with the spiral ones forming new stars; that galaxies are not serene objects with collisions and mergers the norm often leading to black holes – “although the Milky Way hasn’t had a major collision in over 8 billion years”; the role of gravitational forces in bringing galaxies together and all the glorious ever-changing interlinkages and strands of the Cosmic Web.

He explained that the work has revolutionised our understanding of how galaxies like the Milky Way grow and evolve, regulated by a complex interplay between dark matter establishing the Cosmic Web, the formation of stars that go supernova and drive powerful winds, and supermassive black holes that drive a continual exchange of matter and energy within large-scale cosmic ecosystems.

“The current model is that eventually the universe will accelerate expansion faster and faster until it runs out of fuel and dies down.” But all of this is many, many years away!

Enter MeerKAT

Davé’s work in South Africa is about bringing Simba and the MeerKAT telescope together to learn more. MeerKAT is a radio telescope, completed in 2018 and situated 90 km outside the Northern Cape town of Carnarvon. It consists of 64 interlinked antennae with plans to expand this by another 16. It’s the most sensitive radio telescope in the Southern Hemisphere and the precursor of SKA – the Square Kilometre Array telescope – which will eventually incorporate 200 dishes, involve other African countries and be the world’s largest radio telescope.

“MeerKat is very interesting,” said Davé. “People will remember it. Radio observations from MeerKAT are playing a role in elucidating our cosmic origins story. It has elevated South Africa to a global player as the world’s most advanced interferometric radio array capable of detecting gas and black holes in galaxies over most of cosmic time.”

Davé is involved in several MeerKAT projects for which he develops and employs simulations to contextualise and connect MeerKAT’s radio observations with other multi-wavelength data from X-rays to infrared. “My STIAS project employs Simba simulations to elucidate what MeerKAT data tells us about some key mysteries such as the role of supermassive black holes and tenuous atomic hydrogen in shaping the universe.”

And asked why all this work is important, Davé responded: “Curiosity – the question ‘why’ was not invented by science, all children ask it. It’s our natural inclination to know why. It’s a never-ending story. Where we came from and where our place is in the universe sparks the human imagination. I can’t believe I’m paid to do this.”

And will humans find another habitable planet? “There are a millions of galaxies with billions of stars. Although the conditions for life on earth are very specific and we probably don’t understand life well enough yet, it’s probable that there will be one.”

“With this work we as humans have a chance to understand the whole observable universe,” he concluded.


Michelle Galloway: Part-time media officer at STIAS
Photograph: Ignus Dreyer

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