Is the Universe finite or infinite? Is it unique? Why does it allow life to exist? Is there intelligent life elsewhere? These are some of the questions at the heart of the Colloquium entitled ‘The Nature of the Universe: What We Know and What We Don’t Know’ which will be delivered by cosmologist George Ellis, Emeritus Professor at the University of Cape Town and Fellow of the Royal Society, on 18 November at 3.30 pm in the ‘Paolo Budinich’ Main Lecture Hall at SISSA.
The Colloquium forms part of a day dedicated to celebrating the centenary of the birth of Dennis Sciama, one of the founding fathers of modern cosmology and mentor to great scientists, including George Ellis himself, Stephen Hawking, Martin Rees, Brandon Carter and David Deutsch. Sciama was director of the astrophysics group at SISSA from 1983 to 1998 and play a key role in the growth and international recognition of this field at the School. Many of the research topics currently being pursued at SISSA – ranging from cosmology to astrophysics to gravitational astroparticle physics – are the natural continuation of a cultural tradition he established, both in terms of subject matter and research methodology.
To mark this anniversary, SISSA has revamped the exhibition ‘The History of the Universe at a Glance: 13.8 billion years in 130 metres’, which had previously been on display in the corridor on the sixth floor of the institute named after him. Susan Sciama, one of the scientist’s daughters, will also be taking part in the celebrations.
SISSA Colloquium "The nature of the Universe: What we know and what we don’t know"
Abstract. Cosmology is a science based in known physical interactions and observations of astronomical objects (galaxies, radio sources, quasars, pulsars) and, crucially, the power spectrum and angular power spectrum of the Cosmic Microwave Background Radiation. They have revealed the vast scale and age of the Universe. The universe has a simple geometry: on very large scales, it is both spatially homogenous and isotropic. Consequently, we are not at the centre of the Universe: there is no centre. The universe is evolving geometrically and physically. Inflation in the very early universe is followed by a Hot Big Bang phase, decoupling of matter and radiation, growth of structure (galaxies, stars, and planets) and emergence of life. The Universe will probably expand forever.
Three major unknowns are the nature of the Inflaton, Dark Matter, and Dark Energy (Dark energy proposals are unphysical if they claim w < -1). There are tensions: The Hubble Tension, the clustering parameter tension, the dipole tension, and apparent occurrence of galaxies and black holes at too early times. There is also a Cosmological Constant problem – the vacuum energy density predicted by quantum field theory is wrong by 120 orders of magnitude. This problem is solved if the underlying theory of gravity is unimodular gravity.
There are a series of effective horizons restricting what we know: the Last Scattering Surface (LSS) prevents seeing earlier times, the visual horizon limits what we can observe, the physics horizon limits the energies we can test. We don’t know if the Universe is spatially finite or infinite, or if it has an exotic spatial topology. We don’t know if the Universe had a beginning (one possibility is there was a bounce from a previous collapse phase). If it did have a beginning, what happened “before” is not described by science: physics did not exist then.
The ultimate underlying issue is the Uniqueness of the Universe: we can’t do experiments on other universes or observe them - if they exist. Consequently, we cannot establish laws governing the existence of universes, why the universe exists, and why it has the nature it has. An important question is, Why does the Universe allow life to exist? (the “Anthropic” question). A proposed solution is existence of a Multiverse; but that has significant problems, in addition to being unproveable. Is there intelligent life elsewhere in our Galaxy/Universe? Almost certainly, Yes in both cases. Can we interact with them? No (there are no “wormholes” in spacetime). Finally, No, we don’t live in a simulation.
Biography. George Ellis, Fellow of the Royal Society, is Professor Emeritus of Applied Mathematics at the University of Cape Town. He obtained his PhD at the University of Cambridge in 1964, where he worked on general relativity theory and cosmology. He has been based at the University of Cape Town since 1974, but has been visiting Professor at Texas, Chicago, Hamburg, Boston, Edmonton, London, and Oxford, and for a while, was Professor of Cosmic Physics at the International School of Advanced Studies (SISSA), Trieste. In recent years he has been working on the philosophy of cosmology and the emergence of complexity, the nature of causation, and aspects of neuroscience.
In 1999 he was awarded the Star of South Africa Medal by President Nelson Mandela, and in 2004, the Templeton Prize. His books include The Large Scale Structure of Space Time (1973, with Stephen Hawking), The Moral Nature of the Universe: Cosmology, Theology, and Ethics (1996, with Nancy Murphy), Downward Causation and the Neurobiology of Free Will (2009, Ed, with Nancey Murphy and Tim O’Connor), How Can Physics Underlie the Mind: Top-Down Causation in the Human Context (2017), Beyond Evolutionary Psychology (2018, with Mark Solms), Relativistic Cosmology (2021, with Roy Maartens and Malcolm MacCallum) and How we come to be: Almost everything that leads to our existence (2026).