The nature of the Universe: What we know and what we don’t know - George Ellis - Colloquium

The next SISSA colloquium is scheduled for Wednesday, November 18 from 15:30 to 16:30, in Aula Magna Budinich, SISSA, with Professor George Ellis, FRS, Emeritus of Applied Mathematics University of Cape Town.

The nature of the Universe: What we know and what we don’t know

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, FRS, 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).