Distinguished scientist: Prof George Ellis gave a crash course on cosmology.

In trying to pin down the origins and nature of the universe, cosmologists - reams and reams of maths and data notwithstanding - occasionally have to fall back on philosophy and metaphysics, explained Emeritus Professor George Ellis at the Faculty of Science's Distinguished Alumni Lecture, on 16 September.

In his breakneck introduction to the topic, Ellis noted that astronomers and cosmologists have made great strides in their knowledge of the physical universe, from its early Hot Big Bang era - when it was made up of nothing other than hot gas and radiation - to today's expanding universe, crammed with billions of galaxies, each in turn containing billions of stars and planets. Mathematical models and observations - such as those pertaining to the cosmic microwave background, the leftover radiation from the Big Bang - bear out this Standard Model of Cosmology.

But even in these "data-rich" times, there are glaring gaps in the cosmology canon. For example, cosmologists 'know' hypothetically that dark matter and dark energy account for the vast majority of the mass of the observable universe - yet they do not know what dark matter and dark energy are.

There are other scientifically-unanswerable issues - such as how the universe started, and why it has the very special nature that makes it a hospitable environment for life to exist on planet Earth.

"The problem is, you can't do the relevant experiments - you can't rerun the universe, you can't reach the energies and densities," said Ellis.

Instead, cosmologists and mathematicians have to resort to conjecture and speculation to fill these gaps in knowledge. So, for example, some have mooted the idea of a multiverse - a set of multiple universes, all of which exist at once; and each, perhaps, with its own set of physical laws.

But because this hypothesis can't be tested, it's not quite science, argued Ellis. Questions remain, though.

"From now on," offered Ellis, "cosmology is going to have to interact with philosophy much more, and much more consciously, than it has done in the past."