Before the Big Bang
Professor Jeff Forshaw
6th October 2017
Jeff Forshaw is a professor of theoretical physics at the University of Manchester, specialising in the phenomenology of particle physics. He is passionate about communicating the excitement of the pursuit of science, earning the Institute of Physics Kelvin Prize (2013) for “…outstanding contributions to the public understanding of physics”, whose previous winners include Brian Cox and Jim Al-Khalili.
Jeff’s involvement in cosmology, apart from its need for particle physics expertise, was sparked, he revealed, by agreeing to give a course to 4th year students in Manchester. He began, though, in way that could have been entitled, “How do we know that?” by walking us through some examples of how we corroborate our ideas by a process of prediction and measurement: how big is the Earth? how old is the Atlantic Ocean? and the Earth? and the sun? until some of them become “nailed-on facts”, as he put it. In that category is the Big Bang. The “archaeological evidence” from many independent lines of prediction and observation lead us to think with an overwhelming degree of certainty that the observable universe we see about us was once a very hot dense gas of particles in a volume no bigger than the Waterfront Hall. Already, at that time, it had slight fluctuations of density whose slightly higher density portions gradually collapsed under gravity to form galaxies in filaments and voids observed today. It expanded into the present state of affairs in a manner that is entirely predictable and understandable in terms of the properties of particles and gravity that we know about from our laboratory experiments. That hot gas and its subsequent expansion he defined as the Big Bang. It is also called the Standard Cosmological Model. What happened before that he would come to later. The clinching evidence was a “photograph”, as he put it, of the last vestiges of free charged particles (plasma) 380,000 years after the Big Bang, the radiation (photons) from which forms the Cosmic Microwave Background. 90% of the photons from that era have travelled through the mostly empty space of our universe for nearly 14 billion years without collision. They have cooled from a few thousand degrees Celsius to about -270 C (2.725 degrees above absolute zero, to be more accurate) on average due the intervening expansion of the universe, which stretches their wavelengths. From the “colour”, i.e., the energy, of the photons one can infer the temperature of the original plasma; it varies across the sky by minute amounts in a strikingly beautiful mottled fashion whose structure is spectacularly accurately predicted by the Model using measurements and properties from completely independent observations. It would have been so easy to be otherwise. Jeff himself had written a computer program for the 4th year students to make that prediction—simple, he said, compared to predicting the outcome of kicking a bucket of water, for example.
So how did the Big Bang, that hot dense gas the size of the Waterfront Hall with slight density perturbations, come to be? Here, he admitted, we were moving into the realm of speculation, but ideas that were seen as highly speculative 5-10 years ago are, through the process of working out bit-by-bit, predicting and comparing, becoming increasingly accepted as feasible. He postulated the existence of some “cosmic treacle” called the inflaton field. There exists a present-day analogue that surrounds us all the time, the Higgs field that is responsible for generating mass, predicted 50 years ago by Peter Higgs (and others) and whose excitations were recently observed at the Large Hadron Collider at CERN, Geneva. A special property of the inflaton field is that its energy drives a huge inflation of the universe and guarantees certain properties of the present universe, for example “flatness”, that are difficult to explain otherwise. The Waterfront-Hall-sized region of hot dense gas, which eventually became our observable universe, may well have been a region no bigger than a billionth the size of a proton. Some quanta of the field (inflatons), after a period of huge inflation, decay into “ordinary matter”—and the rest is history, as they say. Jeff showed us a computer simulation he had written of the fluctuating field expanding and decaying into particles, then subsequently condensing into galaxies. The inflaton field is subject to quantum fluctuations, which give rise to just the right size of density fluctuations at the start of the Big Bang.
Finally he speculated that inflation is a continuous process and that our universe is just one of many, possibly infinitely many, universes with the same—or possibly different—physical properties. This is the multiverse conjecture. It seems perhaps that the Big Bang and the Steady State Model may be companion theories after all.
Much of this is in his new book, “Universal: A Guide to the Cosmos”, co-authored with Brian Cox.
The audience gave Jeff spontaneous applause for his passionate and entertaining presentation. He fielded a few questions and afterwards was surrounded by inquirers until we dragged him and his partner away for dinner.
With thanks to John Allison for this report
9th October 2017