An Infinity of Worlds

8 November 2022

An Infinity of Worlds by Will Kinney, MIT Press 

An Infinity of Worlds

Cosmology, along with quantum mechanics, is probably among the most misunderstood physics topics for the layperson. Many misconceptions exist, for instance whether the universe had a beginning or not, what the cosmic expansion is, or even what exactly is meant by the term “Big Bang”. Will Kinney’s book An Infinity of Worlds: Cosmic Inflation and the Beginning of the Universe clarifies and corrects these misconceptions in the most accessible way.

Kinney’s main aim is to introduce cosmic inflation – a period of exponential expansion conjectured to have taken place in the very early universe – to a general audience. He starts by discussing the Standard Model of cosmology and how we know that it is correct. This is done most successfully and in a very succinct way. In only 24 pages, the book clarifies all the relevant concepts about what it means for the universe to expand, its thermal history and what a modern cosmologist means by the term Big Bang.

The book continues with an accessible discussion about the motivation for inflation. There are plenty of comments about the current evidence for the theory, its testability and future directions, along with discussions about the multiverse, quantum gravity, the anthropic principle and how all these combine together.

A clear understanding

There are two main points that the author manages to successfully induce the reader to reflect on. The first is the extreme success of the cosmic microwave background (CMB) as a tool to understand cosmology: its black-body spectrum established the Big Bang; its analysis demonstrated the flatness of the universe and its dark contents and motivated inflation; its fluctuations play a large part in our understanding of structure formation in the universe; and, along with the polarisation of the CMB, photons provide a window into the dynamics of inflation. Kinney notes that there are also plenty of features that have not been measured, which are especially important for inflation, such as the B-modes of the CMB and primordial gravitational waves, meaning that CMB-related observations have a long way to go.

The second main point is the importance of a clear understanding of what we know and what we do not know in cosmology. The Big Bang, which is essentially the statement that the universe started as a hot plasma of particles and cooled as it expanded, is a fact. The evidence, which goes well beyond the observation of cosmic expansion, is explained very well in Kinney’s book. Beyond that there are many unknowns. Despite the excellent motivation for and the significant observational successes of inflationary models, they are yet to be experimentally verified. It is probably safe to assume, along with the author, that we will know in the future whether inflation happened or not. Even if we establish that it did and understand its mechanism, it is not clear what we can learn beyond that. Most inflationary models make statements about elements, such as the inflationary multiverse, that in principle cannot be observed.

Steven Weinberg once commented that we did not have to wait to see the dark side of the moon to conclude that it exists. Whether this analogy can be extended successfully to include inflation or string theory is definitely debatable. What is certain, however, is that there will be no shortage of interesting topics and discussions in the years to come about cosmology and fundamental physics in general. Kinney’s book can serve as a useful introduction for the general public, but also for physics students and even physicists working in different fields. As such, this book is a valuable contribution to both science education and dissemination.

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