What does it actually mean to say something is “likely” in science?

The Random Universe: How Models and Probability Help Us Make Sense of the Cosmos explores how scientists use probability to draw conclusions about the universe from limited, uncertain data—and what we really mean when we call a theory “likely.”

Get the book: Amazon US | Amazon UK | Yale University Press | Yale UK | Local bookshop

If you read it, a short review on Goodreads or Amazon really helps others find it.

The Random Universe tells the story of how scientists use data to interpret and model the universe, combining philosophy and the history of science—from David Hume’s question “can we know anything more than what we experience?” to the work of Thomas Bayes and modern cosmology.

With examples such as Eddington’s 1919 eclipse expedition confirming Einstein’s theory of gravity, it shows how models and probability are used to predict and explain—and why scientific conclusions are always, in some sense, uncertain. My aim is to show how scientists build and test models of the world, and why that process matters for how we decide what is true.

Prefer a quick introduction? Watch a short talk about the book here, or find recent and upcoming appearances.

Reviews

Nature Astronomy book review. (14 May 2026):

The book works. It takes you from the reliability of London buses to the Big Bang in a circuitous but continuous path, leaving you with an accomplished feeling of having slotted parts of the Universe and parts of our understanding into their logical places. The contextualizing of statistics and randomness in human history adds flavour to what could otherwise have been quite obscure and textbookish subject matter, and the handful of equations and plots are explained in straightforward terms. While not a casual read, it is an accomplished one.

Nature, Modelling the cosmos and imagining a future without meat: Books in brief (6 March 2026):

“The sun and the stars and the edge of the Universe are inaccessible, but no more so than the interior lives of other people,” writes astrophysicist Andrew Jaffe in his intriguing book about epistemology, probability and cosmology. We can observe the Universe in detail with scientific instruments — but without devising plausible models of how it works, such as the theory of relativity, quantum mechanics and even the interpretation of our Universe as one component of a ‘multiverse’, we cannot understand it.

Unfortunately, some randomness has crept into the book: errata (inevitably!) can be found here.