Book Review: “The Cosmic Cocktail” by Katherine Freese

The Cosmic Cocktail: Three Parts Dark Matter
Katherine Freese
Princeton University Press (May 4, 2014)

Katherine Freese’s “Cosmic Cocktail” lays out the current evidence for dark matter and dark energy, and the status of the relevant experiments. The book excels in the chapter about indirect and direct detection of WIMPs, a class of particles that constitutes the presently best motivated and most popular dark matter candidates. “The Cosmic Cocktail” is is Freese’s first popular science book.

Freese is a specialist in the area of astroparticle physics, and she explains the experimental status for WIMP detection clearly, not leaving out the subtleties in the data interpretation. She integrates her own contributions to the field where appropriate; the balance between her own work and that of others is well met throughout the book.

The book also covers dark energy, and while this part is informative and covers the basics, it is nowhere near as detailed as that about dark matter detection. Along the way to the very recent developments, “The Cosmic Cocktail” introduces the reader to the concepts necessary to understand the physics and relevance of the matter composition of the universe. In the first chapters, Freese explains the time-evolution of the universe, structure formation, the evolution of stars, and the essentials of particle physics necessary to understand matter in the early universe. She adds some historical facts, but the scientific history of the field is not the main theme of the book.

Freese follows the advice to first say what you want to tell them, then tell them, then tell them what you just told them. She regularly reminds the reader of what was explained in earlier chapters, and repeats explanations frequently throughout the book. While this makes it easy to follow the explanations, the alert reader might find the presumed inattention somewhat annoying. The measure of electron volts, for example, is explained at least four times. Several sentences are repeated almost verbatim in various places, for example that “eventually galaxies formed… these galaxies then merged to make clusters and superclusters…” (p. 31) “…eventually this merger lead to the formation of galaxies and clusters of galaxies...” (p. 51) or “Because neutrons are slightly heavier than protons, protons are the more stable of the objects...” (p. 70), “neutrons are a tiny bit heavier than protons… Because protons are lighter, they are the more stable of the two particles.” (p. 76), “Inflation is a period of exponential expansion just after the Big Bang”. “inflationary cosmology… an early accelerating period of the history of the Universe” (p. 202), and so on.

The topics covered in the book are timely, but do not all contribute to the theme of the book, the “cosmic cocktail”. Freese narrates for example the relevance and discovery of the Higgs and the construction details of the four LHC detectors, but does only mention the inflaton in one sentence while inflation itself is explained in two sentences (plus two sentences in an endnote). She covers the OPERA anomaly of faster-than-light neutrino (yes, including the joke about the neutrino entering a bar) and in this context mentions that faster-than-light travel implies violations of causality, confusing readers not familiar with Special Relativity. On the other hand, she does not even name the Tully-Fisher relation, and dedicates only half a sentence to baryon acoustic oscillations.

The book contains some factual errors (3 kilometers are not 5 miles (p. 92), the radius of the Sun is not 10,000 kilometers (p. 95), Hawking radiation is not caused by quantum fluctuations of space-time (p.98), the HESS experiment is not in Europe (p. 170), the possible vacua in the string theory landscape do not all have a different cosmological constant (p. 201)). Several explanations are expressed in unfortunate phrases, eg: “[T]he mass of all galaxies, including our own Milky Way, must be made of dark matter.” (p. 20) All its mass? “Imagine drawing a circle around the [gravitational lens]; the light could pass through any point on that circle.” (p. 22). Circle in which plane?

The metaphors and analogies used by Freese’s are common in the popular science literature: The universe is an expanding balloon or a raisin bread, the Higgs field is “a crowded room of dancing people” or some kind of molasses (p.116). Some explanations are vague “The multiverse perspective is strengthened by theories of inflationary cosmology” (which?) others are misleading, eg, the reader may be left with the idea that Casimir energy causes cosmic acceleration (p. 196) or that “Only with a flat geometry can the universe grow old enough to create the conditions for life to exist.” (p. 44). One has to be very careful (and check the endnote) to extract that she means the spatial geometry has to be almost flat. Redshift at the black hole horizon is often illustrated with somebody sending light signals while falling through the horizon. Freese instead uses sound waves, which adds confusion because sounds needs a medium to travel.

These are minor shortcomings, but they do limit the target group that will benefit from the book. The reader who brings no background knowledge in cosmology and particle physics I am afraid will inevitably stumble at various places.

Freese’s writing style is very individual and breaks with the smooth – some may find too smooth – style that has come to dominate the popular science literature. It takes some getting used to her occasionally quite abrupt changes of narrative direction in the first chapters, but the later chapters are more fluently written. Freese interweaves anecdotes from her personal life with the scientific explanations. Some anecdotes document academic life, others seem to serve no particular purpose other than breaking up the text. The book comes with a light dose of humor that shows mostly in the figures, which contain a skull to illustrate the ‘Death of MACHO’s’, a penguin, and a blurry photo of a potted plant.

The topic of dark energy and dark matter has of course been covered in many books, one may mention Dan Hooper’s “Dark Cosmos” (Smithsonian Books, 2006) and Evalyn Gates “Einstein’s Telescope” (WW Norton, 2009). These two books are meanwhile somewhat out-of-date because the field has developed so quickly, making Freese’s book a relevant update. Both Gates’ and Hooper’s book are more easily accessible and have a smoother narrative than “The Cosmic Cocktail”. Freese demands more of the reader but also gets across more scientific facts.

I counted more than a dozen instances of the word “exciting” throughout the book. I agree that these are indeed exciting times for cosmology and astroparticle physics. Freese’s book is a valuable, non-technical and yet up-to-date review, especially on the topic of dark matter detection.

[Disclaimer: Free review copy. Page numbers in the final version might slightly differ.]