THE KNOWN UNKNOWNS

Jeremiah P. Ostriker and Simon Mitton, Heart of Darkness: Unraveling the Mysteries of the Invisible Universe, Princeton University Press, 2013.

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Katherine Freese, The Cosmic Cocktail: Three Parts Dark Matter, Princeton University Press, 2014

It’s ironic that one of the achievements of which modern science is most proud is discovering that it doesn’t know what the vast majority of the universe is made of.

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 As Ostriker and Mitton sum up in Heart of Darkness: ‘We seem to have been forced into one of the oddest situations ever encountered in science. We have a model for the universe that really works in the sense that it truly passes every empirical test; yet it is founded on two mysterious, invisible components whose influence is palpable but whose nature is totally obscure to us.’

These ‘known unknowns’ are dark matter and dark energy, which together make up some 95% of the universe, leaving the ordinary atomic matter that constitutes the visible universe a puny 5% of all creation. Despite the terminology dark matter and dark energy are unrelated. (At least probably; nothing is certain in this ocean of uncertainty.)

These two books, both part of Princeton University’s ‘Science Essentials’ series that aims to ‘bring cutting-edge science to a general audience,’ tell the story of this discovery of how much there is left to be discovered. They complement, rather than compete with, each other, as recognition of the ‘dark side’ of the universe came from two converging scientific pathways, astronomical observation and theoretical particle physics. Ostriker and Mitton put the emphasis on the former, Katherine Freese on the latter.

The bulk of both books deal with dark matter, since that is a little better understood, in the sense that cosmologists at least know what they don’t know about it and have some ideas about how to find out. On the other hand, as Freese writes, ‘Given our current knowledge of physics, dark energy doesn’t make any sense.’ Scientists know there is a force – a kind of antigravity - that is responsible for accelerating the expansion of the universe, the strength of which, unlike every other known force, increases with distance, but they have no idea what it is or where it comes from. Consequently, both books devote only a chapter to dark energy; there just isn’t that much to say about it.

Heart of Darkness is written by an American and a Brit. Jeremiah P. Ostriker is professor of astrophysics at Columbia University, and was one of the first to draw attention to the dark matter problem in the mid-1970s. Simon Mitton is a research scholar in the history and philosophy of science at Cambridge University and former Vice President of the Royal Astronomical Society.

They take a chronological approach to the development of cosmology’s ‘modern paradigm’, ‘a flat, hot, big bang model dominated by dark matter and dark energy.’ They begin with Einstein and the new ‘toolkit’ his theories and mathematics provided, which coincided with the recognition that the fuzzy blurs known as nebulae were not clouds of gas but in fact other galaxies beyond our own Milky Way, in what amounted to a second Copernican revolution. Ostriker and Mitton then show how this led to the discovery – against the resistance of Einstein, who called the concept an ‘abomination’ - that the universe is expanding, which caused a dramatic paradigm shift in 1930 and led to the development of the big bang theory.

However, the complacency engendered by the belief that science had nailed how the universe works was jolted when efforts to work out the detail – such as the rate of expansion - as well as ever more accurate data from space-based platforms such as the Hubble Space Telescope, revealed, first, the existence of dark matter and then dark energy. The conclusion of Ostriker and Mitton’s final chapter, which reviews the current state of play, is that ‘an honest look at our current model shows that we are profoundly ignorant about the basic underpinnings of the modern paradigm.’

A fascinating aspect of the story is how much of today’s model was anticipated by the first, inter-war generation of cosmologists and theoretical physicists. The first to propose the existence of dark matter (dunkle Materie), way back in 1937, was the ‘brilliant but zany’ Swiss astronomer Fritz Zwicky. His idea was ‘all but forgotten’ until the mid-1970s, when cosmologists stopped brushing the problem aside and began to take it seriously.

Similarly, it was only in the mid-1990s that cosmologists were ‘dragged kicking and screaming’ to acceptance of dark energy, even though theoretical physicists such as Einstein and the remarkable Belgian Catholic priest and physicist Georges Lemaître, whose proposal of the expanding universe led to the 1930 paradigm shift, had ‘seemingly via precognition’ discerned it in their equations seventy years before. Ostriker and Mitton write that the closeness of Lemaître’s ‘rank speculation’ to today’s model of the origin and evolution of the cosmos – which even included the concept of vacuum energy, something only accepted by science in the 1990s (and one of the favoured candidates for dark energy) - is ‘more than a little unsettling.’

Ostriker and Mitton deliberately emphasise the contributions of scientists who are not so well known to the public, such as Zwicky, Lemaître, George Gamow (who brought particle physics into cosmology in the 1940s and 50s) and the ‘daring and perspicacious’ New Zealander Beatrice Tinsley. In the early 1970s, Tinsley played a major role in puncturing cosmological complacency by making the common-sense point, missed entirely by her male peers and accepted only grudgingly, that cosmologists need to take the way galaxies have evolved into account when attempting to work out values such as the expansion of the universe, the radical consequences of which forced them to confront the dark matter problem.

The female perspective is, unsurprisingly, also brought out in Katherine Freese’s The Cosmic Cocktail, together with a greater sense that physics can be fun, the first sign of both being the jacket’s author photo, showing Freese wrapped in a lilac feather boa. Hers is a more personal account than Ostriker and Mitton’s, as she mixes her ‘personal trajectory as a scientist’ with the quest to understand the universe’s dark side. The title reflects the sense of fun, both in physics itself and in the lifestyle of a physicist - her many anecdotes always seem to involve cocktails, champagne, whisky, dancing, bars and nightclubs.

Freese, a professor of physics at Michigan University, notes that women tend to be attracted to dark matter research, which she attributes to the smaller size of research teams in that field giving them more chance of being able to make their mark in what is still largely a man’s world. She relates how working as a hostess in a Tokyo bar during her post-graduation travels taught her how ‘to deflect men’s advances and demand to be treated professionally – skills that later proved invaluable in the male-dominated physics world.’

She is firmly in the theorist camp, happiest working with the mathematics of particle physics (‘better than any cocktail’) to tease out clues for the ‘experimentalists’ to test, an area in which she has made major contributions to dark matter research. However, she begins her account with the astronomical evidence for dark matter, starting with its first proposal by Zwicky and taking the reader through the observations that allowed astronomers to infer its existence even though it cannot be viewed directly. She then turns to dark matter’s role in the evolution of the universe, showing how scientists have reached their current conclusions about the ratios of ordinary matter to dark matter and energy.

Freese goes on to consider the various theories of what dark matter is, and their relative strengths and weaknesses, before concentrating on the most popular theory, that they are made up a type of particle termed WIMPs (Weakly Interacting Massive Particles). Not that WIMPs have actually been discovered, existing only in supersymmetry theory, which itself hasn’t yet been established, but if they are proven to exist they will be the best candidates for the type of particle that makes up dark matter. No wonder that Freese writes that ‘Theoretical physicists always have this uneasy feeling that they may be working on science fiction.’

At the end Freese ponders (as most readers will have done by this point) whether either dark energy or dark matter really exist at all, rather than just being fudges to cover up science’s ignorance about the nature of the universe. She concludes that ‘the case for dark matter is so strong, so consistent, and so easy to resolve with a new fundamental particle’ that science is probably right about it, whereas ‘Dark energy is a little more disturbing, because scientists really don’t know how to begin to explain it.’

Although both sets of authors have done their best to make their accounts entertaining – Freese more successfully - I didn’t find either book a particularly easy read, both being heavy going in parts. This is perhaps unavoidable, as while the headline facts about the mysteries of the universe’s dark side are exciting and easily grasped, the detail of how science came to recognise them requires a lot of complicated and technical explanation. I don’t think either would make a good general introduction to the subject, as a fair amount of background knowledge of scientific principles is assumed.

Freese – again unavoidably for a book based on the theoretical approach – uses a lot of equations, which can be off-putting for the non-mathematical reader. While Ostriker and Mitton save most of the maths for an appendix, they too throw the odd equation into the main text. This is slightly ironic, as Mitton was the editor of Stephen Hawking’s A Brief History of Time who famously advised him that he would lose half his readers for every equation he included.

Neither book deals with the deeper philosophical (still less the theological) implications of modern cosmology – what it might tell us about why the universe exists and how it came to be - which is perhaps understandable but also a shame, as these questions are likely to be of interest to the general readers the books are aimed at.

Ostriker and Mitton acknowledge the philosophical issues but pronounce them outside their remit. Chief among those questions, raised several times as the story in Heart of Darkness unfolds, is the ‘Goldilocks Problem’: the recognition that, to a degree that can’t be shrugged off as mere coincidence, cosmological and quantum forces seem to have ‘fine-tuned’ in order to produce a universe that is just right for the evolution of intelligent observers, carrying the awkward implication that it is, in some sense, designed to be that way. Dark energy is itself the prime example of this – another ‘embarrassing coincidence’ - since it is, to a ludicrous degree of precision, exactly the right value to produce such a universe.

They briefly discuss the celebrated anthropic principle, which attempts to address this problem, but consider it ‘outside the realm of science’, and therefore their book, because it is untestable by observation or experiment. However, unlike many science writers they are consistent in dismissing the only viable scientific alternative to design, the multiverse theory, on exactly the same grounds.

Freese also declares herself ‘not a fan of the multiverse,’ again on the grounds that it doesn’t make any testable predictions (‘saying that we happen to live in one universe of many doesn’t absolve us of explaining why our world behaves the way it does’). For her part, Freese gives the slightest of nods to the fine-tuning problem, when she discusses the ‘bizarre’ timing of the period in the universe’s expansion six billion years ago when dark energy overcame the attractive forces of ordinary matter: ‘The epoch when dark energy kicked in as the dominant component coincided with the epoch where the conditions became ripe for the existence of life. Cosmologists are struggling to explain this strange coincidence.’

What does come through in both books, though, is a real sense of the excitement that these scientists feel in not knowing everything, together with the confidence that one day we’ll work it out. 

• Clive Prince.