Celebrate our new astronomical insight into the ultimate fate of the Universe.
The Runaway Universe: The Race to Find the Future of the Cosmos
- Donald Goldsmith
Like Lord Byron's deep and dark blue ocean, the cosmos rolls on. The cosmic expansion, as we see it relative to our own inconsequential position, proceeds at a speed of 70 kilometres per second (give or take) for every megaparsec of distance away from us. This number is the cosmologists' ‘Hubble constant’, its magnitude a subject of fierce dispute until the last decade, during which the likely range has so narrowed (65 to 75, say) as to make the remaining debate essentially scholastic — if no less interesting to specialists.
But is this year's Hubble constant the same as last year's? I don't mean this year's preferred measurement, but this year's actual value. Is the Universe expanding more or less rapidly this year than last year? And, if so, by how much? And can we even hope to measure what must surely be a tiny differential change?
Astonishingly, observational cosmologists are only now able to measure this effect — although there is still debate about how accurately. Even more astonishing is that the answer has turned the expectation of most theorists — not only cosmologists but also their cousins in particle and field theory — on its head.
The trick to doing the measurement is to increase the baseline: instead of comparing today's expansion with that of one year ago, compare it with that of 109 years ago by looking ‘back in time’ at very distant objects. But not just any objects will do; they must be objects with identical brethren in the here-and-now, because the differential measurement requires what amounts to a control population (astronomers refer to ‘standard candles’). In just the past few years, two groups, one based at the University of California at Berkeley and the other at Harvard University, have perfected a technique for correcting the raw data from a certain kind of supernova, the ‘Type Ia’. With this technique, they have created a superb control population for measuring the cosmic expansion and bringing within feasibility the measurement of its change with time.
Reputable theorists all expected expansion to be slowing down. That is because gravitation is attractive, so the initial expansion of the cosmos is slowed over time by the force of gravity fighting the expansion. Until recently, the debate was always framed thus: is gravity going to win, leading to a ‘Big Crunch’, or is the expansion sufficiently robust that gravity will slow it only incrementally, leaving the cosmos free to ‘coast’ serenely to infinite expansion?
The Type Ia supernovae seem to show something much more peculiar. They do indeed indicate that the expansion is slowing, but not by as much as the known force of gravity should demand — for any plausible amount of matter. The evidence, found independently by the two groups, each with a lot of data, is that the known gravitation must be partly offset by a repulsive force (cosmologists delight in this much-repeated pun).
And so we look to the zoo of speculative repulsive forces, starting with the ‘cosmological constant’ invented by Einstein (and later repudiated by him), and moving to recent suggestions called ‘quintessence’ by their supporters (that is, a fifth essence, since physicists traditionally speak of four forces). By virtue of its seniority, although not its plausibility, the cosmological constant is given pride of place. Although now only a fractional offsetting of gravity on cosmological distances, it is destined (if it exists) to become more and more dominant with time, driving the Universe to an exponential expansion that, in a mere 1011 years, will drive the galaxies so far apart from one another as to make them virtually isolated sub-universes.
Hence the “runaway Universe” in the title of Donald Goldsmith's excellent exposition of these and several related matters. Goldsmith is one of the great explainers of the astronomy of our time; and an explainer's craft is of a higher order than that of a mere popularizer. In this book, the science is in no way watered down or reduced to fuzzy analogies. Rather, the author explains, carefully and precisely, in ordinary language, the entire chain of discovery and logical reasoning that underlie the broad outline given above. This makes for some pretty dense prose from time to time, and some challenging graphs to study (no equations), but the text is enlivened, mercifully, by a requisite number of scurrilous personal stories, pieces of historical context and even a good measure of plain, dry wit.
About halfway through the book, when the implications of the new supernova data are becoming weighty, the author writes: “Let us pull ourselves from the slough of despond in which the implications of the new observations threaten to drown us. Rather let us lift our spirits by celebrating the new astronomical powers of insight that have brought us the latest news about the universe.” A splendid idea. A few hours spent with this book is just the way to get those spirits hoisted.