Going Supernova

David Kaiser

  • BuyCycles of Time by Roger Penrose
    Bodley Head, 288 pp, £25.00, September 2010, ISBN 978 0 224 08036 1
  • BuyHow Old Is the Universe? by David Weintraub
    Princeton, 370 pp, £20.95, ISBN 0 691 14731 0

Twenty years ago, the science writer Dennis Overbye published a marvellous book, Lonely Hearts of the Cosmos, in which he traced the development of cosmology – the scientific study of the universe as a whole – during the second half of the 20th century. The cosmologists in Overbye’s book were lonely for two reasons. They included the last remnants of a generation of astronomers who, before large groups of collaborators and automated data collection had become routine, used to sit up all night, alone, under unheated domes, squinting through huge telescopes to catch the faintest glimpses of light from faraway galaxies. And for much of the period that Overbye covered, the field of cosmology hung on the margins of respectability among physicists, a neglected stepchild in the shadow of such flashy fields as high-energy particle physics, with its hulking accelerators and skyrocketing budgets.

Overbye captured the cosmologists’ struggles to measure basic features of our universe. Usually their answers could be trusted only to within a factor of two – that is, each measurement carried roughly 100 per cent uncertainty. Were galaxies receding from each other at such-and-such a speed, or twice that fast? The answer bore directly on how old our observable universe could be – another key quantity that could be pinned down only to within a factor of two. (The instructor on one of my undergraduate courses in the subject merrily informed us on the first day that we could use the equation ‘1 = 10’, since most quantities of interest had comparable uncertainties. We were not, however, allowed to square that equation.) No wonder cosmologists had to suffer for so long: those huge uncertainties appeared downright amateurish when compared with the quantitative triumphs in other branches of physics. For the energy levels of a hydrogen atom, for example, theory and experiment had long since converged, agreeing right down to 14 decimal places.

Since even the basic pace of the universe’s expansion was so difficult to determine, cosmologists often threw up their hands (or argued at length) over follow-up questions, such as whether the expansion was speeding up or slowing down. The answer to that question would reveal how much stuff the universe contained. A densely packed universe, with lots of matter and energy per cubic metre, should eventually halt its expansion and collapse back onto itself, a big crunch at the end to mirror the big bang at the beginning. A universe with less matter and energy stuffed into each unit of volume would expand for ever at a quickening pace, becoming progressively more dilute. Balanced right between, Einstein’s equations predicted a Goldilocks solution: some critical amount of stuff per volume for which the rate of expansion would slowly fade but the universe would never recollapse, gently coasting into oblivion instead. The fate of the universe hung on numbers like the present-day expansion rate and the amount of stuff per unit volume. Yet try as they might – and their efforts were often extraordinarily clever – cosmologists simply couldn’t measure the universe’s basic features with the requisite confidence or precision.

That began to change, and to change fast, soon after Lonely Hearts of the Cosmos appeared. We cosmologists feel a lot less lonely these days. The field is booming, attracting new recruits, using fantastic new instruments, and producing a plethora of exciting new ideas. In the autumn of 1992, my fellow undergraduate physics students and I joined our professors for a champagne study break to mark the first release of data from the Cosmic Background Explorer (COBE) satellite. From its perch high above the atmosphere, the satellite had measured the first light released after the big bang: photons that had been streaming freely through the universe since the moment when electrons first began to combine with protons to form stable, neutral hydrogen atoms, about 380,000 years after the universe began. (Before that moment, ambient temperatures were too high to allow stable hydrogen to form.) From the subtle bumps and wiggles in the distribution of those photons, cosmologists could discern that the temperature of outer space today is just 2.725 degrees above absolute zero, and is consistent across the entire sky to one part in 100,000.

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