Plucked from the Vacuum

COMMENT BOOKS & ARTS quantum scales during its earliest stages. The later accelerating expansion might be driven by the ‘dark energy’ of the vacuum itself — a seething ocean of virtual particle pairs popping in and out of existence as a result of quantum uncertainty. Furthermore, Krauss points out, our Uni- verse seems to have a net gravitational energy that is suspiciously close to zero: its exist- HEIDELBERG UNIV. SPRINGEL, HITS, V. ence may ‘cost’ nothing, requiring no energy input. This raises the possibility of the ulti- mate free lunch — of a cosmos that is merely a piece of borrowed stuff, having appeared spontaneously, like a virtual particle, and been filled with matter and radiation sim- ply as a consequence of the energy of empty The cooling Universe enabled galaxies to form, as this simulation shows. space. Ours may be one of an infinite array of universe-like things, just one instance in COSMOLOGY a multiverse. It would be easy for this remarkable story to revel in self-congratulation, but Krauss steers it soberly and with grace, taking time to let the Plucked from reader digest the material. His discussion of the multiverse is a good example: he lays out the possibilities and scientific and philosophi- cal implications without beating the drum the vacuum for any one hypothesis. His asides on how he views each piece of science and its chances of being right are refreshingly honest. A tale of multiverses, cosmic inflation and dark energy He notes that a number of vital empirical grips Caleb Scharf. discoveries are, ominously, missing from our cosmic model. Dark matter is one. Despite decades of astrophysical evidence for its n less than 100 years, humanity has seen lithium, is the result of presence, and plausible options for its origins, the predictions of fundamental physics primordial nucleosyn- physicists still cannot say much about it. We converge with the observation of nature thesis in the first min- don’t know what this major mass component Ion cosmic scales. The result is an increas- utes after the Big Bang, of the Universe is, which is a bit of a predica- ingly convincing picture that we, and the when newly stable ment. We even have difficulty accounting Universe as a whole, owe our existence to protons and neutrons for every speck of normal matter in our local the instability of ‘empty’ space at a quantum combined in a frenzy Universe. This does not mean that some- level. Moreover, space and the laws of phys- of nuclear fusion. thing is wrong with the current picture, but ics may be just one variant among many pos- Another milestone that we astronomers should be uncomfort- sible ways to assemble universes, separated in the story is the A Universe able about embracing a phenomenon such as by dimension or by scale. measurement, in 1992, from Nothing: dark energy when we still have a mess to tidy Why There is How physicists came up with the current of hot and cold patches Something Rather up elsewhere. model of the cosmos is quite a story, and to in the sea of microwave Than Nothing Krauss eases such concerns by showing tell it in his elegant A Universe From Nothing, radiation that was scat- LAWRENCE M. KRAUSS that the Universe’s accelerating expansion physicist Lawrence Krauss walks a carefully tered as the plasma of a Free Press: 2012. fits astonishingly well with a host of its other laid path. The evolving Universe of finite age 380,000-year-old Uni- 224 pp. $24.99, characteristics. He shows how the theories of has pushed physicists to tackle ever more pro- verse cooled enough £17.99 the physical sciences — from the subatomic found and challenging questions using some to form atoms. These to the cosmological — unite sublimely. of the most innovative tools ever forged. patterns reveal the seeds of galactic structures What does this mean for humanity? In a Krauss begins in 1916, when Albert Ein- in today’s Universe and offer striking confir- provocative afterword, evolutionary biologist stein fudged the field equations in his general mation of current cosmological theories. and atheist Richard Dawkins writes that the theory of relativity to ensure that they mod- But just as all seemed to be fitting together apparent near-inevitability of something aris- elled a static Universe. To do this he added a well, astronomers turning their telescopes ing out of unstable nothingness, as described small constant repulsive force, equivalent to to distant, dimmed supernovae saw that the by Krauss, is devastating for theologians and endowing empty space with energy. Within expansion of the space between the stars was creationists. Dawkins is right. But it is also a few years, Edwin Hubble and other astron- accelerating rather than decelerating. Like invigorating for the rest of us, because in this omers demonstrated that, in fact, the Uni- Einstein, physicists are again stepping back nothingness there are many wonderful things verse is expanding and dynamic. Einstein to look for a way to tie the strands together. to see and understand. ■ removed the force from his equations. In the Mathematical investigations of the atomic following decades came the recognition that and subatomic world provide a plausible Caleb Scharf is director of astrobiology at most matter is dark and only weakly inter- solution. The overall smoothness and geo- Columbia University, New York, USA. His acting, and that the observed cosmic mix of metrical regularity of the Universe could forthcoming book is Gravity’s Engines. light elements, such as hydrogen, helium and have resulted from its rapid inflation from e-mail: [email protected] 440 | NATURE | VOL 481 | 26 JANUARY 2012 © 2012 Macmillan Publishers Limited. All rights reserved.

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