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PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 651 August 28, 2003 by Phillip F. Schewe, Ben Stein, and James Riordon

THE BIG RIP: A NEW COSMIC DOOMSDAY scenario takes the present acceleration of the expansion of the universe to new extremes. Dartmouth physicist Robert Caldwell and his colleagues Marc Kamionkowski and Nevin Weinberg at Caltech have determined that if the supposed dark energy responsible for the acceleration is potent enough not only will the space between galaxies continue to increase but that the galaxies themselves will fly apart as will, at successive times stars, planets, and even atoms and nuclei. Since the acceleration idea became established with astronomers a few years ago in the wake of observations of distant supernovae, it has been conventional to apportion the supposed energy inventory of the universe as follows: 5% in the form of conventional baryon matter (out of which atoms are made), 25% in the form of dark matter, and the biggest part, 70%, in the form of dark energy. Not a lot is known about dark matter, and even less about dark energy. Cosmologists have taken to discussing the enigmatic properties of the dark energy with the use of a new parameter, w, which is the ratio of its average pressure to energy density. The degree of this runaway expansion impulse is expressed by w. What is the nature of dark energy and how does it overcome the attractive pull of gravitation in order to speed up the cosmic expansion, and what is the proper value of w? In the best known model, the "cosmological constant" in Einstein's famous equations of general relativity corresponds to energy and pressure of the universal quantum vacuum, and is constant in space and time. Here the value of w is -1. In a second popular model, the "quintessence"model, the dark energy is associated with a universal quantum field relaxing towards some final state. Here the energy density and pressure of the dark energy are slowly decreasing with time, and the value of w is somewhere between -1/3 and -1 (w must be smaller than -1/3 in order for cosmic acceleration to occur). In Caldwell's "phantom energy" model, there is no stable vacuum quantum state and the energy density and the expansionary pressure exerted on the universe seems to increase even as the spacetime itself expands (with ordinary gases, pressure falls with expansion). In this scenario w is less than -1. The implications of this new type of cosmology are that bound systems should in the course of time be ripped up (see figure at http://www.aip.org/mgr/png/2003/200.htm ). For example, at a w value of -1.5 the universe would last for 35 billion years before being ripped apart. About 60 million years before the end, the Milky Way would be torn apart. About 3 months before the end the solar system would become undone. About 30 minutes before that the Earth would explode. And about 10^-19 seconds before the ultimate moment of doom, atoms would be pulled apart. Caldwell (robert.r.caldwell@dartmouth.edu, 603-646-2742) suggests that deciding between this model and the others might be possible in coming years with much better data coming from microwave background, supernovae, and galaxy measurements. (Caldwell et al., Physical Review Letters, 15 August 2003; text at www.aip.org/physnews/select)

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