"Some say the world will end in fire, Some say in ice" wrote the poet Robert Frost in 1916. A third, even more fantastic possibility for the death of the universe has just been proposed. The acceleration of the expansion of the universe might eventually become so dramatic that in the foreseeable future galaxies, stars, planets and even atoms and nuclei are ripped apart.

The recent results of the Wilkinson Microwave Anisotropy Probe (WMAP) confirmed that the universe is mainly made of "dark energy" thought to be responsible for the current acceleration of its expansion (CERN Courier April 2003 p11 ). But what would happen if the rate of acceleration increased with time? A "Big Rip" is the answer according to Robert Caldwell, Marc Kamionkowski and Nevin Weinberg of Dartmouth University, New Hampshire. In this scenario, the acceleration of the expansion of the universe becomes infinite in finite time, finally overcoming all forces, including the nuclear force that binds the quarks in neutrons and protons together.

If and when this might happen depends on the equation-of-state parameter w describing the nature of dark energy, where w = p/ρ, the ratio of the spatially homogeneous dark energy pressure p to its energy density ρ. The simplest explanation of dark energy is a cosmological constant, in which case w = -1. Other possibilities are "quintessence" with w > -1 and "phantom energy" with w < -1. para> If dark energy is in the form of a cosmological constant or quintessence, the universe's expansion will accelerate, but at a constant or decreasing rate, respectively. This standard scenario of an ever-expanding universe would lead distant galaxies to disappear progressively behind the horizon of the universe (CERN Courier April 2002 p11). The anti-gravity force of such a kind of dark energy cannot disrupt galaxies. In the case of phantom energy, however, the force increases with time and becomes infinite in a finite time depending on the value of w. For w = -1.5 this might happen in only about 20 billion years.

The countdown towards this Big Rip would be as follows: at 60 million years before the Big Rip, our galaxy is disrupted; at three months before, the solar system is unbound; at 30 minutes before, the Earth explodes; and at 10-19 seconds before, atoms dissociate.

This scenario cannot be excluded on the basis of observational constraints so far available. Current results from WMAP give only an upper limit of -0.78 for w, although its future observations may provide some additional constraints. Otherwise we will have to wait for the European Planck mission to be launched in 2007 to further constrain the nature of the dark energy that controls the ultimate fate of the universe.

Further reading

R Caldwell, M Kamionkowski and N Weinberg www.arxiv.org/abs/astro-ph/0302506.

Pictures of the month

The X-ray image (left) from NASA's Chandra satellite reveals many details of the supernova remnant DEM L71 located in the Large Magellanic Cloud some 180,000 light-years away. A hot inner cloud (light blue) is surrounded by an outer blast wave also visible in the optical image (right). The inner cloud is made of glowing iron and silicon at a temperature of 10 million degrees, suggesting that the star that exploded several thousand years ago was actually a white dwarf. Blowing apart these compact stars (typically the mass of the Sun for the size of the Earth) requires a gigantic thermonuclear explosion which arises when the white dwarf pulls too much material from a nearby companion star onto itself. These explosions are referred to as Type Ia supernovae to distinguish them from the more common Type II supernovae that end the life of massive stars. Because Type Ia supernovae have roughly the same luminosity, their detection in distant galaxies provided the first evidence in 1998 for the current acceleration of the expansion of the universe. (X-ray: NASA/CXC/Rutgers/J Hughes et al.; optical: Rutgers Fabry-Perot.)