Upcoming experiments could resolve a sticking point in physicists' understanding of dark energy, the force driving the universe¿s apparent expansion. If the strength of this energy is constant, then we are currently sitting on a cusp¿an era when dark energy¿s outward push has gone from barely noticeable to on par with, and set to surpass, matter¿s inward pull. Physicists call this current state of affairs the cosmic coincidence problem¿in short, why should this balance exist right now?¿and have proposed many theories of dark energy to circumvent it.

In a paper published in yesterday¿s Physical Review Letters, Neal Dalal and his colleagues at the University of California at San Diego show that most of these cosmological theories can be expressed in terms of three variables: the current strength of the dark energy, how hard it¿s pushing against gravity, and how strongly matter and dark energy interact. Experimental observations will limit the possible ranges for these values, which in turn will hem in all those models of dark energy.

The Supernova/Acceleration Probe (SNAP) satellite, set to launch later this decade, could determine once and for all if the cosmic coincidence problem really exists, the authors note, or if matter and dark energy have always been in balance. In the meantime, statistical tests performed on pairs of quasars and fast-moving supernovae, as well as studies of the abundance of galactic clusters in the universe¿s past, could help narrow down the choices

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