Will the Universe perpetually expand, or eventually collapse? Is space flat, or curved? On page 51 of this issue, Perlmutter et al. report observations of the explosion of a star so distant that its demise preceded the birth of our planet — observations that begin to answer these grand questions, and promise a final answer soon. If we inhabit a nice, simple Universe, the destiny of the Universe is determined by its density. The present expansion is being decelerated by gravity, at a rate governed only by the average density of matter, Ω_M. If Ω_m < 1, space is negatively curved (like a saddle), and will expand for ever, Ω_M > 1 means positive curvature (like a sphere) and eventual collapse, Ω_M = 1 is supreme simplicity: space is flat and expansion slows asymptotically towards zero.
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