Saul Perlmutter announced the Supernova Cosmology Project's evidence for a cosmological constant at the annual meeting of the American Astronomical Society in Washington, D.C., on January 8, 1998. On February 18 of that year, Gerson Goldhaber and Perlmutter discussed the SCP evidence at the UCLA conference on Dark Matter in Los Angeles, where Alexei Fillipenko announced similar results from the High-Z Supernova Search Team.

What they had observed was the accelerating expansion of the universe, presumably caused by Einstein's cosmological constant (lambda). Initially a purely mathematical term in the equations of General Relativity — which Einstein later dropped — theorists by the end of the 20th century had come to regard the cosmological constant as a manifestation of the vacuum energy described by quantum mechanics.

Yet a straightforward formulation indicates that the vacuum energy is hundreds of orders of magnitude too powerful to account for observed cosmic acceleration. Thus acceleration was, as theorist Frank Wilczek, then at the Institute for Advanced Study in Princeton, put it, "maybe the most fundamentally mysterious thing in basic science."

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