Very Distant Supernovae Suggest that the Cosmic Expansion is Speeding Up

To determine the rate at which the cosmos is expanding, one has to measure the redshifts and brightnesses of objects very far away. To seek out the derivative of that rate—the expected deceleration, or perhaps even an acceleration, of the cosmic expansion—one has to look out much farther still. That's precisely what two competing groups of astronomers and physicists undertook to do a few years ago. And now both groups have come up with the same astonishing, if still somewhat tentative, conclusion: The present mean mass density $(ρ0)$ that acts to slow down the Hubble expansion by gravitational braking appears to be overmatched by some ethereal agency that is actually speeding it up.