cosmologies. Something else must be hidden in the blackness of space. That extra component was named dark energy.

The simplest way of incorporating dark energy into cosmology is to reinstate the cosmological constant term, also known by the Greek letter (lambda). Although that makes general relativity less elegant, it also makes it more accurate. A mathematical clarification is one thing, but a true physical explanation is another. Theorists scrambled to try to explain the origins of cosmological antigravity.

It would be incorrect, though, to picture the universe as always speeding up in its expansion. Additional supernova measurements by Schmidt’s group and Perlmutter’s group have revealed that the universe began to accelerate relatively recently in its history—within a few billion years of the present day. Before then the universe was dense enough so that matter terms dominated the cosmological constant term. The attraction of gravity overpowered the repulsion of lambda, slowing the expansion. Therefore, space was decelerating before it began to accelerate. Only when the universe’s matter was dispersed enough did lambda begin to dominate and the universe start to speed up.

As Steinhardt has pointed out, the outstanding coincidence that we live within a few billion years of the turnaround time of the universe seems to contradict the Copernican ideal that humankind occupies no special place or time. Thus, the new results cry out for a wholesale rethinking of our basic concept of the universe. As he has remarked: