January 9, 1998 (a Friday)

Models

The story of how Dark Energy was discovered is a classic case of nature confounding expectations.

Ever since astronomers had accepted the idea of the Big Bang, they had been out hunting for its subsequent cosmic deceleration.

The idea was simple.

While the Big Bang blows space apart (it literally stretches all points of space-time away from each other), the gravitational pull of matter should, over time, slow down that initial burst of cosmic expansion. Two research groups, one at Berkeley and the other at Harvard, were racing to find the magnitude of deceleration in the universe. It was a critical project since the rate of cosmic braking is directly related to the total density of mass (and energy) in the universe.

Things didn’t go quite as planned. As data was gathered and analyzed, both the Harvard and Berkeley groups were stunned to find no evidence for deceleration. Instead, according to observations, the expansion of the universe was speeding up — it was accelerating. After exhaustively checking and rechecking their data, both groups bit the bullet and announced their results on this date (9 January 1998) at a meeting of the American Astronomical Society in New York City.

“All the indications from our observations of supernovas spanning a large range of distances are that we live in a universe that will expand forever,” said the leader of one team, Dr. Saul Perlmutter of Lawrence Berkeley National Laboratory in California. “Apparently there isn’t enough mass in the universe for its gravity to slow the expansion, which started with the Big Bang, to a halt.”

Dr. Peter Garnavich of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., a leader of the other team, said the low deceleration of the expansion rate means that the universe is much older, about 15 billion years old, than had been calculated by some astronomers, whose estimates had ranged as low as 8 billion years. If the universe was not expanding at a faster rate earlier, then it has taken longer for it to reach its present size.

In an independent study of the expansion rates of 14 distant radio galaxies, Dr. Ruth A. Daly, a Princeton University astronomer, said “our results are basically almost identical” to those of the Berkeley and Harvard-Smithsonian supernova observations. “We are 95 percent confident that the universe is going to expand forever,” Dr. Daly said.

Remarking on the close agreement of the different studies, Dr. Neta A. Bahcall, a Princeton astrophysicist, said this reinforced confidence that the conclusions are correct, or else everyone has overlooked some hidden flaw.

Where does Dark Energy appear in all this? As Newton showed 400 years earlier, accelerations need forces. And, as the physicists of the eighteenth and nineteenth centuries demonstrated, forces need energy. The discovery of cosmic acceleration meant that space was being forced apart and must, therefore, be pervaded by a new form of energy acting as “anti-gravity.” While Newtonian gravity only produces attractions, Einstein’s more complete description of gravity — as the shape of space-time — demonstrated that repulsion and gravity could go hand-in-hand.

The discovery of cosmic acceleration and Dark Energy upended cosmology almost overnight. In spite of the community’s incredulity, further studies, including studies of cosmic geometry, gave new support (multiple lines of evidence) for the reality of Dark Energy. Like it or not, this unanticipated form of anti-gravity was now a powerful actor on cosmology’s stage.

“Like it or not” is the key phrase. The history of science is stacked high with ideas and discoveries nobody was expecting, or even wanted. From the discovery of the muon (“who ordered that?” asked physicist I.I. Rabi) to climate change (the ultimate inconvenient truth), we don’t get to dictate to nature how it should behave. The greatest and strangest beauty of science is, in fact, this constant reminder of just how wrong we can be.

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