Gravity in reverse: the tale of Albert Einstein's "greatest blunder"

Natural History, Dec, 2003 by Neil deGrasse Tyson

At no time since Hubble discovered the expanding universe has any team of observers ever reliably measured omega to be anywhere close to one. Adding up all the mass and energy they could measure, dark matter included, the biggest values from the best observations topped out at about 0.3. Since that's less than one, as far as observers were concerned, the universe was "open" for the business of expansion, riding a one-way saddle into the future.

Meanwhile, beginning in 1979, Alan H. Guth, a physicist at MIT, and others advanced an adjustment to big bang theory that cleared up some nagging problems. In brief, Guth explained why things look about the same everywhere in the universe. A fundamental by-product of this update to the big bang was that it drove omega toward one. Not toward one-half. Not toward two. Certainly not toward a million. Toward one.

Scarcely a theorist in the world had a problem with that requirement, because it helped get the big bang to account for the global properties of the known universe. There was, however, another little problem: the update predicted three times as much mass and energy as observers could find. Undeterred, the theorists said the observers just weren't looking hard enough.

At the end of the tallies, visible matter alone could account for very little of the critical density. How about the mysterious dark matter? Nobody knows what dark matter is, but observers knew there is five times as much dark matter as visible matter. They added that in as well. Alas, still way too little mass-energy. The observers were at a loss. "Guys," they protested, "there's nothing else out there." And the theorists answered, "Just keep looking."

Both camps were sure the other camp was wrong--until the discovery of dark energy. That single component raised the mass-energy density of the universe to the critical level. Yes, if you do the math, the universe holds three times as much dark energy as anything else.

A skeptical lot, the community of astrophysicists decided they would feel better about the result if there were some way to corroborate it. The Wilkinson Microwave Anisotropy Probe (WMAP) was just what the doctors ordered and needed. This NASA satellite, launched in 2001, was the latest and best effort to measure and map the cosmic microwave background, the big bang's blueprint for the amount and distribution of matter and energy in the universe. Astrophysicists can now say with confidence that omega is indeed equal to one: the matter-energy density of the universe we know and love is equal to the critical density. The tabulation? The cosmos holds 73 percent dark energy, 23 percent dark matter, and a measly 4 percent ordinary matter, the stuff you and I are made of.

For the first time ever, the theorists and observers kissed and made up. Both, in their own way, were correct. Omega is one, just as the theorists demanded of the universe, even though you can't get there by adding up all the matter--dark or otherwise--as they had naively presumed. There's no more matter running around the cosmos today than had ever been estimated by the observers. Nobody had foreseen the dominating presence of cosmic dark energy, nor had anybody imagined it as the great reconciler of differences.