June 8, 2008

Dark matters muchly

"Although cosmologists have adopted a cute name, dark energy, for whatever is driving this apparently antigravitational behavior on the part of the universe, nobody claims to understand why it is happening, or its implications for the future of the universe and of the life within it, despite thousands of learned papers, scores of conferences and millions of dollars’ worth of telescope time. It has led some cosmologists to the verge of abandoning their fondest dream: a theory that can account for the universe and everything about it in a single breath.

“The discovery of dark energy has greatly changed how we think about the laws of nature,” said Edward Witten, a theorist at the Institute for Advanced Study in Princeton, N.J.

When it came time for one physicist to discuss new ideas about dark energy, he showed a blank screen.

“We are placing a large bet,” Dr. Mountain said, “using our credibility as collateral, that we as a community know what we are doing.”

Through myriad techniques and observations, cosmologists have recently arrived, after decades of strife, at a robust but dark consensus regarding a cosmos in which stars and galaxies, as well as the humans who gawk at them, amount to barely more than a disputatious froth. It was born 13.7 billion years ago in the Big Bang. By weight it is 4 percent atoms and 22 percent so-called dark matter of unknown identity — perhaps elementary particles that will be discovered at the Large Hadron Collider starting up outside Geneva this year. That leaves 74 percent for the weight of whatever began causing the cosmos to accelerate about five billion years ago.

“Dark energy has the somewhat unusual property that it was embarrassing before it was discovered,” he said.

In 1917, Einstein invented a fudge factor known as the cosmological constant, a sort of cosmic repulsion to balance gravity and keep the universe in balance. He abandoned his constant when the universe was discovered to be expanding, but quantum physics resurrected it by showing that empty space should be foaming with energy that had the properties of Einstein’s constant.

Alas, all attempts to calculate the amount of this energy come up with an unrealistically huge number, enough energy to blow away the contents of the cosmos like leaves in a storm before stars or galaxies could form. Nothing could live there.

Dr. Witten and other physicists used to think this conundrum “would somehow go away.”

“It seems now that the answer is not really zero,” Dr. Witten said.

Einstein’s constant is the most economical explanation for dark energy, Dr. Witten said. The others, involving new force fields or tinkering with Einstein’s gravity, are hard to make work and raise more questions than they answer. But if dark energy is the cosmological constant, it is smaller than predicted by a shocking factor of 1060.

As a result, he said, maybe physicists should give up trying to explain that number and look instead for a theory that generates all kinds of universes, a so-called multiverse.

That idea has been given mathematical form by string theory, which portrays the constituents of nature as tiny wriggling strings, an elegant idea that in principle explains all the forces of nature but in practice leads to at least 10500 potential universes.

This maze was an embarrassment for string theory. As Dr. Witten, one of the leaders of the field, said, “I am tempted to say this was an embarrassment of my youth.”

“Who needs that mess?” he recalled thinking. “There is just one world we live in.”

Now, Dr. Witten allowed, dark energy might have transformed this fecundity from a vice into a virtue, a way to generate universes where you can find any cosmological constant you want. We just live in one where life is possible, just as fish only live in water.

“This interpretation of string theory might be close to the truth,” Dr. Witten said. But that truth comes at a cost.

“Before the discovery of the dark energy, quantum physicists tended to assume that the ‘vacuum’ we live in has some deep meaning that reflects nature’s deepest secrets,” Dr. Witten said. But if ours is only one of a zillion in a haystack, there is nothing special about it, no secret to be found.

“As for how I feel personally, I am not sure what to say,” he said in an e-mail message. “I wasn’t terribly enthusiastic the first, or even second, time I heard the proposal of a multiverse. But none of us were consulted when the universe was created.”

“We really need new theory, and we have none,” Dr. Krauss said.

Dr. Krauss said, “It would be crazy to talk ourselves out of this.”

He added: “You have to do what you can. You would be crazy not to look.”

Dark, perhaps forever

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