Sunday, June 29, 2008

Hawking & Colleagues

WHY was the big bang so very big? It has been a struggle to explain why the infant universe expanded so rapidly.

The idea that the universe expanded at a blistering rate in the first 10-34 seconds after the big bang was proposed to explain why regions of the universe separated by vast distances have such a similar background temperature: before inflation occurred, these regions would have been close together with similar properties.

But just why the universe inflated in the first place remains a mystery. WHY was the big bang so very big? It has been a struggle to explain why the infant universe expanded so rapidly.

Now Stephen Hawking at the University of Cambridge, and colleagues, think they are close to perfecting an answer - by treating the early cosmos as a quantum object with a multitude of alternative universes that gradually blend into ours.

Quantum mechanics is awash with strange ideas and can shed new light on inflation, which came in the wake of when the universe itself was around the size of an atom.

By quantum lore, when a particle of light travels from A to B, it does not take one path but explores every one simultaneously, with the more direct routes being used more heavily.

This is called a sum over histories and Prof Hawking and Prof Hertog propose the same thing for the cosmos.

In this theory, the early universe can be described by a mathematical object called a wave function and, in a similar way to the light particle, the team proposed two years ago that this means that there was no unique origin to the cosmos: instead the wave function of the universe embraced a multitude of means to develop.

This is very counter intuitive: they argued the universe began in just about every way imaginable (and perhaps even some that are not). Out of this profusion of beginnings, like a blend of a God’s eye view of every conceivable kind of creation, the vast majority of the baby universes withered away to leave the mature cosmos that we can see today.

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But, like any new idea, there were problems. The professors found that they could not explain the rapid expansion - inflation - of the universe, evidence of which is left behind all around us in what is called the cosmic microwave background, in effect the echo of the big bang, a relic of creation that can be measured with experiments on balloons and on space probes.

Now, in a paper in Physical Review Letters with Prof James Hartle of the University of California, Santa Barbara, they realised that their earlier estimates of inflation were wrong because they had not fully thought through the connection between, on the one hand, their theoretical predictions and, on the other, our observations of the echo.

At first, they found that the most probable history of the cosmos had only undergone "a little bit of inflation at the beginning, contradicting the observations," said Prof Hertog. Now, after a correction to take account of how the data we have on inflation is based on only a view of a limited volume of the universe, they find that the wave function does indeed predict a long period of inflation.

"This proposal, with volume weighting, can explain why the universe inflated," Prof Hawking tells New Scientist. By taking into account that we have a parochial view of the cosmos, the team has come up with a radical new take on cosmology.

Most models of the universe are bottom-up, that is, you start from well-defined initial conditions of the Big Bang and work forward. However, Prof Hertog and Prof Hawking say that we do not and cannot know the initial conditions present at the beginning of the universe. Instead, we only know the final state - the one we are in now.

Their idea is therefore to start with the conditions we observe today - like the fact that at large scales one does not need to adopt quantum lore to explain how the universe (it behaves classically, as scientists say) - and work backwards in time to determine what the initial conditions might have looked like.

In this way, they argue the universe did not have just one unique beginning and history but a multitude of different ones and that it has experienced them all.

The new theory is also attractive because it fits in with string theory - the most popular candidate for a "theory of everything."

String theory allows the existence of an" unimaginable multitude of different types of universes in addition to our own," but it does not provide a selection criterion among these and hence no explanation for why our universe is, the way it is", says Prof Hertog.

"For this, one needs a theory of the wave function of the universe."

And now the world of cosmology has one. The next step is to find specific predictions that can be put to the test, to validate this new view of how the cosmos came into being.
Proof that a mysterious force called “dark energy” is pushing the universe to expand endlessly at a faster and faster rate was selected as the “Breakthrough of the Year” by the editors of Science magazine.

The bizarre idea that some unknown force exists in the universe that is opposing gravity and flinging galaxies away from each other at an accelerating clip was first proposed in 1998. New studies in 2003 proved that the force does exist, and this discovery captured the top prize by the editors of Science as the year’s most important scientific development.

“It is one of the ultimate discoveries in basic science,” said Don Kennedy, editor-in-chief of the journal. “It stirs our imagination even though it challenges our ability to understand.”

“No longer are scientists trying to confirm the existence of dark energy,” the journal reported in its Friday issue. “Now they are trying to find out what dark energy is made of, and what it tells us about the birth and evolution of the universe.”

The editors also selected nine other research advances, ranging from gamma ray research to the evidence of global warming's effects to the effects of RNA on genes. All the selections, the journal said in a statement, were chosen “for their profound implications for society and the advancement of science.”

Expanding universe
Proof for the existence of the “dark force” came from two studies that probed the very early universe — back to less than 400,000 years after the Big Bang — and confirmed that the universe was expanding at a faster rate.

The Wilkinson Microwave Anisotropy Probe telescope analyzed the cosmic microwave radiation background, an echo from the Big Bang, and determined the age and composition of the universe. It confirmed that only 4 percent is ordinary matter, the stuff seen every day, and 23 percent is a cold, dark matter composed of unknown particles. The rest, some 73 percent, of the universe is dark energy.

The study also narrowed the proven age of the universe to 13.7 billion years, plus or minus a few hundred thousand years. Prior estimates had been between 12 billion and 15 billion years.
Another study, the Sloan Digital Sky Survey, mapped the distribution of a quarter-million galaxies and confirmed again the domination of dark energy.

Of the dark energy finding, the journal said: “It is, perhaps, a sign that scientists will finally begin to understand the beginning.”


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