Ultima Thule

In ancient times the northernmost region of the habitable world - hence, any distant, unknown or mysterious land.

Tuesday, May 09, 2006

Cyclic universe could explain cosmological constant

By Aussiegirl

Here is truly a mind-boggling idea, that time has always existed, and that the universe regularly undergoes Big Bangs followed by Big Crunches. I'm not really sure what to make of this theory, but it does seem interesting and shows that science never stops thinking about things. It's like a drama, where the current star, the theory, is center stage, but there's another wannabe star, a new theory, standing behind the curtain waiting its chance for applause...and so forth down an infinite line of wannabe explanatory theory-stars. (Here's the link to the Wikipedia article on the cosmological constant.)

Cyclic universe could explain cosmological constant (May 2006) - News - PhysicsWeb

Two theoretical physicists have developed a model that could explain why the cosmological constant takes the small, positive value that it does in today's universe. The value of the constant is responsible for the observed acceleration in the expansion of the universe. However, the new model, developed by Paul Steinhardt of Princeton University in the US and Neil Turok at Cambridge University in the UK, will be controversial. It requires that time existed before the Big Bang, assumes that the universe is older than the 14 billion years we think it is, and says that the universe regularly undergoes repeating "cycles" of big bangs and big crunches (Sciencexpress 1126231).

The cosmological constant, or Λ, was first introduced by Einstein in 1917 to explain why the universe did not appear to be expanding. Edwin Hubble later showed that the universe was expanding, causing Einstein to call the constant his "biggest blunder". But when scientists first measured a value for Λ in 1998, they found it had a tiny, positive value -- indicating that acceleration of the universe is speeding up.

However, it is unclear why this value is an incredible 120 orders of magnitude smaller than would be expected if the universe formed under the "standard" Big Bang theory. Solving this mystery is one of the most important challenges in cosmology today. [....]

Steinhard and Turok's new theory assumes we live in a cyclic universe, where each cycle from Big Bang to big crunch takes about a trillion years. It postulates the existence of a long sequence of vacuum states, in which Λ changes in a small series of steps, or cycles, of steadily decreasing cosmological constant. The constant is assumed to start out large and positive and hops down the steps to ever lower values.

Each hop takes longer and longer so that the entire universe spends vastly more time at the lowest positive value of Λ, which we see today, than at any other value (figure 2). The last jump, to a negative value, terminates the cycling behaviour of the universe so that it rapidly ends in a big crunch. [....]

There will, however, be a way of testing the new theory. According to the standard model of the universe, there was a period of rapid expansion shortly after the Big Bang, known as inflation, that bathed the universe with gravitational waves. A series of experiments are currently underway to detect these waves, which have never been seen before. However, Steinhardt and Turok's model says the gravitational waves generated if their model is correct would be too small to be detected. So if gravitational waves are found in the next few years, it would rule out their theory.


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