Wednesday, September 12, 2007

Can string theory accommodate inflation?

This from New Scientist:

String theory is having trouble producing inflation – the rapid expansion of space thought to have occurred in the early universe – at least in some of the theory's simplest incarnations, according to a new study.

The work suggests squaring string theory with the well-accepted notion of inflation will be challenging at best – and some even say that one or both theories may have to be abandoned.

String theory is a leading contender for the "theory of everything", which would unify all the forces of physics in one framework. Though there are many different versions of string theory, all posit that elementary particles are actually tiny vibrating strings, and that the universe contains extra spatial dimensions beyond the three that we can see.

Now, a new study suggests it may be difficult to reconcile string theory with the widely accepted theory of inflation, which explains several key cosmological observations – such as why the universe appears to have the same properties in whichever direction astronomers look.

The study was carried out by a team of researchers led by Mark Hertzberg of MIT in Cambridge, US. The team tried to produce inflation in three versions of string theory in which the extra dimensions are shaped like a doughnut – the simplest possibility. But they found that the conditions needed for inflation appear to be impossible to achieve in these simple versions.

Room for doubt

Many inflation scenarios have been proposed within more complex versions of string theory. But Hertzberg says all of them leave some room for doubt, because not all of the details that underpin them have been verified with complete calculations, though progress has been made in this direction.

He stresses that it could still be possible to robustly produce inflation within string theory. "I don't want to run around saying, 'Oh no, there's no inflation in string theory,'" he told New Scientist. "But since we haven't performed a complete search involving more complicated extra dimensions, we don't know."

Paul Steinhardt of Princeton University in New Jersey, US, who helped to pioneer the theory of inflation, says the findings are in agreement with work that he and others have done using other versions of string theory.

"I think the fact that it is difficult to combine inflation and string theory is very interesting," he told New Scientist. "It could mean they are completely incompatible, which would force us to abandon at least one of them."

Dark energy

He nevertheless cautions that there is still a chance that someone may find a robust way to achieve inflation in string theory in the future.

Another inflation pioneer, Andrei Linde of Stanford University in California, US, is more critical of the work, however.

He says the results only apply to a class of string theory versions called type 2a, which are irrelevant to the real universe because they have been shown to be incompatible with dark energy, the mysterious force causing the universe's expansion to accelerate.

"Why would you even try to describe inflation in a theory that cannot describe our universe?" Linde says.

Max Tegmark of MIT, a member of Hertzberg's team, counters that versions of type 2a string theory should not be written off just yet.

Narrow down

That is because neither it nor another type of string theory that does account for dark energy gives a perfect description of the real universe, he argues, since they predict that the universe should be filled with exotic "supersymmetric" versions of familiar particles like electrons – which are not actually observed.

More sophisticated models of type 2a theories may yet be able to support dark energy, he says.

String theorist Gary Shiu of the University of Wisconsin in Madison, US, says the fact that not every version of string theory is compatible with inflation is good news, because it will help narrow down which versions of the theory should be further investigated from the vast number of possibilities.

"Constructing a universe consistent with observations is not an easy act," he told New Scientist.


  1. I suggest that both string theory and inflation theory need to be abandoned and replaced by a theory that acts nonlocally and from extra large scale spatial dimensions in addition to all the forces.

    Given a nonlocal hidden variables interpretation, the nature of the further cause can be deduced and diagramatically represented from the evidence of wave and entanglement behaviour as described in quantum mechanics.

    The action of the cause can explain the formation of large scale structure as cosmic voids, galaxies and galactic clusters, as well as the close measured relationship between the cosmic expansion acceleration rate and Milgrom' law of rotating spiral galaxies. Milgrom's law thus being the result of an additional effect to that of gravity rather than of MOND.

    A nonlocal causal theory can thus also obviate the need for non-baryonic dark matter. The behavior of spiral galaxies, galaxy clusters and the amount of cosmic lensing thus being large scale effects of a cause that also produces the quantum wave.

  2. It sounds like at some point in the future, things are shaping up nicely for a Joe vs. Jared, Cosmology vs. String Theory, no holds barred, physics throw down. Jared may be a lean, mean, karate machine, but nobody - and I mean nobody - walks around with white board markers like Joe.

    I'd definitely pay to see Joe and Jared go at it for their respective fields. May the best theoretician win.

  3. Well, when string theory is producing Nobel Prizes for experimental evidence for the theory:

    And the highest cited papers in a ~ five year period in all of science:

    Then we might actually have a reasonable battle on our hands. Right now I think inflationary LambdaCDM cosmology is on very, very safe and secure ground.

    However, maybe string theory is still safe. The actual paper says it may be able to produce inflation in more complex versions of string theory. But, at some point you spend more time coming up with a more and more complex theory that all the physics that could have been done with more simple Effect field theories is being wasted.

    There are a couple professors and one new post-doc here that deal with string theory issues. I want to talk with them about this.

  4. By the way, the number of Nobel Prizes and citations a theory produces from experimental evidence papers is the true measure of how "true" your theory is.

    This is Science-Truthiness. Back off Steven Colbert.


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