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TOPIC: Superstrings
Blobrana


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RE: Superstrings
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Physicists Develop Test for 'String Theory'

String theory attempts to unify nature's four fundamental forces — gravity, electromagnetism, and the strong and weak forces — by positing that everything at the most basic level consists of strands of energy that vibrate at various rates and in multiple, undiscovered dimensions. These "strings" produce all known forces and particles in the universe, thus reconciling Einstein's theory of general relativity (the large) with quantum mechanics (the small).
 Proponents say that string theory is elegant and beautiful. Dissenters argue that it does not make predictions that can be tested experimentally, so the theory cannot be proven or falsified. And no particle accelerator yet exists that can attain the high energies needed to detect strings. Because of this technical limitation, tests of string theory have remained elusive until now...

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Blobrana


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 Physicists Develop Test for 'String Theory'
 For decades, scientists have taken issue with “string theory”—a theory of the universe which contends that the fundamental forces and matter of nature can be reduced to tiny one-dimensional filaments called strings—because it does not make predictions that can be tested.
But researchers at the University of California, San Diego, Carnegie Mellon University, and The University of Texas at Austin have now developed an important test for this controversial “theory of everything.”
Described in a paper that will appear in the January 26 issue of the journal Physical Review Letters, their test involves measurements of how elusive high-energy particles scatter during particle collisions. Most physicists believe those collisions will be observable at the Large Hadron Collider, or LHC, a subatomic particle collider scheduled to be operating later this year at the European Laboratory for Particle Physics, or CERN.

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Blobrana


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String Cosmology
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Title: String Cosmology
Authors: James M. Cline
(revised v3)

A pedagogical introduction to aspects of string cosmology, including the landscape (BPBT) solution to the cosmological constant problem, brane-antibrane inflation, warped compactification, the KKLMMT model, the eta problem of SUGRA models, DBI inflation, Kahler modulus and racetrack inflation, the D3-D7 model, cosmic superstrings, and the problem of reheating. Also includes basic methods for phenomenology of multifield models with nonstandard kinetic terms.

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Blobrana


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Albert Einstein theorized long ago that moving matter would warp the fabric of four-dimensional space-time, sending out ripples of gravity called gravitational waves. No one has observed such a phenomenon so far, but University of Washington researchers believe it is possible to detect such waves coming from strange wispy structures called cosmic superstrings.
Many physicists consider a complex and sometimes-controversial premise called string theory to be a leading candidate to unify their understanding of the four basic forces of nature -- gravity, electromagnetic, weak and strong. String theory is sometimes criticized for being untestable or even unscientific, but some versions now predict an exotic behavior with observable effects: the formation of cosmic superstrings, narrow tubes of energy left from the beginning of the universe that have been stretched to enormous lengths by the expansion of the universe, said UW cosmologist Craig Hogan.
If the theories are correct, there are countless cosmic superstrings stretched like a galactic-sized rubber band. They resemble ultra-thin tubes with some of the vacuum of the early universe preserved inside, Hogan said. The strings can form into loops that "flop around" and emit gravitational waves as they decay and eventually disappear.

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Blobrana


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BBC's 'Parallel Universes'
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"BBC Horizon - Parallel Universes" é um documentário que trata da mais nova teoria da física, que uniu a teoria das super cordas com a ... teoria da super gravidade, a chamada "Teoria M", que tenta explicar o que veio ANTES do Big Bang. Os físicos chegaram a conclusão que, para o universo ser sustentável matematicamente, ele precisa necessariamente ter 11 dimensões. E que na verdade o big-bang não foi a explosão de uma singularidade, mas sim o choque entre duas dimensões. Conclui que nosso universo é na verdade um Multiverso, com infinitos universos.

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Blobrana


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Colliding Branes
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Title: Colliding Branes in Heterotic M-theory
Authors: Jean-Luc Lehners, Paul McFadden, Neil Turok

We study the collision of two flat, parallel end-of-the-world branes in heterotic M-theory. By insisting that there is no divergence in the Riemann curvature as the collision approaches, we are able to single out a unique solution possessing the local geometry of (2d compactified Milne)/Z_2 x R_3, times a finite-volume Calabi-Yau manifold in the vicinity of the collision. At a finite time before and after the collision, a second type of singularity appears momentarily on the negative-tension brane, representing its bouncing off a zero of the bulk warp factor. We find this singularity to be remarkably mild and easily regularised. The various different cosmological solutions to heterotic M-theory previously found by other authors are shown to merely represent different portions of a unique flat cosmological solution to heterotic M-theory.

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Blobrana


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Brane Inflation
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Title: Predictions of Dynamically Emerging Brane Inflation Models
Authors: Thorsten Battefeld, Natalia Shuhmaher
(revised v2)

We confront the recent proposal of Emerging Brane Inflation with WMAP3+SDSS, finding a scalar spectral index of n_s=0.9659^{+0.0049}_{-0.0052} in excellent agreement with observations. The proposal incorporates a preceding phase of isotropic, non accelerated expansion in all dimensions, providing suitable initial conditions for inflation. Additional observational constraints on the parameters of the model provide an estimate of the string scale.
A graceful exit to inflation and stabilization of extra dimensions is achieved via a string gas. The resulting pre-heating phase shows some novel features due to a redshifting potential, comparable to effects due to the expansion of the universe itself. However, the model at hand suffers from either a potential over-production of relics after inflation or insufficient stabilisation at late times.

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Blobrana


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String theory
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A provocative branch of physics called string theory might explain everything in the universe, such as how matter came into being and why space and time exist. Or, the theory might explain nothing. But for more than 30 years, it has captivated some of the best minds in physics.
This intellectual feud that has spilled into the greater world -- and onto a soccer field in Santa Barbara, California. There, physicists who spend their days pondering a universe that has nine or 10 dimensions of space, or who suspect that our universe is just one of jillions, get a chance to experience a true-to-life, three-dimensional ball.

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Blobrana


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Brane Inflation
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Title: Cosmology of the Tachyon in Brane Inflation
Authors: Louis Leblond, Sarah Shandera

In certain implementations of the brane inflationary paradigm, the exit from inflation occurs when the branes annihilate through tachyon condensation. We investigate various cosmological effects produced by this tachyonic era. We find that only a very small region of the parameter space (corresponding to slow-roll with tiny inflaton mass) allows for the tachyon to contribute some e-folds to inflation. In addition, non-adiabatic density perturbations are generated at the end of inflation. When the brane is moving relativistically this contribution can be of the same order as fluctuations produced 55 e-folds before the end of inflation. The additional contribution is exactly scale-invariant and enhances the tensor/scalar ratio. Additional non-gaussianities will also be generated, sharpening current constraints on DBI-type models which already predict a significantly non-gaussian signal.

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Blobrana


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RE: Superstrings
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Title: Brane Inflation : String Theory viewed from the Cosmos
Authors: S.-H. Henry Tye

Brane inflation is a specific realization of the inflationary universe scenario in the early universe within the brane world framework in string theory. The naturalness and robustness of this realistic scenario is explained. Its predictions on the cosmological observables in the cosmic microwave background radiation, especially possible distinct stringy features, such as large non-Gaussianity or large tensor mode that deviates from that predicted in the slow roll scenario, are discussed. Stringy KK modes as hidden dark matter is also a possibility. Another generic consequence of brane inflation is the production of cosmic strings towards the end of inflation. These cosmic strings are nothing but superstrings stretched to cosmological sizes. The properties of these cosmic superstrings and their subsequent cosmological evolution into a scaling network open up their possible detections in the near future, via cosmological, astronomical and/or gravitational wave measurements. At the moment, cosmological data is already imposing strong constraints on the details of the scenario. Finding distinctive stringy signatures in cosmological observations will go a long way in revealing the specific brane inflationary scenario and validating string theory as well as the brane world picture. Precision measurements may even reveal the structures of the flux compactification. Irrespective of the final outcome, we see that string theory is confronting data and making predictions.

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