## Posts Tagged ‘**String Theory**’

## String-theory calculations describe ‘birth of the universe’

Researchers in Japan have developed what may be the first string-theory model with a natural mechanism for explaining why our universe would seem to exist in three spatial dimensions if it actually has six more. According to their model, only three of the nine dimensions started to grow at the beginning of the universe, accounting both for the universe’s continuing expansion and for its apparently three-dimensional nature.

Expanding universe as a classical solution in the Lorentzian matrix model for nonperturbative superstring theory

Sang-Woo Kim, Jun Nishimura, Asato Tsuchiya

Recently we have shown by Monte Carlo simulation that expanding (3+1)-dimensional universe appears dynamically from a Lorentzian matrix model for type IIB superstring theory in (9+1)-dimensions. The mechanism for the spontaneous breaking of rotational symmetry relies crucially on the noncommutative nature of the space. Here we study the classical equations of motion as a complementary approach. In particular, we find a unique class of SO(3) symmetric solutions, which exhibits the time-dependence compatible with the expanding universe. The space-space noncommutativity is exactly zero, whereas the space-time noncommutativity becomes significant only towards the end of the expansion. We interpret the Monte Carlo results and the classical solution as describing the behavior of the model at earlier time and at later time, respectively…… http://arxiv.org/pdf

String theory is a potential “theory of everything”, uniting all matter and forces in a single theoretical framework, which describes the fundamental level of the universe in terms of vibrating strings rather than particles. Although the framework can naturally incorporate gravity even on the subatomic level, it implies that the universe has some strange properties, such as nine or ten spatial dimensions. String theorists have approached this problem by finding ways to “compactify” six or seven of these dimensions, or shrink them down so that we wouldn’t notice them. Unfortunately, Jun Nishimura of the High Energy Accelerator Research Organization (KEK) in Tsukuba says “There are many ways to get four-dimensional space–time, and the different ways lead to different physics.” The solution is not unique enough to produce useful predictions.

These compactification schemes are studied through perturbation theory, in which all the possible ways that strings could interact are added up to describe the interaction. However, this only works if the interaction is relatively weak, with a distinct hierarchy in the likelihood of each possible interaction. If the interactions between the strings are stronger, with multiple outcomes equally likely, perturbation theory no longer works……

Read more: http://physicsworld.com

## 11/11/11, Portal to Another Universe?

According to World News Forecast, 11:11am on 11/11/11 could, if Uri Geller is right, be a portal to another universe. This is from Geller’s web-page on the subject:

String theoryis said to be the theory of everything. It is a way of describing every force and matter regardless of how large or small or weak or strong it is. There are a few eleven’s that have been found in string theory.I find this to be interesting since this theory is supposed to explain the universe! The first eleven that was noticed is that string theory has to have 11 parallel universes (discussed in the beginning of the “11.11″ article) and without including these universes, the theory does not work.

The second is that

Brian Greenehas 11 letters in his name. For those of you who do not know, he is a physicist as well as the author of The Elegant Universe, which is a book explaining string theory. (His book was later made into a mini series that he hosted.) Another interesting find is that Isaac Newton (who’s ideas kicked off string theory many years later) has 11 letters in his name as well asJohn Schwarz. Schwarz was one of the two men who worked out the anomalies in the theory. Plus, 1 person + 1 person = 2 people = equality.

Whether or not a portal to another universe does open up, there will be a film opening that day about the topic, see here. In possibly related news, Brian Greene’s *Fabric of the Cosmos* series will start appearing on broadcast TV 11/2/11, with the first episode already available here if you are an iperson.

Peter Woit – www.math.columbia.edu

## String theory: a perspective over the last 25 years

Sunil Mukhi

This article provides some historical background and then reviews developments in string theory over the last twenty-five years or so. Both perturbative and non-perturbative approaches to string theory are surveyed and their impact on how we view quantum gravity is analysed….

Read more: http://arxiv.org

## Superluminal neutrinos from String Theory

**1. Crete Center for Theoretical Physics**

A remarkable claim has been made by the OPERA experiment, that takes a neutrino beam from CERN and studies its interactions inside the Gran Sasso laboratory in central Italy.

As described in their paper http://fr.arxiv.org/abs/1109.4897 submitted to the ArXiV, they have measured the velocity of the neutrinos and found it to be 2.5 X 10^(-5) times larger than that of light in vaccum, using a clever technique proposed in http://arxiv.org/abs/0805.0253.

The result is more than 6 sigma away from the expected value of the velocity of neutrinos, namely the velocity of light.

The result is so unexpected that it will take some time untill it passes further scrutiny and is confirmed by other experiments.

It should be noted that Supernova 1987a data already excluded this state of affairs if the phenomenon is energy independed. However as supernova neutrinos had an energy of about 10 MeV while OPERA neutrinos have 10 GeV, it suggests that the effect has quadratic dependence on energy and is therefore due to a non-renormalizable operator.

A potential loophole that has not obviously been considered is the use of general relativistic corrections to the path of neutrinos.

These may correct the result wich takes as the distance the straight line between emission and interaction points.

A simple newtonian simplified solvable model indicates that such corrections give an effective speed that is **larger** than than the standard one.

In Scwartzschild modeling of the gravitational field f the earth, the (small) parameter that controls the gravitational corrections is x=GM/(Lc^2) where G is Newton’s constant, M the mass of the earth, L is the length of the path (about 750 Km) and c is the speed of light in vacuum.

We expect the result to have a regular expansion in x. Putting the values of the various constants we obtain that x~10^(-8) and is probably too small to affect the result.

There are not many theoretical ideas on how to achieve this state of affairs from first principles assuming that the experimental result is correct. Of course one could just postulate that the speed of neutrinos is different (larger) than c, but this is excluded by supernova data. To make it compatible with such data , Lorentz invariance should be broken.

There however another way that is possible: to have the SM live on a brane-world and the neutrinos to be able to travel in a warped bulk.

This phenomenon was found first as a side effect in a holographic study of the effective potential in N=4 superYM using probe branes by CCTP member E. Kiritsis in hep-th/9906206. It was observed that on a brane, embeded in the background of other black branes, a velocity of light is induced on the brane, that is variable and depends on its position in the bulk. Moreover the velocity of light is smaller that that of the bulk. S. Alexander shortly afterwards provided some brane models that stabilize such a velocity of light to arbirary small values.

This line of ideas lend to the developement of the idea of Mirage Cosmology.

A bit later and probably independently, Chung and Freeze suggested that faster propagation outside of branes could be used to solve the horizon problem in cosmology. This subject was subsequently developed by Caldwell and Langlois.

Finally Herdeiro and Gibbons have shown in full generality that the speed of light on D-branes is always smaller or equal to that of the bulk.

This line of reasoning suggests that models where (a) the SM is living on a brane, and (b) neutrinos can move in a approapriately warpoed bulk can natuarally produce a superluminal speed for neutrinos.

Such models where entertained in the context of orientifold in string theory in hep-ph/0004214 and in more detail in hep-th/0210263 by I. Antoniadis, E. Kiritsis, I. Rizos and T. Tomaras.

In these works the foundations of the bottom-up approach to string theory model building were introduced. The right-handed neutrinos were leaving in the bulk and they were mixing with the doublet neutrinos on the brane. This could potentially provide a first principles model compatible with the OPERA data.

http://hep.physics.uoc.gr/news.shtml

**2. Luboš Motl
Superluminal neutrinos from noncommutative geometry**

In two previous blog entries, I discussed possible mistakes in the Opera experiment and theoretical reasons why they probably exist. But it seems pretty likely to me that a Fermilab experiment will confirm the Opera result – either because there is new physics or, more likely, because it will be affected by the same glitch in the GPS system 😉 – and theorists will be increasingly pushed to give an explanation. Imagine that we’re really forced to admit that the neutrinos are faster than the photons.

What changes will we apply to our theoretical picture of the world? What’s the most sensible setup to rebuild our understanding of the reality? Does string theory offer some semi-natural tools to account for the different speeds? Well, I will mostly promote the famous 3,000-citation 1999 article by Nathan Seiberg and Edward Witten, «**String Theory and Noncommutative Geometry**» to get a flavor of some semi-realistic attempts to assign different speeds to the massless and very light particles…..

….. Read more: http://motls.blogspot.com/2011/09/superluminal-neutrinos-from.html

## An Introduction to String Theory

These notes are based on lectures given by Michael Green during Part III of the Mathematics Tripos (the Certificate for Advanced Study in Mathematics) in the Spring of 2003. The course provided an introduction to string theory, focussing on the Bosonic string, but treating the superstring as well. A background in quantum field theory and general relativity is assumed. Some background in particle physics, group theory and conformal field theory is useful, though not essential. A number of appendices on more advanced topics are also provided, including an introduction to orientifolds in various brane configurations which helps to populate a relatively sparse part of the literature…..

Read more: http://arxiv.org/PS_cache/arxiv/pdf/1107/1107.3967v2.pdf