How Our Universe Was Born With Three Spatial Dimensions From Ten Dimensional Superstring Universe

If one goes far enough back in time, space is indeed extended in 9 directions, but then at some point only 3 of those directions start to expand rapidly. This result demonstrates, for the first time, that the 3-dimensional space that we are living in indeed emerges from the 9-dimensional space that superstring theory predicts.

This calculation was carried out on the supercomputer Hitachi SR16000 (theoretical performance: 90.3 TFLOPS) at the Yukawa Institute for Theoretical Physics of Kyoto University.

The significance of the research
It is almost 40 years since superstring theory was proposed as the theory of everything, extending the general theory of relativity to the scale of elementary particles. However, its validity and its usefulness remained unclear due to the difficulty of performing actual calculations. 

The newly obtained solution to the space-time dimensionality puzzle strongly supports the validity of the theory. 

Furthermore, the establishment of a new method to analyze superstring theory using computers opens up the possibility of applying this theory to various problems. For instance, it should now be possible to provide a theoretical understanding of the inflation*5 that is believed to have taken place in the early universe, and also the accelerating expansion of the universe*6, whose discovery earned the Nobel Prize in Physics this year. It is expected that superstring theory will develop further and play an important role in solving such puzzles in particle physics as the existence of the dark matter that is suggested by cosmological observations, and the Higgs particle, which is expected to be discovered by LHC 

experiments*7.

 

Glossary

*1 Superstring theory
There are four fundamental interactions among elementary particles: electromagnetic force, weak interaction, strong interaction and gravity. There is a theory that describes three interactions but not gravity, and a particle theory including gravity is still under investigation from various directions.
In superstring theory, all the elementary particles are represented as various oscillation modes of strings of tiny length, and thus the four interactions including gravity are described in a unified manner.

*2 Cosmic microwave background
Microwaves that come uniformly from the universe in all directions.

They are considered to be the glow of the embers of the Big Bang, providing strong evidence for Big Bang cosmology, which claims that the universe is expanding after a big explosion.

The cosmic microwave background was discovered accidentally by Penzias and Wilson of the United States, who received the Nobel Prize in Physics in 1978. In recent years, more precise observation is being performed by the WMAP (Wilkinson Microwave Anisotropy Probe) launched by NASA (the National Aeronautics and Space Administration), and its observations have led to many important new understandings in cosmology.

*3 9 dimensions
While the space we are living in has 3 dimensions, it is known in superstring theory that the space has to have 9 dimensions for consistency with quantum mechanics. Therefore, 6 of the 9 dimensions must somehow be invisible.

A popular approach is called “compactification”, in which the extra 6 spatial dimensions curl up very small, but this presents a problem because there are so many ways to actually do it.

*4 IKKT matrix model
A new formulation of superstring theory was proposed by Ishibashi, Kawai, Kitazawa and Tsuchiya in 1996. (IKKT represents the initials of the authors.) The conventional formulation of superstring theory is valid only when the interaction between strings is weak, so it is not suitable for application to real physical phenomena. The IKKT matrix model, which uses large matrices to represent the fundamental dynamical degrees of freedom, has been formulated in a way that is also valid when the interaction between strings is strong.

Earlier investigations were restricted to the analysis of the model that treats time as an imaginary variable for technical reasons, and the relationship to the real world was not clear from the results. In the present research, the technical difficulty of treating time as a real variable has been overcome, enabling for the first time the application of the model to cosmology by explicit calculations.

*5 Inflation
Inflation is a rapid accelerating expansion, which is considered to have taken place within a very short time interval after the birth of our universe. It was proposed by Katsuhiko Sato and Alan Guth independently in the early 1980s. Inflation not only provides natural solutions to various problems in Big Bang cosmology, but also succeeds in explaining some detailed properties of the cosmic microwave background.

However, the mechanism that drove the inflation has only been discussed with respect to various models, and the derivation of inflation from a fundamental theory like superstring theory still remains an important challenge.

*6 Accelerating expansion of the universe
According to Big Bang cosmology, the universe has been expanding since it originated about 13.7 billion years ago.

It was thought that this cosmic expansion was decelerating until it was actually found to be accelerating in the present epoch based on observational data from the last decade or so. The Nobel Prize in Physics for 2011 was awarded for contributions that led to the discovery of the accelerating expansion based on observations of type Ia supernovae.

However, the observed accelerating expansion implies that a mysterious energy (“dark energy”), which is not diluted by expansion, should comprise more than 70 percent of the total energy of the universe, and this requires some clarification from the viewpoint of theoretical physics.

*7 The Higgs particle, which is expected to be discovered by LHC experiments
LHC is the abbreviation for the Large Hadron Collider, the largest accelerator in the world, which started operation in autumn 2008 at CERN (the European Organization for Nuclear Research), located in a suburb of Geneva.

“LHC experiments” refers to all the experiments that are carried out with the accelerator. One of the biggest aims of the LHC experiments is to discover the Higgs particle, which is the only undiscovered particle in the Standard Model of particle physics, and to get some hints for solving the related problems in theoretical particle physics. The Higgs particle has been introduced to provide the mechanism that gives mass to elementary particles, but it entails some peculiarities from the viewpoint of theoretical particle physics including gravity, which should be clarified from both experimental and theoretical sides.

Contacts and sources: 
Jun Nishimura
High Energy Accelerator Research Organization, JapanYouhei Morita, Public Relations Office,
High Energy Accelerator Research Organization, Japan 
Asato Tsuchiya
Shizuoka University, Japan 
Yoko Kitagawa, Public Relations Section, Public Relations Staff
Shizuoka University, Japan 

Sang-Woo Kim
Osaka University, Japan Masato Ikeda, Media Relations
Kazuhiko Nishinoue
Osaka University, Japan