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Superstring Cosmology

String theory was originally developed to bridge the gap between general relativity and quantum mechanics.
String theory has a reputation for difficult and abstract concepts. Certainly, a rigorous treatment of the subject involves very advanced mathematics. The theory is extraordinarily intricate, revealing numerous deep and rich mathematical and physical structures. It is important to remember that the theory is far from being fully formulated and is not properly understood as yet.




So what has string got to do with anything?
You may wondering what these “strings” look like. Well there is no way you will ever be able to see them, even with the most powerful microscope.
As a string moves through time it can vibrate at different frequencies. The frequency determines what type of elementary particle the string appears as.  

String theory predicts that space-time has ten dimensions - but the extra dimensions are curled up so tightly we are unaware of their existence. Superstring theory conjectures that all fundamental forces are united within a multidimensional framework at the Planck time (5.4 x 10-44 seconds) after the Big Bang. For this reason, the characteristic scale of string theory is expected to be close to the Planck scale.

There are five different 10-dimensional string theories and there is a conjecture that all of these theories arise as different limits of a single theory, which has come to be known as M theory.

Further Reading

The Official String Theory web site

Non technical SUPERSTRINGS! tutorial by John M. Pierre

A good introduction to string theory at graduate level. By John H. Schwarz.

What about Observational Evidence?

An experiment called IceCube will search for a distortion in the relative numbers of ‘down’ and ‘up’ neutrinos. This may lead to evidence for the extra dimensions predicted by string theory.

M-theory
The eleven-dimensional description of the underlying theory is called "M theory." As yet, it is less well understood than string theories. No-one knows what the "M" in "M-theory" means - possibilities include magical, mystery or membrane.

The 11th dimension in M theory is related to the string coupling strength; the size of this dimension grows as the coupling becomes strong.

Further Reading

M-theory from Wikipedia

M-theory site

 A Layman’s Guide to M-Theory -  by M. J. Duff

Braneworld Models
These are phenomenological models that reflect at least some of the features of M theory

 Pre Big Bang cosmologies
 The observable universe could be a surface (the “brane”) embedded in a n-dimensional spacetime (the “bulk”), with Standard Model particles and fields trapped on the brane while gravity is free to access the bulk. The dilution of gravity via the extra dimensions weakens gravity on the brane.
There are several inflation models in brane world scenarios: -
1) Chaotic inflation
 2) Natural inflation
 3) Hybrid inflation I : D-term inflation
 4) Hybrid inflation II : F-term inflation
The Ekpyrotic scenario assumes that our visible Universe is a boundary brane in a five-dimensional bulk and that the hot Big Bang occurs when a nearly supersymmetric five-brane travelling along the fifth dimension collides with our visible brane.

 The Multiverse
Retroduction, Multiverse Hypotheses and Their Testability (2006) - by William R. Stoeger

Cosmic strings

A Tutorial on Links between Cosmic String Theory and Super string Theory (2005)
- By Mahbub Majumdar




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