String Cosmology
Landscape
In string theory, not only did spacetime can be compactified, but the flux can be compactified, which named "flux compactification".
According to the estimation, there are approximately 500 different internal cycles, namely, Calabi-Yau space. Each of the cycles can have fluxes that are integer valued numbers. There is some overall constraints on the fluxes which don't allow each flux to be much larger than say, 10. Then we will get the largest possible number:
According to the estimation, there are approximately 500 different internal cycles, namely, Calabi-Yau space. Each of the cycles can have fluxes that are integer valued numbers. There is some overall constraints on the fluxes which don't allow each flux to be much larger than say, 10. Then we will get the largest possible number:
The value is corresponding to the possible number of stable vacua, our universe is just borned on a stable vacuum. It implies that there are such numerous multiverse existed !
Fig 2.1 String landscape: There are numerous stable vacua existed, which corresponding to the valley in the picture. Our universe is also located on a valley.
Fig.2.2 The prediction of string landscape can be derived from F-theory, which is a product of type IIB string theory, compactified on Calabi-Yau 4-manifold.
Cosmic String
"Cosmic string" is a hypothetical object in physics. It is a topological defect formed during the symmetry breaking in the early universe.
The cosmic strings are likely to form just after inflation. As the model, cosmic strings must are extreme long microscopic objects and have extreme high density and mass. They're also have associations with time travel.
The cosmic strings are likely to form just after inflation. As the model, cosmic strings must are extreme long microscopic objects and have extreme high density and mass. They're also have associations with time travel.
Fig. 2.3 Cosmic strings
Brane Cosmology
In theoretical physics, it’s unknown that why gravity is much weaker than weak force, namely, “hierarchy problem”.
To solve the problem, scientists have put forward a new model which interpreted by extra dimensional branes, called “Randall-Sundrum model”.
In this theory, universe can be interpreted as a 3-dimensional brane embedded in higher dimensions, whereas gravity is on the other brane. Based on warped spacetime, gravity is much weaker on our brane than on another brane(gravitational brane).
Besides, only closed string, which is regarded as gravity, can move in the extra dimensions. According to the contemporary brane cosmology, our universe is only a brane embedded in extra dimensions (bulk). The collision of two branes is viewed as Big Bang.
To solve the problem, scientists have put forward a new model which interpreted by extra dimensional branes, called “Randall-Sundrum model”.
In this theory, universe can be interpreted as a 3-dimensional brane embedded in higher dimensions, whereas gravity is on the other brane. Based on warped spacetime, gravity is much weaker on our brane than on another brane(gravitational brane).
Besides, only closed string, which is regarded as gravity, can move in the extra dimensions. According to the contemporary brane cosmology, our universe is only a brane embedded in extra dimensions (bulk). The collision of two branes is viewed as Big Bang.
Fig. 2.4 Randall-Sundrum model (the first brane cosmological model).