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4

One of ideas associated with string theory is the ekpyrotic universe. This starts with brane cosmology i.e. the idea that our universe is a four dimensional brane floating around in the ten dimensional string theory spacetime. There will be many such brane worlds and the ekpyrotic idea is that a collision between two branes would appear just like the Big ...


3

I think the idea is that there are no open strings in IIA, IIB in perturbation theory around the vacuum. As you say, we know D-branes exist in IIA and IIB, and these are defined as the submanifolds where open strings can end, so if a theory has D-branes it should also have open strings. But to have an open string means that you necessarily have a heavy, ...


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Let's start focusing on Bosonic String Theory. There are two kind of strings: open and closed. The closed strings can freely propagate in the 26 dimensional space of the theory. The 26 arises from a precise calculation, in order for the theory to be consistent, Lorentz invariant and other stuff. In the spectrum of the closed string there is always a ...


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The point is that there is no "part of the string" for a fundamental string. So there is no need for a "carrier particle" or something like that. Given the action for a free string, the Nambu-Goto action, you find the equation of motion that the string must satisfy an you discover that the solution contains a translational part associated to the center of ...



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