We study the role that string and brane winding modes may have played in cosmology. Such windings tend to impede the growth of a dimension, and dimensional counting implies that a pair of winding modes will only interact in at most 4 spacetime dimensions. This may explain why we observe 3 large spatial dimensions.; We first generalize this proposal to more phenomenologically realistic backgrounds, known as orbifolds, in which "pseudo-wound" strings can unwind. We find that the windings can persist for many "Hubble times" in some of these spaces, suggesting that they may affect the dynamics in the same way as genuinely wound strings.; Since string theory is merely a perturbative expansion of M-theory, it is important to reevaluate the proposal in this context. We divide our analysis into early- and late-time components, asking whether the late-time behavior allows 3 large dimensions, and then determining if the early-time behavior makes such an outcome likely. Working in the low-energy limit of M-theory we assume the universe is a homogeneous but anisotropic 10-torus containing wrapped 2-branes and a supergravity gas. The biggest hierarchy that could evolve from an initial thermal fluctuation produces three large unwrapped dimensions. We consider the thermodynamic and cosmological properties of brane gases in the early universe. We find that for allowed initial volumes all branes typically annihilate before freeze-out can occur.; We finally consider the situation in string theory in which the universe is taken to be a homogeneous but anisotropic 9-torus filled with a gas of excited strings. We study the evolution of the system both analytically and numerically to determine the late-time behavior. We find that, although dynamical evolution can indeed lead to three large spatial dimensions, such an outcome is not statistically favored.
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