Freedom Of String Theory Research Articles

Brane gases in the early Universe

Over the past decade it has become clear that fundamental strings are not the only fundamental degrees of freedom in string theory. D-branes are also part of the spectrum of fundamental states. In this paper we explore some possible effects of D-branes on early Universe string cosmology, starting with two key assumptions: firstly that the initial state of the Universe corresponded to a dense, hot gas in which all degrees of freedom were in thermal equilibrium, and secondly that the topology of the background space admits one-cycles. We argue by t duality that in this context the cosmological singularities are not present. We derive the equation of state of the brane gases and apply the results to suggest that, in an expanding background, the winding modes of fundamental strings will play the most important role at late times. In particular, we argue that the string winding modes will only allow four space-time dimensions to become large. The presence of brane winding modes with $p>1$ may lead to a hierarchy in the sizes of the extra dimensions.

Thinning out of relevant degrees of freedom in scattering of strings

By comparing the scattering of point-like and string-like particles in a massless shock wave, we show that only the former process is sensitive to ripples in the metric living on scales shorter than λ s ≡ 2α′ℏ . We take this result as evidence for irrelevance of very-short-distance degrees of freedom in string theory.

The Hagedorn transition and the number of degrees of freedom of string theory

Extending recent discussions of the Hagedorn transition in string theory, we argue that this transition is a first-order phase transition with a very large heat (corresponding to a genus-zero contribution to the free energy that appears above the critical temperature). Formally analyzing the k-loop contributions to the free energy at temperatures far above the Hagedorn temperature leads to a number of interesting speculations about the underlying degrees of freedom in string theory.

Weyl gauge fixing and BRS symmetry in string theory

The Weyl freedom in string theory is incorporated into the BRS symmetry in the conformal-type gauge fixing. This clarifies the origins of the requirements such as D = 26 and the intercept condition. The physical equivalence of the reparametrization invariant and Weyl invariant path integral measures at D = 26 is established. The anomaly associated with the BRS transformation is explicitly evaluated, and the possible theory at D < 26 from the BRS viewpoint is also briefly discussed.