Manifestly O(d,d)O(d,d) Invariant Approach to Space-Time Dependent String Vacua

An $O(d,d)$ symmetry of the manifold of string vacua that do not depend on $d$ (out of $D$) space-time coordinates has been recently identified. Here we write down, for $d=D-1$, the low energy equations of motion and their general solution in a manifestly $O(d,d)$-invariant form, pointing out an amusing similarity with the renormalization group framework. Previously considered cosmological and black hole solutions are recovered as particular examples.Comment: 9 page

The Price of an Exact, Gauge-Invariant RG-Flow Equation

We combine old ideas about exact renormalization-group-flow (RGF) equations with the Vilkovisky-De Witt (VDW) approach to reparametrization invariant effective actions and arrive at a new, exact, gauge-invariant RGF equation. The price to be paid for such a result is that both the action and the RGF equation depend explicitly upon the base point (in field space) needed for the VDW construction. We briefly discuss the complications originating from this fact and possible ways to overcome them.Comment: 12 pages. To appear in Phys. Lett.

Structures in the microwave background radiation

We compare the actual WMAP maps with artificial, purely statistical maps of the same harmonic content to argue that there are, with confidence level 99.7 %, ring-type structures in the observed cosmic microwave background.Comment: 4 pages, 2 figure

Constructing Phantom Cosmologies from Standard Scalar Field Universes

We illustrate how form-invariance transformations can be used for constructing phantom cosmologies from standard scalar field universes. First, we discuss how to relate two flat Friedmann-Robertson-Walker cosmologies with different barotropic indexes $\gamma$ and $\bar \gamma$. Then, we consider the particular case $\bar \gamma=-\gamma$, and we show that if the matter content is interpreted in terms of self-interacting scalar fields, then the corresponding transformation provides the link between a standard and a phantom cosmology. After that, we illustrate the method by considering models with exponential potentials. Finally, we also show that the mentioned duality persists even if the typical braneworld modifications to the Friedmann equation are considered.Comment: 3 pages, revtex4, new title and minor changes in the main text and references to match published versio

Quantum Inhomogeneities in String Cosmology

Within two specific string cosmology scenarios --differing in the way the pre- and post-big bang phases are joined-- we compute the size and spectral slope of various types of cosmologically amplified quantum fluctuations that arise in generic compactifications of heterotic string theory. By further imposing that these perturbations become the dominant source of energy at the onset of the radiation era, we obtain physical bounds on the background's moduli, and discuss the conditions under which both a (quasi-) scale-invariant spectrum of axionic perturbations and sufficiently large seeds for the galactic magnetic fields are generated. We also point out a potential problem with achieving the exit to the radiation era when the string coupling is near its present value.Comment: 30 pages, RevteX, epsfig, 5 figure