A No-Go Theorem in String Cosmology

A no-go theorem pertaining to the graceful exit problem in Pre-Big-Bang inflation is reviewed. It is shown that dilaton self-interactions and string fluid sources fail to facilitate branch changing necessary to avoid singularities. A comment on the failure of the higher genus corrections to induce graceful exit is also included.Comment: 7 pages, latex, no figures, contributed talk at the 6th Canadian Conference on General Relativity and Relativistic Astrophysics, University of New Brunswick, Fredericton, NB, May 24-2

Topological Mass Generation in Three-Dimensional String Theory

The effective action of string theory in three dimensions is investigated, incorporating the Lorentz and gauge Chern-Simons terms in the definition of the Kalb-Ramond axion field strength. Since in three dimensions any three-form is trivial, the action can be reformulated by properly integrating the axion out. The circumstances under which it can be recast in form of topologically massive gravity coupled to a topologically massive gauge theory are pointed out. Finally, the strong coupling limit of the resulting action is inspected, with the focus on the roles played by the axion and dilaton fields.Comment: 11 pages LaTeX, Alberta preprint Alberta-Thy-46-9

A Closed Bianchi I Universe in String Theory

A special Bianchi I universe is constructed in $~4D~$ string theory. Geometrically it represents a $~3D~$ anti-de-Sitter space crossed with a flat direction whereas in terms of an associated conformal field theory it is an extremal case of a gauged WZWN theory with target the coset $~SU(1,1) \times R^2 / R~$. Some of its properties are discussed.Comment: 9 pages plain TeX (More typos corrected, a reference added in Ref. [3], and a new equation (Eq. 7) and some clarifications included

Spherical Cows in the Sky with Fab Four

We explore spherically symmetric static solutions in a subclass of unitary scalar-tensor theories of gravity, called the `Fab Four' models. The weak field large distance solutions may be phenomenologically viable, but only if the Gauss-Bonnet term is negligible. Only in this limit will the Vainshtein mechanism work consistently. Further, classical constraints and unitarity bounds constrain the models quite tightly. Nevertheless, in the limits where the range of individual terms at large scales is respectively Kinetic Braiding, Horndeski, and Gauss-Bonnet, the horizon scale effects may occur while the theory satisfies Solar system constraints and, marginally, unitarity bounds. On the other hand, to bring the cutoff down to below a millimeter constrains all the couplings scales such that `Fab Fours' can't be heard outside of the Solar system.Comment: 15 pages, LaTe

Anisotropies in Non-Thermal Distortions of Cosmic Light from Photon-Axion Conversion

Ultralight axions which couple sufficiently strongly to photons can leave imprints on the sky at diverse frequencies by mixing with cosmic light in the presence of background magnetic fields. We explore such direction dependent grey-body distortions of the CMB spectrum, enhanced by resonant conditions in the IGM plasma. We also find that if such axions are produced in the early universe and represent a subdominant dark radiation component today, they could convert into X-rays in supervoids, and brighten them at X-ray frequencies.Comment: 11 pages, 1 figur

Vacuum Energy Sequestering: The Framework and Its Cosmological Consequences

Recently we suggested a reformulation of General Relativity which completely sequesters from gravity {\it all} of the vacuum energy from a protected matter sector, assumed to contain the Standard Model. Here we elaborate further on the mechanism, presenting additional details of how it cancels all loop corrections and renders all contributions from phase transitions automatically small. We also consider cosmological consequences in more detail and show that the mechanism is consistent with a variety of inflationary models that make a universe big and old. We discuss in detail the underlying assumptions behind the dynamics of our proposal, and elaborate on the relationship of the physical interpretation of divergent operators in quantum field theory and the apparent `acausality' which our mechanism seems to entail, which we argue is completely harmless. It is merely a reflection of the fact that any UV sensitive quantity in quantum field theory cannot be calculated from first principles, but is an input whose numerical value must be measured. We also note that since the universe should be compact in spacetime, and so will collapse in the future, the current phase of acceleration with $w_{DE}\approx-1$ is just a transient. This could be tested by future cosmological observations.Comment: 39 pages LaTeX, 1 .pdf figure v3: a small correction, version published in PR

Tailoring T-Duality Beyond the First Loop

In this article we review a recent calculation of the two-loop $\sigma$-model corrections to the T-duality map in string theory. Using the effective action approach, and focusing on backgrounds with a single Abelian isometry, we give the $O(\alpha')$ modifications of the lowest-order duality transformations. The torsion plays an important role in the theory to $O(\alpha')$, because of the Chern-Simons couplings to the gauge fields that arise via dimensional reduction.Comment: 9 pages, Latex, no figures. Based on the talks given by N. Kaloper at the conferences Black Holes: Theory and Mathematical Aspects at Banff, AB, Canada and the $7^{th}~C^2GR^2A$ in Calgary, AB, Canada, May 31 - June 7, 199