Post-Newtonian Feasibility of the Closed String Massless Sector.

We perform post-Newtonian analysis of double field theory as a test of string theory in gravitational sector against observations. We identify the Eddington-Robertson-Schiff parameters ß_{PPN}, γ_{PPN} with the charges of electric H flux and dilaton respectively, and further relate them to stress-energy tensor. We show ß_{PPN}=1 from weak energy condition and argue that the observation of γ_{PPN}≃1 signifies the ultrarelativistic equation of state in baryons, or the suppression of gluon condensate.

Identifying Riemannian Singularities with Regular Non-Riemannian Geometry.

Admitting non-Riemannian geometries, double field theory extends the notion of spacetime beyond the Riemannian paradigm. We identify a class of singular spacetimes known in general relativity with regular non-Riemannian geometries. The former divergences merely correspond to coordinate singularities of the generalized metric for the latter. Computed in the string frame, they feature an impenetrable non-Riemannian sphere outside of which geodesics are complete with no singular deviation. Approaching the non-Riemannian points, particles freeze and strings become (anti)chiral.

String Theory and Non-Riemannian Geometry.

The O(D,D) covariant generalized metric, postulated as a truly fundamental variable, can describe novel geometries where the notion of Riemannian metric ceases to exist. Here we quantize a closed string upon such backgrounds and identify flat, anomaly free, non-Riemannian string vacua in the familiar critical dimension, D=26 (or D=10). Remarkably, the whole Becchi-Rouet-Stora-Tyutin closed string spectrum is restricted to just one level with no tachyon, and matches the linearized equations of motion of double field theory. Taken as an internal space, our non-Riemannian vacua may open up novel avenues alternative to traditional string compactification.

Standard Model as a Double Field Theory.

We show that, without any extra physical degree introduced, the standard model can be readily reformulated as a double field theory. Consequently, the standard model can couple to an arbitrary stringy gravitational background in an O(4,4) T-duality covariant manner and manifest two independent local Lorentz symmetries, Spin(1,3)×Spin(3,1). While the diagonal gauge fixing of the twofold spin groups leads to the conventional formulation on the flat Minkowskian background, the enhanced symmetry makes the standard model more rigid, and also stringy, than it appeared. The CP violating θ term may no longer be allowed by the symmetry, and hence the strong CP problem can be solved. There are now stronger constraints imposed on the possible higher order corrections. We speculate that the quarks and the leptons may belong to the two different spin classes.

Spacetime emergence of the robertson-walker universe from a matrix model.

Using a novel, string theory-inspired formalism based on a Hamiltonian constraint, we obtain a conformal mechanical system for the spatially flat four-dimensional Robertson-Walker Universe. Depending on parameter choices, this system describes either a relativistic particle in the Robertson-Walker background or metric fluctuations of the Robertson-Walker geometry. Moreover, we derive a tree-level M theory matrix model in this time-dependent background. Imposing the Hamiltonian constraint forces the spacetime geometry to be fuzzy near the big bang, while the classical Robertson-Walker geometry emerges as the Universe expands. From our approach, we also derive the temperature of the Universe interpolating between the radiation and matter dominated eras.