arXiv subjects

Guilherme Franzmann

Publications and source records attributed to Guilherme Franzmann.

26 records · Page 2Linked to original sources

arXiv2019

T-dual cosmological solutions in double field theory

We investigate the cosmological solutions coming from the double field theory equations of motion after coupling a matter source to them. Assuming constant dilaton and imposing the section condition with respect to the regular coordinates leads to a universe dominated by momentum modes while imposing the section condition with respect to the dual coordinates naturally leads to a universe dominated by momentum modes. We show that both regimes have asymptotic behaviours related by T-duality. This hints towards defining a duality between the clocks considered in each regime and interpreting winding modes as being radiation from the point of view of a Euclidean time.

hep-th↗

arXiv2019

T-Dual Cosmological Solutions of Double Field Theory II

In this paper we present cosmological solutions of Double Field Theory in the supergravity frame and in the winding frame which are related via T-duality. In particular, we show that the solutions can be viewed without the need of complexifying the cosmological scale factor.

hep-th↗

arXiv2018

Dual Space-Time and Nonsingular String Cosmology

Making use of the T-duality symmetry of superstring theory, and of the double geometry from Double Field Theory, we argue that cosmological singularities of a homogeneous and isotropic universe disappear. In fact, an apparent big bang singularity in Einstein gravity corresponds to a universe expanding to infinite size in the dual dimensions.

hep-th↗

arXiv2018

Point Particle Motion in DFT and a Singularity-Free Cosmological Solution

We generalize the action for point particle motion to a double field theory background. After deriving the general equations of motion for these particle geodesics, we specialize to the case of a cosmological background with vanishing antisymmetric tensor field and constant dilaton. We then show that the geodesics can be extended to infinity in both time directions once we define the appropriate physical clock. Following this prescription, we also show the existence of a singularity-free cosmological solution.

hep-th↗

arXiv2017

Running of the Spectrum of Cosmological Perturbations in String Gas Cosmology

We compute the running of the spectrum of cosmological perturbations in String Gas Cosmology, making use of a smooth parametrization of the transition between the early Hagedorn phase and the later radiation phase. We find that the running has the same sign as in simple models of single scalar field inflation. Its magnitude is proportional to $(1 - n_s)$ ($n_s$ being the slope index of the spectrum), and it is thus parametrically larger than for inflationary cosmology, where it is proportional to $(1 - n_s)^2$.

hep-th↗

arXiv2017

Varying fundamental constants: a full covariant approach and cosmological applications

We build a minimal extension of General Relativity in which Newton's gravitational coupling, $G$, the speed of light, $c$, and the cosmological constant, $Λ$, are spacetime variables. This is done while satisfying the contracted Bianchi identity as well as the local conservation of energy momentum tensor. A dynamical constraint is derived, which shows that variations of $G$ and $c$ are coupled to the local matter-energy physical content, while variation of $Λ$ is coupled to the local geometry. This constraint presents a natural cosmological screening mechanism that brings new perspective concerning the current observations of a cosmological constant, $Λ_0$ in cosmological observations. We also explore early universe background cosmology and show that the proposal provides alternatives to obtain an accelerated expansion, similar to those coming from Varying Speed of Light theories.

gr-qc↗

arXiv2015

Atomic beings and the discovery of gravity

We aim to bring a new perspective about some aspects of the current research in Cosmology. We start with a brief introduction about the main developments of the field in the last century; then we introduce an analogy that shall elucidate the main difficulties that observational sciences involve, which might be part of the issue related to some of the contemporary cosmological problems. The analogy investigates how microscopic beings could ever discover and understand gravitational phenomena.

astro-ph.CO↗

arXiv2015

Can Back-Reaction Prevent Eternal Inflation?

We study the effects which the back-reaction of long wavelength fluctuations exert on stochastic inflation. In the cases of power-law and Starobinsky inflation these effects are too weak to terminate the stochastic growth of the inflaton field. However, in the case of the cyclic Ekpyrotic scenario, the back-reaction effects prevent the unlimited growth of the scalar field.

hep-th↗