Gabriel German

Low scale inflation

We show that the scale of the inflationary potential may be the electroweak scale or even lower, while still generating an acceptable spectrum of primordial density perturbations. Thermal effects readily lead to the initial conditions necessary for low scale inflation to occur, and even the moduli problem can be evaded if there is such an inflationary period. We discuss how low scale inflationary models may arise in supersymmetric theories or in theories with large new space dimensions.

A de Sitter tachyon thick braneworld

Among the multiple 5D thick braneworld models that have been proposed in the last years, in order to address several open problems in modern physics, there is a specific one involving a tachyonic bulk scalar field. Delving into this framework, a thick braneworld with a cosmological background induced on the brane is here investigated. The respective field equations — derived from the model with a warped 5D geometry — are highly non-linear equations, admitting a non-trivial solution for the warp factor and the tachyon scalar field as well, in a de Sitter 4D cosmological background. Moreover, the non-linear tachyonic scalar field, that generates the brane in complicity with warped gravity, has the form of a kink-like configuration. Notwithstanding, the non-linear field equations restricting character does not allow one to easily find thick brane solutions with a decaying warp factor which leads to the localization of 4D gravity and other matter fields. We derive such a thick brane configuration altogether in this tachyon-gravity setup. When analyzing the spectrum of gravity fluctuations in the transverse traceless sector, the 4D gravity is shown to be localized due to the presence of a single zero mode bound state, separated by a continuum of massive Kaluza-Klein (KK) modes by a mass gap. It contrasts with previous results, where there is a KK massive bound excitation providing no clear physical interpretation. The mass gap is determined by the scale of the metric parameter H. Finally, the corrections to Newtons law in this model are computed and shown to decay exponentially. It is in full compliance to corrections reported in previous results (up to a constant factor) within similar braneworlds with induced 4D de Sitter metric, despite the fact that the warp factor and the massive modes have a different form.

Fine-tuning and the ratio of tensor to scalar density fluctuations from cosmological inflation

The form of the inflationary potential is severely restricted if one requires that it be natural in the technical sense, i.e. terms of unrelated origin are not required to be correlated. We determine the constraints on observables that are implied in such natural inflationary models, in particular on r, the ratio of tensor to scalar perturbations. We find that the naturalness constraint does not require r to be large enough to be detectable by the forthcoming searches for B-mode polarization in cosmic microwave background maps. We show also that the value of r is a sensitive discriminator between inflationary models.

Hybrid natural inflation from non Abelian discrete symmetry

Abstract A spontaneously broken global discrete symmetry may have pseudo Goldstone modes associated with the spontaneous breaking of the approximate continuous symmetry of the low dimension terms in the Lagrangian. These provide natural candidates for an inflaton that can generate slow-roll inflation. We show that, in the case of a non-Abelian discrete symmetry, the pseudo Goldstone modes readily couple to further scalar fields in a manner that the end of inflation is determined by these additional scalar fields, generating hybrid inflation. We give a simple parameterisation of the inflationary potential in this case, determine the inflationary parameters resulting, and show that phenomenological successful inflation is possible while keeping the scale of symmetry breaking sub-Planckian. Unlike natural inflation the inflation scale can be very low. We construct two simple hybrid inflation models, one non-supersymmetric and one supersymmetric. In the latter case no parameters need be chosen anomalously small.

Hybrid Natural Low Scale Inflation

We discuss the phenomenological implications of Hybrid Natural Inflation models in which the inflaton is a pseudo-Goldstone boson but inflation is terminated by a second scalar field. A feature of the scheme is that the scale of breaking of the Goldstone symmetry can be lower than the Planck scale and so gravitational corrections are under control. We show that, for supersymmetric models, the scale of inflation can be chosen anywhere between the Lyth upper bound and a value close to the electroweak breaking scale. The observed density perturbations and spectral index are readily obtained by the choice of the free parameters. The tensor to scalar ratio and the spectral tilt are extremely small.

Study of field fluctuations and their localization in a thick braneworld generated by gravity non-minimally coupled to a scalar field with the Gauss-Bonnet term

In this work we study a scenario with a warped 5D smooth braneworld with 4D Minkowski geometry builded from bulk scalar matter non-minimally coupled to gravity with an additional Gauss-Bonnet term. We present exact solutions for the full braneworld configuration in contrast to previous results where only approximate solutions were constructed due to the highly non-linear character of the relevant differential equations. These solutions allow us to study the necessary conditions for the finiteness of the 4D Planck mass and additionally, enables us to perform a more rigorous analysis of 4D gravity localization compared to approximate approaches. It is remarkable that all the constructed braneworld configurations lead to standard 4D gravity localization since they contain a localized massless tensor mode (the graviton). We also analyze the localization properties of scalar, vector and tensor fluctuation modes for the constructed field configurations. We show that for the considered backgrounds, only the massless tensor mode, i.e. the 4D graviton, is localized on the brane, while the vector and scalar modes are not confined to the brane.

Tachyon inflation in the large-N formalism

We study tachyon inflation within the large-

Testing Hybrid Natural Inflation with BICEP2

N

Collapse threshold for a cosmological Klein Gordon field

formalism, which takes a prescription for the small Hubble flow slow--roll parameter

On gravity localization in scalar braneworlds with a super-exponential warp factor

\epsilon_1