arXiv subjects

Konstantinos Dimopoulos

Publications and source records attributed to Konstantinos Dimopoulos.

At least 55 records · Page 3Linked to original sources

arXiv2017

Initial conditions for inflation

Within the $α$-attractors framework we investigate scalar potentials with the same pole as the one featured in the kinetic term. We show that, in field space, this leads to directions without a plateau. Using this, we present a proposal, which manages to overcome the initial conditions problem of inflation with a plateau. An earlier period of proto-inflation, beginning at Planck scale, accounts for the Universe expansion and arranges the required initial conditions for inflation on the plateau to commence. We show that, if proto-inflation is power-law, it does not suffer from a sub-Planckian eternal inflationary stage, which would otherwise be a problem. A simple model realisation is constructed in the context of $α$-attractors, which can both generate the inflationary plateau and the exponential slopes around it, necessary for the two inflation stages. Our mechanism allows to assume chaotic initial conditions at the Planck scale for proto-inflation, it is generic and it is shown to work without fine-tuning.

astro-ph.CO↗

arXiv2017

Quintessential Inflation with $α$-attractors

A novel approach to quintessential inflation model building is studied, within the framework of $α$-attractors, motivated by supergravity theories. Inflationary observables are in excellent agreement with the latest CMB observations, while quintessence explains the dark energy observations without any fine-tuning. The model is kept intentionally minimal, avoiding the introduction of many degrees of freedom, couplings and mass scales. In stark contrast to $Λ$CDM, for natural values of the parameters, the model attains transient accelerated expansion, which avoids the future horizon problem, while it maintains the field displacement mildly sub-Planckian such that the flatness of the quintessential tail is not lifted by radiative corrections and violations of the equivalence principle (fifth force) are under control. In particular, the required value of the cosmological constant is near the eletroweak scale. Attention is paid to the reheating of the Universe, which avoids gravitino overproduction and respects nucleosynthesis constraints. Kination is treated in a model independent way. A spike in gravitational waves, due to kination, is found not to disturb nucleosynthesis as well.

gr-qc↗

arXiv2017

Thermal Inflation with a Thermal Waterfall Scalar Field Coupled to a Light Spectator Scalar Field

A new model of thermal inflation is introduced, in which the mass of the thermal waterfall field is dependent on a light spectator scalar field. Using the $δN$ formalism, the "end of inflation" scenario is investigated in order to ascertain whether this model is able to produce the dominant contribution to the primordial curvature perturbation. A multitude of constrains are considered so as to explore the parameter space, with particular emphasis to key observational signatures. For natural values of the parameters, the model is found to yield a sharp prediction for the scalar spectral index and its running, well within the current observational bounds.

hep-ph↗

arXiv2016

Active galaxies can make axionic dark energy

AGN jets carry helical magnetic fields, which can affect dark matter if the latter is axionic. This preliminary study shows that, in the presence of strong helical magnetic fields, the nature of the axionic condensate may change and become dark energy. Such dark energy may affect galaxy formation and galactic dynamics, so this possibility should not be ignored when considering axionic dark matter.

astro-ph.HE↗

arXiv2016

How Thermal Inflation can save Minimal Hybrid Inflation in Supergravity

Minimal hybrid inflation in supergravity has been ruled out by the 2015 Planck observations because the spectral index of the produced curvature perturbation falls outside observational bounds. To resurrect the model, a number of modifications have been put forward but many of them spoil the accidental cancellation that resolves the eta-problem and require complicated Kahler constructions to counterbalance the lost cancellation. In contrast, in this paper the model is rendered viable by supplementing the scenario with a brief period of thermal inflation, which follows the reheating of primordial inflation. The scalar field responsible for thermal inflation requires a large non-zero vacuum expectation value (VEV) and a flat potential. We investigate the VEV of such a flaton field and its subsequent effect on the inflationary observables. We find that, for large VEV, minimal hybrid inflation in supergravity produces a spectral index within the 1-sigma Planck bound and a tensor-to-scalar ratio which may be observable in the near future. The mechanism is applicable to other inflationary models.

hep-ph↗

arXiv2016

Modelling Inflation with a Power-law Approach to the Inflationary Plateau

A new family of inflationary models is introduced and analysed. The behaviour of the parameters characterising the models suggest preferred values, which generate the most interesting testable predictions. Results are further improved if late reheating and/or a subsequent period of thermal inflation is taken into account. Specific model realisations consider a sub-Planckian inflaton variation or a potential without fine-tuning of mass scales, based on the Planck and grand unified theory scales. A toy model realisation in the context of global and local supersymmetry is examined and results fitting the Planck observations are determined.

hep-ph↗

arXiv2016

Reheating in Gauss-Bonnet-coupled inflation

We investigate the feasibility of models of inflation with a large Gauss-Bonnet coupling at late times, which have been shown to modify and prevent the end of inflation. Despite the potential of Gauss-Bonnet models in predicting favourable power spectra, capable of greatly lowering the tensor-to-scalar-ratio compared to now-disfavoured models of standard chaotic inflation, it is important to also understand in what context it is possible for post-inflationary (p)reheating to proceed and hence recover an acceptable late-time cosmology. We argue that in the previously-studied inverse power law coupling case, reheating cannot happen due to a lack of oscillatory solutions for the inflaton, and that neither instant preheating nor gravitational particle production would avoid this problem due to the persistence of the inflaton's energy density, even if it were to partially decay. Hence we proceed to define a minimal generalisation of the model which can permit perturbative reheating and study the consequences of this, including heavily modified dynamics during reheating and predictions of the power spectra.

astro-ph.CO↗

arXiv2015

Shaft Inflation and the Planck satellite observations

A new family of inflation models is introduced and studied. The models are characterised by a scalar potential which, far from the origin, approaches an inflationary plateau in a power-law manner, while near the origin becomes monomial, as in chaotic inflation. The models are obtained in the context of global supersymmetry starting with a superpotential, which interpolates from a generalised monomial to an O'Raifearteagh form for small to large values of the inflaton field respectively. It is demonstrated that the observables obtained, such as the scalar spectral index, its running and the tensor to scalar ratio, are in excellent agreement with the latest observations, without any fine-tuning. Moreover, by widening mildly the shaft in field space, it is shown that sizable tensors can be generated, which may well be observable in the near future.

hep-ph↗

arXiv2014

Non-Bunch-Davies Initial State Reconciles Chaotic Models with BICEP and Planck

The BICEP2 experiment has announced a signal for primordial gravity waves with tensor-to-scalar ratio $r=0.2^{+0.07}_{-0.05}$ [arXiv:1403.3985]. There are two ways to reconcile this result with the latest Planck experiment [arXiv:1303.5082]. One is by assuming that there is a considerable tilt of $r$, $\mathcal{T}_r$, with a positive sign, $\mathcal{T}_r=d\ln r/d\ln k\gtrsim 0.57^{+0.29}_{-0.27}$ corresponding to a blue tilt for the tensor modes of order $n_T\simeq0.53 ^{+0.29}_{-0.27}$, assuming the Planck experiment best-fit value for tilt of scalar power spectrum $n_S$. The other possibility is to assume that there is a negative running in the scalar spectral index, $dn_S/d\ln k\simeq -0.02$ which pushes up the upper bound on $r$ from $0.11$ up to $0.26$ in the Planck analysis assuming the existence of a tensor spectrum. Simple slow-roll models fail to provide such large values for $\mathcal{T}_r$ or negative runnings in $n_S$ [arXiv:1403.3985]. In this note we show that a non-Bunch-Davies initial state for perturbations can provide a match between large field chaotic models (like $m^2ϕ^2$) with the latest Planck result [arXiv:1306.4914] and BICEP2 results by accommodating either the blue tilt of $r$ or the negative large running of $n_S$.

hep-th↗

arXiv2014

Shaft Inflation

A new family of inflation models is introduced and studied. The models are characterised by a scalar potential which, far from the origin, approximates an inflationary plateau, while near the origin becomes monomial, as in chaotic inflation. The models are obtained in the context of global supersymmetry starting with a superpotential, which interpolates from a generalised monomial to an O'Raifearteagh form for small to large values of the inflaton field respectively. It is demonstrated that the observables obtained, such as the scalar spectral index and the tensor to scalar ratio, are in excellent agreement with the latest observations. Some discussion of initial conditions and eternal inflation is included.

hep-ph↗

arXiv2014

Reconciliation of High Energy Scale Models of Inflation with Planck

The inflationary cosmology paradigm is very successful in explaining the CMB anisotropy to the percent level. Besides the dependence on the inflationary model, the power spectra, spectral tilt and non-Gaussianity of the CMB temperature fluctuations also depend on the initial state of inflation. Here, we examine to what extent these observables are affected by our ignorance in the initial condition for inflationary perturbations, due to unknown new physics at a high scale $M$. For initial states that satisfy constraints from backreaction, we find that the amplitude of the power spectra could still be significantly altered, while the modification in bispectrum remains small. For such initial states, $M$ has an upper bound of a few tens of $H$, with $H$ being the Hubble parameter during inflation. We show that for $M\sim 20 H$, such initial states always (substantially) suppress the tensor to scalar ratio. In particular we show that a general choice of initial conditions can satisfactorily reconcile the simple $1/2 m^2 ϕ^2$ chaotic model with the Planck data.

hep-th↗

arXiv2013

Inflationary buildup of a vector field condensate and its cosmological consequences

Light vector fields during inflation obtain a superhorizon perturbation spectrum when their conformal invariance is appropriately broken. Such perturbations, by means of some suitable mechanism (e.g. the vector curvaton mechanism), can contribute to the curvature perturbation in the Universe and produce characteristic signals, such as statistical anisotropy, on the microwave sky, most recently surveyed by the Planck satellite mission. The magnitude of such characteristic features crucially depends on the magnitude of the vector condensate generated during inflation. However, the expectation value of this condensate has so-far been taken as a free parameter, lacking a definite prediction or a physically motivated estimate. In this paper, we study the stochastic evolution of the vector condensate and obtain an estimate for its magnitude. Our study is mainly focused in the supergravity inspired case when the kinetic function and mass of the vector boson is time-varying during inflation, but other cases are also explored such as a parity violating axial theory or a non-minimal coupling between the vector field and gravity. As an example, we apply our findings in the context of the vector curvaton mechanism and contrast our results with current observations.

hep-ph↗

arXiv2013

D-branes and cosmic structure

We outline the embedding of the vector curvaton scenario as a promising mechanism to generate statistical anisotropy within Type IIB string theory, where the vector field on a single D-brane plays the role of the vector curvaton. We first consider a toy model in the context of open string inflation, and then begin to construct a concrete model in the context of closed string inflation.

hep-th↗

arXiv2013

Correlated curvature perturbations and magnetogenesis from the GUT gauge bosons

In the context of hybrid inflation at the scale of grand unification, it is shown that, if (some of) the gauge bosons of the unbroken grand unified group obtain a flat superhorizon spectrum of perturbations from inflation they can affect or even generate both the observed curvature perturbation and simultaneously produce a sizable primordial magnetic field capable of explaining galactic magnetism. The mechanism employs the vector curvaton idea for a supermassive grand unified gauge boson after the breaking of grand unification.

hep-ph↗

arXiv2012

Statistical Anisotropy from Vector Curvaton in D-brane Inflation

We investigate the possibility of embedding the vector curvaton paradigm in D-brane models of inflation in Type IIB string theory in a simple toy model. The vector curvaton is identified with the U(1) gauge field that lives on the world volume of a D3-brane, which may be stationary or undergoing general motion in the internal space. The dilaton is considered as a spectator field which modulates the evolution of the vector field. In this set up, the vector curvaton is able to generate measurable statistical anisotropy in the spectrum and bispectrum of the curvature perturbation assuming that the dilaton evolves as e^(-ϕ)\propto a^2 where a(t) is the scale factor. Our work constitutes a first step towards exploring how such distinctive features may arise from the presence of several light fields that naturally appear in string theory models of cosmology.

hep-th↗

arXiv2012

Parity Violating Statistical Anisotropy

Particle production of an Abelian vector boson field with an axial coupling is investigated. The conditions for the generation of scale invariant spectra for the vector field transverse components are obtained. If the vector field contributes to the curvature perturbation in the Universe, scale-invariant particle production enables it to give rise to statistical anisotropy in the spectrum and bispectrum of cosmological perturbations. The axial coupling allows particle production to be parity violating, which in turn can generate parity violating signatures in the bispectrum. The conditions for parity violation are derived and the observational signatures are obtained in the context of the vector curvaton paradigm. Two concrete examples are presented based on realistic particle theory.

hep-ph↗

arXiv2012

The hybrid curvaton

We study both oscillating and inflating curvaton scenarios when the curvaton mechanism is caused by a hybrid potential. The source of the curvature perturbation is the inhomogeneous phase transition that causes the modulation of the onset of the oscillation. For the supergravity-motivated curvaton there is a possibility of finding natural coincidence of the energy density that is needed to affect non-Gaussianity in the curvaton scenario.

hep-ph↗

arXiv2012

Statistical Anisotropy and the Vector Curvaton Paradigm

The vector curvaton paradigm is reviewed. The mechanism allows a massive vector boson field to contribute to or even generate the curvature perturbation in the Universe. Contribution of vector bosons is likely to generate statistical anisotropy in the spectrum and bispectrum of the curvature perturbation, which will soon be probed observationally. Two specific models for the generation of superhorizon spectra for the components of an Abelian vector field are analysed. Emphasis is put on the observational signatures of the models when the vector fields play the role of vector curvatons. If future observations support the vector curvaton mechanism this will open a window into the gauge field content of theories beyond the standard model.

hep-ph↗