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Speakers: Y. Welling (DESY), V. Guidetti (Bologna/DESY), D-G Wang (Leiden), E. Bellini (Oxford), T. Bjorkmo (DAMTP, Cambridge), P. Christodoulidis (Groningen). Location: Leiden.

Event details of Theoretical Cosmology meeting
Date 5 April 2019
Time 10:30 -17:00


10:30 - 11:00 Y. Welling (DESY) - Orbital inflation: inflating along an angular isometry
 of field space

11:00 - 11:30 Coffee 

11:30 - 12:00 V. Guidetti (Bologna/DESY) - Geometrical destabilisation of light fields in string inflation 

12:00 - 12:30 D-G Wang  (Leiden) - Shift-symmetric Orbital inflation: single-field or multi-field? 

12:30 - 14:00 Lunch 

14:00 - 15:00 E. Bellini (Oxford) - Sheer shear: weak lensing with one mode 

15:00 - 15:30 Coffee 

15:30 - 16:10 T. Bjorkmo (DAMTP, Cambridge) - The rapid-turn inflationary attractor

16:15 - 16:55 P. Christodoulidis (Groningen) - Attractors, bifurcations and curvature in multi-field inflation 

17:00 - end Borrel


Y. Welling (DESY)

Orbital Inflation: inflating along an angular isometry of field space 

Motivated by the recent observation that multi-field inflation mimics the predictions of single field inflation if the inflaton is efficiently coupled to a second massless degree of freedom (see Gang's talk), we introduce 'Orbital Inflation' as a natural generalisation. We present a simple framework that allows us to reconstruct simultaneously a two-field action and an inflationary trajectory, which proceeds along an ‘angular’ direction of field space, and that has a controlled mass of ‘radial’ isocurvature perturbations. In single field inflation a single function V(φ) determines the behavior of both the background and perturbations. Similarly, in this class of models, once we know the geometry of field space, a single function H(φa) determines the behavior of both the background and perturbations. We exploit this exquisite analytical control in two ways. First, Orbital Inflation provides a playground for quasi-single field inflation because the couplings between perturbations are controlled. Second, we exactly solve the phenomenology of Orbital Inflation in the limit of a small isocurvature mass and a small radius of curvature. The predictions are single-field-like, although the consistency relations are violated. Moreover, the value of the isocurvature mass dictates how the inflationary predictions fan out in the (ns, r) plane. 

V. Guidetti (Bologna / DESY)  

Geometrical destabilisation of light fields in string inflation 

A typical feature of 4D string models is the presence, at tree-level, of a  plethora of massless fields called moduli. Some of them are axion-like fields which become massive only via tiny non-perturbative effects which tend to make them naturally very light. When these fields live on a curved manifold a geometrica destabilisation of the inflationary trajectory can be induced since the growth of the isocurvature perturbations quickly brings the system in the non-perturbative regime.

D-G Wang  (Leiden) 

Shift-symmetric Orbital inflation: single-field or multi-field?  

Multi-field inflation with curved field manifold may be more naturally realized in the UV completion of inflation. From an observational point of view, however, the current constraints on primordial non-Gaussianity and isocurvature perturbation already/marginally ruled out many of these models. In this talk I will introduce a new class of two-field inflationary attractors, known as ‘shift-symmetric orbital inflation’. It is strongly multi-field, but the phenomenology still mimics single-field inflation, since in the end only one degree of freedom (the one with isocurvature origin) is responsible for the prediction of primordial perturbations. This new regime of multi-field attractors provides a different perspective to explore UV completions of inflation which is free from the problems faced by single field inflation. 

E. Bellini (Oxford)  

Sheer shear: weak lensing with one mode 

The aim of this talk is to show the potential of the Karhunen-Loève (KL) decomposition in reducing the dimensionality of the data vector for cosmic shear measurements. This greatly simplifies the problem of computing covariance matrices. To do so, I shall first introduce the theoretical background of the KL decomposition. After a brief description of the CFHTLenS data we are using, I will focus on the method adopted to calculate the lensing power spectra and estimate the covariance matrix. Finally, I will discuss the constraints obtained on cosmological parameters after a KL compression and what are the potential limits of this method. 

T. Bjorkmo (DAMTP, Cambridge) 

The rapid-turn inflationary attractor 

In recent years, rapidly-turning inflation models have received a lot of attention. Much like slow-roll slow-turn, these theories are in fact related and admit a common description. I will discuss how this can be shown by only requiring a large, slowly varying turn rate, and the existence of a conserved adiabatic mode on superhorizon scales. The results do not require any particular geometry, and rely on a new vielbein basis defined on the target space that vastly simplifies the analysis.

Several recently studied turning inflation models, including  hyperinflation, side-tracked inflation, and a flat field-space model, are examples of this general  class of attractor solutions, and I will discuss how they are related to each other. 

P. Christodoulidis (Groningen)  

Attractors, bifurcations and curvature in multi-field inflation 

Recent years have seen the introduction of various multi-field inflationary scenarios, in which non-trivial parameterization of the scalar geometry can play a crucial role. We outline a simple description that unifies these different proposals and comment on the stability of solutions. We demonstrate how the underlying dynamics is governed by critical points and bifurcations that determine the late-time behaviour of the system. This unifies the late-time attractors of hyperinflation, angular, orbital and side-tracked inflation. Interestingly, the case of hyperinflation is shown to be a special case of side-tracked inflation.


Niels Bohrweg 2, Leiden, Casimir Room (Oort building room 273) 

Local organiser

Ana Achúcarro, Leiden


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