Theoretical Cosmology meetings

To actively encourage the field of theoretical cosmology and to set an informal stage for the exchange of ideas, the Dutch theoretical cosmology community organizes Friday afternoon meetings approximately 6 times a year — usually on the first Friday of the month. The meetings typically start in the afternoon with a main speaker, followed by a short break to continue with another seminar or journal club discussion on some topic of current interest. We end the afternoon with drinks. The supporting institutes in Leiden, Amsterdam, Groningen, Utrecht where recently joined by the strings and cosmology group in Leuven and take turns in hosting the event.

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Leiden Spring 2019

April 5 @ 10:00 - 17:00

Leiden, April 5th 2019

The first THC meeting of the spring takes place in Leiden. The venue is the Oort Building (Niels Bohrweg), in the Casimir room (room 273).


10:30 – 11:00: Talk by Yvette Welling (DESY)

Title: Orbital inflation and the Chamber of Secrets


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. 

11:00 – 11:30: Coffee

11:30 – 12:00: Talk by Veronica Guidetti (Bologna University / DESY)

Title: 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.

12:00 – 12:30: Talk by Dong-Gang Wang (Leiden University)

Title: 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.

12:30 – 14:00: Lunch

14:00 – 15:00: Talk by Emilio Bellini (Oxford University)

Title: 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.

15:00 – 15:30: Coffee

15:30 – 16:10: Talk by Theodor Bjorkmo (DAMTP)

Title: 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.

16:15 – 16:55: Talk by Perseas Christodoulidis (Groningen University)

Title: 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.

17:00: Borrel


April 5
10:00 - 17:00
Event Category:


Ana Achucarro


Institute Lorentz Leiden
Niels Bohrweg 2
Leiden, 2333 CA Netherlands
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