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 Winter 2023

February 3 @ 10:30 - 16:30

Leiden, February 3rd.

A lunch will be offered. Please, register your presence at the following link:
Registration sheet


Here the remote connection for who is joining online:

Time: February 3rd, 2022 10:30 PM Amsterdam Time

Join Zoom Meeting
Zoom Link
Meeting ID: 620 8383 0127
Passcode: +UK$1Tas


In the following the program of the day:


10:30 – 11:00 CET : Welcome + Coffee

11:00-11:40 CET: Talk by Jorinde van de Vis (UU)

Title: Gravitational waves from feebly interacting particles in a first order phase transition


In most studies of gravitational waves from first order cosmological phase transitions, it is assumed that the released vacuum energy gets transformed either to bubble wall collisions, or to sound waves in the plasma. In this talk, I consider an alternative possibility that has so far not been considered: the released energy gets transferred primarily to feebly interacting particles that do not admit a fluid description but simply free-stream individually. I will discuss the formalism to study the production of GWs from such configurations, and demonstrate that such GW signals have qualitatively distinct characteristics compared to conventional sources and are potentially observable with near-future GW detectors.

11:40-12:20 CET: Talk by Alba Kalaja (RUG)

Title: The reconstruction of the CMB lensing bispectrum


Weak gravitational lensing by the intervening large-scale structure (LSS) of the Universe is the leading non-linear effect on the anisotropies of the cosmic microwave background (CMB). The integrated line-of-sight gravitational potential that causes the distortion can be reconstructed from the lensed temperature and polarization anisotropies via estimators quadratic in the CMB modes. While previous studies have focused on the lensing power spectrum, upcoming experiments will be sensitive to the bispectrum of the lensing field, sourced by the non-linear evolution of structure. The detection of such a signal would provide additional information on late-time cosmological evolution, complementary to the power spectrum. However, the reconstruction process introduces a number of noise biases that significantly hinder the signal. Identifying these terms represents a first step towards detection. In this talk, I will discuss how we compute the leading noise biases of the bispectrum in the flat-sky limit, and how these results compare to simulations.

12:20-12:30 CET: THC@NL, twenty years on…

12:30 – 14:00 CET : LUNCH (please register)

14:00-14:40 CET: Talk by Giovanni M. Tomaselli (UvA)

Title: Compact stars in QFT 


Very compact stars seem to be forbidden in General Relativity. While Buchdahl’s theorem sets an upper bound on compactness, further no-go results rely on the existence of two light rings, the inner of which has been associated to gravitational instabilities. However, little is known about the role of quantum fields in these strong gravity regimes. Working in the probe approximation where the backreaction is ignored, we show that the trapping of modes around the inner light ring leads the renormalized stress tensor of Conformal Field Theories to diverge faster than the classical source in the Buchdahl limit. This leads to the violation of the Null Energy Condition, as well as the isotropy assumption used in Buchdahl’s theorem. The backreaction of quantum fields in this regime therefore cannot be ignored. This happens as the star’s surface approaches the Buchdahl radius 9GM/4
rather than the Schwarzschild radius, with the quantum fields having support in a small region around the center, becoming negligible at the surface. These are generic quantum features and do not depend on the details of the interactions. Our findings open a way for further investigation into the role of QFT in astrophysics.

14:40-15:20 CET: Talk by Alice Garoffolo (UL)

Title: Wave-optics limit of the stochastic gravitational-wave background 


The stochastic gravitational waves background is a rich resource of cosmological information, encoded both in its source statistics and its anisotropies induced by propagation effects. During their journey, the gravitational waves constituting the stochastic background encounter cosmic structures, which are able to modify the observed signal. According to the ratio between the wavelength of the wave, and the matter overdensities typical length-scale, interference and diffraction effects may arise, possibly boosting the signal’s amplitude or inducing a non-trivial polarization pattern. In this talk I will present my results about the wave-optics limit of the stochastic gravitational wave background, and describe how we can use them to gain information about the matter content of the Universe.

15:20 – 15:40 CET : Coffee break

15:40-16:20 CET: Talk by Philippa Cole (UvA/GRAPPA)

Title: Disks, spikes and clouds: distinguishing between black hole binary environments with LISA 


LISA, the space-based gravitational wave detector, is due to fly in the mid 2030s. An entire new frequency range will be opened up for discovering gravitational wave sources, including intermediate and extreme mass ratio black hole binaries which will remain in band for up to weeks, months or even years. This offers an exciting new avenue for fundamental physics discoveries because the environment of the binaries will have an effect on the gravitational waveform over this long period of time, and we will be able to measure the properties of the environments from the gravitational wave observations alone. I will show that we can measure the parameters of not only baryonic environments such as accretion disks, but also the properties of dark matter spikes or clouds of scalar fields if they are present around the binaries. I will demonstrate that we can distinguish between different environments with a Bayesian model comparison approach and argue the importance of including environmental effects in waveform modelling. This is so that we don’t miss the opportunity to learn about the nature of dark matter or the structure of accretion disks, but crucially also so that we don’t infer biased parameters by assuming that the system is inspiralling through vacuum.

16:20-16:30 CET: Final Remarks

16:30- CET: Borrel


February 3
10:30 - 16:30
Event Category:


Ana Achucarro
Jan Pieter van der Schaar
Daan Meerburg
Diederik Roest


Casimir room, Oort 273 –the coffee room of the Institute Lorentz– Niels Bohrweg, 2, Leiden