Christian Fidler
Former member
Postdoc in CP3 from October 2014 to September 2017
Current position: Postdoc at RWTH, AAchen University, Germany.
Research statement
I am interested in the dynamics of the Universe, both at early times related to the CMB and at late times for the large scale structure.
I am involved in the development of the non-linear Boltzmann code SONG. This code simulates the early Universe at unrivalled precision including all standard model species.

It currently is used to compute:
-B mode polarisation of the CMB induced from non-linear dynamics after inflation
-Non-Guassianity in the temperature and polarisation of the CMB from non-linear dynamics
- Spectral Distortions in the CMB from reionisation

Working on this project I have gained valuable experience in numerical simulations and programming. I also have a good understanding of numerical mathematics and have designed several new methods fit for the computations performed by SONG.
Theoretical Cosmology
I am working on the theoretical framework related to non-linear simulations.

- My work contributes to developing the framework of second-order cosmological perturbation theory. I have derived the polarised non-linear Boltzmann equations.
- I am working on a new formalism for non-linear propagation of photons in curved spaces.
Past projects
Hide past projects.

I am involved in the following research directions:

Cosmological data

Our expertise on inflation and cosmic strings is involved in the CMB data analysis of the PLANCK satellite.
Our current efforts concern the study of future CMB polarization experiments, ground based, and in space, as the CORE satellite.
We are part of the EUCLID collaboration and interested in the impact of high precision measurements of the matter power spectra of the large scale structures for cosmic inflation.
We are also involved in the LISA project, the giant space interferometer dedicated to gravitational wave astronomy, which should open a new window on cosmic string physics and other early universe phenomena.
Another direction concerns the 21cm cosmological radiation. This radiation is emitted by neutral hydrogen atoms and should shed light into the so-called "dark ages": from the recombination to the reionisation of the universe by the first stars. This new observable is expected to be sensitive to the nature of dark matter as well as to some properties of the inflationary era.

External collaborators: Sébastien Clesse (RWTH, Aachen), V. Vennin (Portsmooth, U.K.), CORE Coll., Euclid Coll., eLISA Coll.

Large Scale Structures

The statistical properties of large scale structures contain a large amount of information on cosmological observables. The abundance of halos of given mass is sensitive to various cosmological observables such as the equation of state of dark energy, to the amount of primordial non-Gaussianity as well as to the mass and cross section of the dark matter particles. Various of our activities and research are equally focused to the future EUCLID satellite mission.

External collaborators: M. Musso.


When computing cosmological predictions it is often assumed that reionisation is homogenous and completely described by only one parameter, it's optical depth. However, reionisation is driven by the local collapse of matter and therefore highly inhomogeneous.
The above method is therefore only an approximation and large corrections can be expected for quantities which depend on the exact dynamics of reionisation.
We study more realistic models on reionisation and their impact on the cosmic microwave background, especially in polarization.

SONG -- Simulations of the early Universe

We work on the development and update of the numerical code SONG which solves the dynamics of the primordial Universe after Inflation. The computational methods used are comparable to the ones employed in the public codes CLASS and CAMB, but we solve the equations of motion beyond the linear order approximation, providing greater precision.
This is crucial for several dynamical effects which are absent in the leading order equations such as the generation of B-mode polarization and non-Gaussianity.

Furthermore, the code plays a central role in the recently developed Newtonian motion gauge framework. In this framework, a Newtonian N-body simulation can be promoted to a full relativistic simulation by interpreting it on the space-time of a specific Newtonian motion gauge. SONG can be used to compute the structure of these space-times up to second order in perturbation theory, thereby including for example the impact of relativity on the dark matter bispectrum.

External collaborators: Guido W. Pettinari, Thomas Tram, Cyril Pitrou (IAP, France).

Publications in CP3
All my publications on Inspire


CMB anisotropies from patchy reionisation and diffuse Sunyaev-Zel'dovich effects
Christian Fidler and Christophe Ringeval
[Abstract] [PDF] [Journal] Published in JCAP 1710 (2017) 026
Refereed paper. 6th September.
General relativistic weak-field limit and Newtonian N-body simulations
Fidler, Christian and Tram, Thomas and Rampf, Cornelius and Crittenden, Robert and Koyama, Kazuya and Wands, David
[Abstract] [PDF] [Journal]
Refereed paper. 4th September.
The effect of early radiation in N-body simulations of cosmic structure formation
Adamek, Julian and Brandbyge, Jacob and Fidler, Christian and Hannestad, Steen and Rampf, Cornelius and Tram, Thomas
[Abstract] [PDF] [Journal]
Refereed paper. 4th September.
Relativistic initial conditions for N-body simulations
Fidler, Christian and Tram, Thomas and Rampf, Cornelius and Crittenden, Robert and Koyama, Kazuya and Wands, David
[Abstract] [PDF] [Journal]
Refereed paper. 4th September.
Kinetic theory of fermions in curved spacetime
Fidler, Christian and Pitrou, Cyril
[Abstract] [PDF] [Journal]
Refereed paper. 4th September.


Cosmological N-body simulations including radiation perturbations
Jacob Brandbyge, Cornelius Rampf, Thomas Tram, Florent Leclercq, Christian Fidler, Steen Hannestad
[Abstract] [PDF] [Journal] Submitted to MNRAS
Refereed paper. 26th October.
Relativistic Interpretation of Newtonian Simulations for Cosmic Structure Formation
Fidler, Christian and Tram, Thomas and Rampf, Cornelius and Crittenden, Robert and Koyama, Kazuya and Wands, David
[Abstract] [PDF] [Journal] to be published in JCAP
Refereed paper. 4th July.
The Intrinsic Matter Bispectrum in $Lambda$CDM
Tram, Thomas and Fidler, Christian and Crittenden, Robert and Koyama, Kazuya and Pettinari, Guido W. and Wands, David
[Abstract] [PDF] [Journal]
Refereed paper. 1st March.


Precise numerical estimation of the magnetic field generated around recombination
Fidler, Christian and Pettinari, Guido and Pitrou, Cyril
[Abstract] [PDF] [Journal]
Refereed paper. 25th November.
General relativistic corrections to $N$-body simulations and the Zel'dovich approximation
Fidler, Christian and Rampf, Cornelius and Tram, Thomas and Crittenden, Robert and Koyama, Kazuya and Wands, David
[Abstract] [PDF] [Journal]
Refereed paper. 21st May.

[UCLouvain] - [SST] [IRMP] - [SC]
Contact : Jérôme de Favereau
Job opportunities Post-doctoral positions in BSM phenomenology at colliders and in astrophysics
Postdoctoral Position on advanced analysis techniques applied to the CMS di-Higgs study
Post-doctoral positions in collider phenomenology and particle astrophysics
EOS be.h : 10 PhD positions