# Master Projects in Astroparticle Physics and Cosmology (2016-2017)

The group of Cosmology and Astroparticle Physics proposes 9 Masterarbeit topics:

- some are more about cosmology (1,2,3,4,5,6,9), some about astropaticles (7,8)
- some are more analytical (2), some more numerical (1,4,5,6,7), and some even designed for big fans of numerics (3,8) and/or scientific outreach (8).

These projects will be supervised Pr. Julien Lesgourgues and the three post-doc of the group:

Maria Archidiacono (archi@phys.au.dk)

Sebastien Clesse (sebastien.clesse@unamur.be)

Alessandro Cuoco (alessandro.cuoco@physik.rwth-aachen.de)

Please contact Julien Lesgourgues (lesgourg@physik.rwth-aachen.de) as soon as possible if you are interested in working in one of these topics, starting from April or October.

Project 1: Signatures of Cosmic Strings in the Cosmic Microwave Background

**Field:** Cosmology (CMB physics)

**Main supervisor:** Julien Lesgourgues

**Other supervisor:** Sebastien Clesse

**Numerical aspects:** significant amount of coding, mainly in C/C++ [estimated difficulty 6/10]

**Starting date:** could be April or October

**Abstract:**

General Relativity predicts the possible existence of strange objects called "cosmic strings". They can be described as gigantic spaghettis travelling in the Universe. More seriously, they are one-dimensional topological defects of the metric tensor, i.e. curves along which the metric has a singularity. This is a well-known model, taken very seriously since about fourty years. We are in contact with two groups of researchers, in Louvain (BE) and Geneva (CH), who have done the best numerical simulations in the world concerning the propagation of a network of cosmic strings in the universe. In collaboration with them, we plan to interface their simulations with the Cosmic Microwave Background code of Julien Lesgourgues, in order to predict the signature of cosmic strings on CMB observations (map of the CMB temperature anisotropies, etc). At the end of the project, we will be able to check whether current data give any hint of the presence of cosmic strings in our universe.

Project 2: Computing Large Scale Structure formation with QFT-inspired method

**Fields:** Cosmology (Large Scale Structure)

**Main supervisor:** Julien Lesgourgues

**Numerical aspects:** none, this is purely analytical project (although it is possible to perform also a numerical implementation at the end, only in option)

**Starting date:** could be April or October

**Abstract:**

Cosmological perturbations in the early universe and/or on large scales can be understood with a theory of linear perturbations, easy to calculate analytically or numerically. When Large Scale Structures form (at late time and/or small scales), perturbations become non-linear. Then, they can only be followed using huge N-body simualtions in real space. There is however some hope to undersand this process analytically, using Qantum Field Theory-inspired methods (renormalisation, resummation, regularisation...) The student will learn and compare the two most promising approaches proposed recently by experts in the field, and think whether one of them can be improved (in terms of accuracy) or generalised (to extended comsological models). This project is the most challenging one from the point of view of theoretical physics. It requires a solid understanding of the QFT courses, strong mathematical skills, and a real passion for theoretical research.

Project 3: Higher Performance Computing with CLASS

**Field:** Numerical cosmology

**Main supervisor:** Julien Lesgourgues

**Other supervisor:** Deanna Hooper (PhD student)

**Numerical aspects:** this is a very numeric-oriented project, for a passionate of high performance computing, numerical methods, parallelisation/vectorisation tools, etc. [estimated difficulty 8/10]

**Starting date:** could be April or October

**Abstract:**

The Aachen group leads the developpement of large and complex public code, CLASS, used worlwide for calculating CMB and Large Scale Structure observables. This code uses parallelisation, and its speed scales almost linearly up to 8 cores. The level of parallelisation needs to be improved in order to get better performance on the most modern CPUs (Intel Xeon up to 64 cores, MIC/Xeon Phi up to ~240 cores). We also want to improve the performances of the code by using a better vectorisation of the operations, and by off-loading some calculations on GPUs (graphical processors). This will be partly possible by adding appropriate commands into the code, but sometimes it will be necessary to revisit completly the choice of numerical methods used in different parts of the code. This project could almost be done by a student in the IT departement. However we can make it appropriate for a physics student, in two ways: first, the student will need to improve his knowledge of cosmology in order to understand the code; this is important in order to make good coding choices! Second, once the code performs very well, we can tackle a few problems not treated before. because the code was too slow; this will lead to a couple of publications in cosmology/astrophysics. Reserved to big fans of numerics!

Project 4: New methods to compute the role of neutrinos in the perturbed universe

**Field:** Cosmology (cosmological perturbations)

**Main supervisor:** Julien Lesgourgues

**Other supervisor:** Maria Archidiacono

**Numerical aspects:** significant amount of coding, mainly in C/C++ [estimated difficulty 5/10]

**Starting date:** could be April or October

**Abstract:**

Public codes computing the spectrum of CMB anisotropies and matter fluctuations (in the regime of linear perturbations) solve the equation of motion of all the species present in the universe. The bottleneck in this calculation is the time spent in the integration of the equation of motion of massive neutrinos: they are the most delicate ones. Maria Archidiacono and Julien Lesgourgues are currrently investigating two new methods that could be used to replace the standard equations by simpler ones, and may speed up the calculation significantly. The student will keep investigating on these two methods, analytically and numerically. He/she will compare their performances, in terms of accuracyt and speed. Once this is settled, the best method will be released within a new version of a public code.

Project 5: Defining an imaginative and interactive smartphone application and/or website for cosmology outreach

**Field:** Cosmology, outreach and numerics

**Main supervisor:** Sebastien Clesse

**Other supervisor:** Sebastien Clesse, Deanna Hooper (PhD student) and others

**Numerical aspects:** requires excellent programming skills, including HTML, java, python, skills with graphical interfaces... [estimated difficulty 8/10]

**Starting date:** could be April or October

**Abstract:**

This is a very unconventional project, with a lot of freedom, and requiring much imagination and creativity. What would you do if you wanted to create a website and/or a smartphone application aimed at stimulating the interest of a very large audience for cosmology? Could invent interactive simulations, graphical animations, pedagogical applications, and code them in an attractive and professional way? The student will participate to brainstormings with several memebrs of the Aachen cosmology group, in order to define several ideas, and will try to implement some of them concretely. There is already a plateform of codes and graphical interfaces developped by some of us. We only need more time, imagination and manpower to turn this into a beautifull project! This topic is unusual for a master project in physics, but it sounds perfectly appropriate, since you will need to improve a lot your level in cosmology (as much as if you were doing tradditional research). Choose this only if you are an enthusiastic person, if you like explain physics to people, and if you have very good coding skills (previous experience with web mastering and/or writing apps would be a plus).

Project 6: SKA forecasts for dark energy and/or modified gravity

**Field:** Cosmology (21cm surveys, dark energy and/or modified gravity)

**Main supervisor:** Sebastien Clesse

**Other supervisors:** Maria Archidiacono, Julien Lesgourgues

**Numerical aspects:** some coding and manipulation of data, with C and Python [estimated difficulty 5/10]

**Starting date:** preferentially October

**Abstract:**

One aim of the Square Kilometre Array (SKA) radiotelescope will be to map the distribution of neutral hydrogen at high redshifts, during the reionization era, by measuring its 21cm hyperfine line. The large scale distribution of neutral hydrogen is related to the matter power spectrum, and thus 21cm observations can be used to probe cosmology. Compared to the CMB, the 21-cm tomography is particularly promising given that a very broad range of redshifts (from the dark ages down to the end of the reionisation) and a much wider range of perturbation wavelengths are potentially accessible. The objective of this project is to forecast the sensitivity of SKA cosmological parameters, for the standard LambdaCDM model, as well as for some typical models of dark energy (such as early and stressed dark energy) and modified gravity (such as f(R), chameleons, symmetrons), by using either Fisher matrix or Monte-Carlo-Markov-Chain statistical methods.

Project 7: Dark Energy signatures in the extra-galactic gamma-ray background

**Field:** Cosmology and gamma-ray astroparticle physics

**Main supervisor:** Alessandro Cuoco

**Other supervisor:** Julien Lesgourgues

**Numerical aspects:** some coding and data manipulation, mainly with python or equivalent (IDL, matlab...) [estimated difficulty 4/10]

**Starting date:** preferentially October

**Abstract:**

The imprint that Dark Energy leaves in the Cosmic Microwave Background can typically be searched through the cross-correlation with tracers of the cosmological gravitational potential, like galaxy catalogues. For this project, we plan to use gamma-ray maps from the Fermi satellite as gravitational tracer, and correlate them with CMB maps from the Planck satellite. The project will involve learning and using the typical tools used for cross-correlation analyses, i.e. cross-correlation functions and angular power spectra, which will be then applied to the Fermi-Planck cross-correlation. The cross-correlation results will be then used to constrain the Dark Energy properties.

Project 8: Investigating annihilating and decaying Dark Matter with polarised radio/microwave observations

**Field:** Astroparticle physics

**Main supervisor:** Alessandro Cuoco

**Other supervisors:** Julien Lesgourgues

**Numerical aspects:** some coding and data manipulation, mainly with python or equivalent (IDL, matlab...) [estimated difficulty 5/10]

**Starting date:** preferentially October

**Abstract:**

Annihilation/decay of Dark Matter in our Galaxy produces electrons which interacting with the Galactic magnetic field can then produce synchrotron radiation in the radio/microwave frequencies. The predicted radio/microwave emission can then be compared with observations, in order to derive DM constraints or search for DM signatures. So far, only the synchrotron intensity has been used to search for DM. On the other hand, thanks to the PLANCK satellite, precise maps of the the radio/microwave polarisation are now available. The project will focus on deriving predictions for the DM synchrotron polarisation signal and on using the PLANCK polarisation observations to put novel constraints on the DM properties.

Project 9: N-body simulations with interacting dark matter

**Field:** Cosmology and astrophysics

**Main supervisor:** Maria Archidiacono

**Other supervisors:** Julien Lesgourgues, Thejs Brinckmann (PhD student)

**Numerical aspects:** you will modify a big C++ code and run large simulations on the RWTH cluster [estimated difficulty 6/10]

**Starting date:** preferentially October

**Abstract:**

N-body codes are designed to simulate the formation of Large Scale Structure in a real-space box representing our universe. Most simulations assume that the universe contains standard Cold Dark Matter. The student will learn how to work with one of these codes, called GADGET. He/she will then try to modify it, in order to investigate some intriging models in which dark matter is interacting with other species, or self-interacting. This work can be seen as a preparation step for the anlysis of future data from the Euclid satellite, which could be able to probe or exclude such dark matter models.