Research

Dark Matter: Theoretical Cosmology, Particle physics, Astrophysics, Gravitational dynamics

Publication list (refereed or submitted articles)

Seminars and Conference Talks

Seminars

  • 2024, January - GRAPPA, Amsterdam - Exotic energy injection: 21cm and CMB as dark matter probes
  • 2021, October - ULB, Bruxelles - Analytical study of particle dark matter structuring on small scales and implications for dark matter searches
  • 2020, October - IAP, Paris, ICAP meetings - Statistics of the subhalo population in the Milky Way for the detection of dark matter point sources
  • 2020, June - LAPTh, Annecy - Dark Matter subhalo population in the Milky Way: From Particle Models to gamma-ray point sources

Conference and workshop talks

  • 2024, June - Rethymnon (Greece), invited, SynCRETism symposium - Probing dark matter with the 21cm line
  • 2024, May - Geneva CERN (Switzerland), invited, EuCAPT symposium - 21cm and CMB as dark matter probes
  • 2024, April - LaThuile (Italy), Moriond Cosmology - HERA sensitivity to dark matter decay
  • 2023, September - Trieste IFPU (Italy), invited, 21cm workshop - 21cm and exotic heating: the example of dark matter decay
  • 2023, September - Brussels (Belgium), invited, News From the Dark - 21cm signal sensitivity to dark matter decay
  • 2023, June - Ghent (Belgium), CosPa - HERA sensitivity to dark matter decay
  • 2023, January - Quy Nhon (Vietnam), invited, Rencontres du Vietnam: Theory meets experiments - Dark matter subhalo boost factor
  • 2022, October - Montpellier (France), Theorie Univers Gravitation workshop - Probing dark matter energy injection with the 21cm power spectrum
  • 2022, August - Vienna (Austria), Identification of DM - Sommerfeld-enhanced and subhalo-boosted dark-matter annihilation
  • 2022, January - LaThuile (Italy), Moriond Cosmology - Statistics of the subhalo population in the Milky Way for the detection of dark matter point sources
  • 2021, November - Annecy LAPTh (France), News From the Dark - An analytical model of subhalo population: mass function and stellar encounters
  • 2021, August - University of Illinois (online), Cosmo'21 - Subhalo properties in a simplified dark matter model and the impact on indirect searches
  • 2020, June - Montpellier (France), News From the Dark - Cold Dark Matter subhalos in the Milky Way: Baryonic tides and impact on indirect searches
  • 2019, December - Sydney (Australia), TeVPa - Statistics of subhalos in the Milky Way for dark matter indirect searches with gamma rays
  • 2019, October - Brussels (Belgium), IRN Terascale - Statistics of the subhalo population in the Milky Way for the detection of dark matter point sources
  • 2019, May - Montpellier (France), News From the Dark - Subhalos: the connection with simplified particle dark matter models

Poster presentations

  • 2019, July - Buenos Aires (Argentina), Dark Side of the Universe - Subhalo properties in a simplified dark matter model
  • 2019, June - Valencia (Spain), Invisible workshop - Subhalo properties in a simplified dark matter model

Attendance to international schools of physics

  • 2019, May - Heidelberg MPIK (Germany), ISAPP - The Dark Side of the Universe
  • 2019, January - Florence GGI (Italy) - Lectures on the Theory of Fundamental Interactions

An introduction on Dark Matter

Cosmology is the study of the Universe as a whole, its past, present and fate. The rigorous scientific and modern approach of this subject only begun in the 1920’s with new powerful observational devices allowing for better and better precision, and when physicists started to fully appropriate Einstein’s General Relativity as a tool to understand the dynamics of the entire Universe. Indeed, using this theory, Alexander Friedmann proposed in 1922 that the Universe had to be in an ongoing expansion. Two years later, in 1924, thanks to the new Hooker telescope of Mount Wilson, Edwin Hubble understood that many object in space that were taken to be nebula were in fact galaxies far beyond our own Milky-Way and he measured their velocities. Then, in 1927 Georges Lemaitre independently found the same result than Friedmann and with Hubble observations he was able to give an approximation of the expansion rate. Eventually, in 1929 Hubble gave a more precise value for this rate and what is now called the Hubble expansion law. Since then modern cosmology has build on these breaktrhough and has been in constant evolution thanks to better and better instruments and experiments. Today, the best description of our Universe today is provided by a model with two puzzling ingredients: dark energy and dark matter (herafter denoted DM). Understanding their nature is therefore a key goal of modern physics. In the following we focus on the DM issue.

From the beginning of the 20th century physicists have tried to evaluate the mass of dark bodies or dark stars (i.e. massive, non luminous matter) in the solar neighborhood. In 1906 Henri Poincaré coined the generic denomination matière sombre (dark matter in english). Following the work of lord Kelvin on the kinetic of gaz applied to gravitationally bound stellar systems he postulated that the mass of matière sombre had to be inferior than the mass of luminous matter(*).

Il y a les étoiles que nous voyons parce qu'elles brillent; mais ne pourrait-il y avoir des astres obscurs qui circulent dans les espaces interstellaires et dont l'existence pourrait rester longtemps ignorée ? Mais alors, ce que nous donnerait la méthode de lord Kelvin, ce serait le nombre total des étoiles, en y comprenant les étoiles obscures; comme son chiffre est comparable a celui que donne le téléscope, c'est qu'il n'y a pas de matière obscure , ou du moins qu'il n'y en a pas tant que de matière brillante. - Henri Poincaré La Voie lactée et la théorie des gaz, 1906

However this computation suffered from large uncertainties and as he pointed out, this result was only a mere order of magnitude. Therefore one often says that the problem of DM appeared three decades later, in 1933 when the astrophysicist Fritz Zwicky, who was interested in the Coma galaxy cluster, tried to evaluate its mass (*). He provided different estimates obtained with two kinds of methods, in the vein of the studies made by lord Kelvin, Poincaré and others in the local environment until then. On the one hand, he used a dynamical estimate from the measurement of the galaxies velocity dispersion and based on the law of gravitation. On the other hand he evaluated the mass of luminous visible matter (via the mass-luminosity relation) in the cluster. Unexpectedly the two resulting masses came out to be different by two orders of magnitude and according to Zwicky, this was a hint for the possible existence of dunkle materie (translation of dark matter in german) lying inside the cluster. Although this was the first experimental appearance of DM, this discovery did not receive much credits because most physicists were still concerned by the measurement errors. The issue became pressing only in the 1970’s in part due to Vera Rubin and Kent Ford who measured the velocity of stars in spiral galaxies and discovered that the result was once again different than the prediction made by the law of gravitation: stars on the outer arms have higher velocities than expected(*). From this point forward the problem of DM has drawn more and more attention in physics. Today we know that introducing DM into the theoretical models could explain many more cosmological and astrophysical observations such as the inhomogeneities in the Cosmic Microwave Background (CMB) the Big-Bang nucleosynthesis (BBN), the formation of structures such as galaxies and cluster of galaxies, gravitational lensing effects etc. No known form of matter could play the role of DM (e.g. the BBN and CMB constraints). There is little doubt on its existence and as mentioned earlier, understanding its nature is an important key to understand the Universe.

Therefore there are two possibilities to tackle the DM problem. The first one is to state that, in fact, Einstein’s law of gravitation is not valid for large length scales and should be modified or replaced. With this point of view DM does not really exist and our misunderstanding of gravity itself is the cause of all the abnormal observations. However, finding an accurate and physically acceptable modification is a real challenge and generally require the introduction of new degree of freedom for the new fundamental thoery of gravity to work. One could mention for instance the empirical theoretical proposal MOND(*) (for MOdified Newtonian Dynamics) that relies on this eventuality but whose "covariantization" remains a very challenging task. The second way to think of this problem is to introduce DM as new massive particles: non baryonic particles. It could be particles predicted by Standard Model (SM) extensions: supersymmetry, Kaluza-Klein theories, string theory, etc, or particles introduced to solve more specific problems like axions or massive neutrinos. Not to forget that it could also be made of other exotic, while macroscopic, objects, like primordial black holes. Today DM particles are actively searched through different strategies, exemplified by a series of experiments or observational programs, e.g.: EDELWEISS, CDMS/SuperCDMS, XENON1T, AMS02, Gaia, Fermi. On the one hand there are the direct detection experiments which try to find DM through its interaction with classical SM matter, assuming that such an interaction exists. On the other hand the indirect detection goal is to find the possible by-products, such as photons or comsic rays, of the DM annihilation into SM particles, coming from different structures (the Milky Way, the dwarf galaxies, the galaxy clusters, ...).

In the commonly accepted paradigm DM is said to be cold - according to observational requirements set on its velocity at given key moments of its history in the Universe. One then talk about the cold-DM (CDM) scenario. However, althouth it is successful to explain the structures on galactic scales and beyond, there are still some issues on subgalactic scales(*), the most famous ones being : Cusp/Core, Missing Satellites, and Too-Big-to-Fail problems. At this scales baryonic physics may also play an important role but today the underlying physics is not sufficiently known to give precise answers. That make crucial the detailed understanding of the structuring of DM and related observational consequences on these scales. This may lead to additional constraints or discovery perspectives for CDM candidates.