Session I: Galaxy Formation

Session II: Dark Matter

Session III: Large-scale Structure

Session IV: Cosmic Microwave Background

Session V: Gravitational Waves

Session VI: Limits of Cosmology

Session VII: Conclusions


Full program (pdf)

Monday, 11 December 2017

08:30 Registration
Chair: Jean Audouze
09:30 Welcome  [PDF]  [video] Francis Bernardeau
09:45 What Joe has taught us?
[abstract I will give my personal view on the impact that Joe had not only as researcher but also as trainer of new cohorts of cosmologists. ]  [PDF]  [video]
Nicola Vittorio
Session I: Galaxy Formation
Chair: Arif Babul
10:15 Scaling Laws for Dark Halos of Galaxies
[abstract How do the properties of dark halos change with the brightness of the galaxies? Galaxies of different masses form at different redshifts, so we would expect some changes. We measure the densities and scale radii for the dark halos of spirals from their rotation curves. For the faintest pressure supported dwarf galaxies, we can use the structure and velocity dispersions of their stars or gas. It turns out that the dark halos have well defined scaling laws between the brightness of the parent galaxy and the density and scale radii of the dark halos, extending over about 20 magnitudes in brightness. The origin of these scaling laws goes back to the fluctuation spectrum of the early universe. We also conclude that there may be a population of dark dwarfs with circular velocities less than about 40 km/s. ]  [PPT]  [video]
Kenneth Freeman
10:45 The Magic Scale of Galaxy Formation: Supernova Feedback, Hot Halo, Black-Hole Growth and Compaction to a Blue Nugget
[abstract I will describe the physical origin of the optimal scale for galaxy formation, in halos of ~10^{12} M_solar, and the contribution of Joe Silk to its understanding. ]  [PDF]  [video]
Avishai Dekel
11:15 Coffee break
Chair: Sadegh Khochfar
11:45 The symphony of quasar and stellar feedback
[abstract Feedback from active galactic nuclei (AGN) is routinely included in semi-analytic models and numerical simulations. They have shown that such feedback is able to quench cooling flows and decrease star formation. However, in cosmological simulations AGN feedback is approximated by only a few parameters and usually tuned to match global observations. In reality, these parameters hide complex physics working on galactic scales that may influence the evolution of these galaxies. I will explore the impact of a more realistic treatment of the quasar mode of AGN feedback, where radiation is emitted from a thin accretion disc surrounding the blackhole, and discuss the intertwined interaction between AGN feedback, the interstellar medium, and stellar feedback. I will further show how stellar feedback plays a crucial role in shaping the efficiency of the radiation-gas coupling and examine the possible impact of both AGN and stellar feedback in high-redshift, gas-rich galaxies. ]  [PDF]  [video]
Rebekka Bieri
12:00 The manifold nature of AGN feedback from 3D simulations
[abstract AGN are source of relativistic jets, which act as a very important feedback mechanism on galactic scales. Thermal jet feedback can directly heat and destroy cold, star-forming gas clumps, thus contribute to the creation of red-and-dead galaxies. At the same time, mechanical jet feedback can explain the properties of the hot and cold outflows observed at all wavelenghts.
We have performed a series of high-resoultion 3D numerical simulations of the interaction of AGN jets with a cold phase of the Inter-Stellar Medium of their host galaxies: after an initial compression, gas is efficiently heated so that the mass of cold gas available for star formation decreases on a scale of few Myr (depending on distance from the AGN), and accelerated to produce multi-phase outflows. Finally, we use the simulations to briefly compare the effects of jet and quasar feedback.
These findings may explain the co-existence of negative and positive feedback in recently observed in high-redshift starburst galaxies.
]  [PDF]  [video]
Salvatore Cielo
12:15 Kinematics and dynamics of molecular gas in galaxies
[abstract Molecular hydrogen (H2) is a fundamental component of galaxies, being the most abundant element in molecular clouds, where stars form, and an important source of radiative cooling at low temperature. With the advent of the ALMA telescope, a large amount of data about the distribution of H2 in galaxies has become available. However, the large majority of numerical simulations on galactic and cosmological scales still lacks the ability to directly follow the formation and dissociation of H2, and must rely on pre-calibrated sub-grid models to compare the results with observations. I will present a new model to self-consistently track the evolution of H2, including gas and dust shielding, H2 self-shielding, star formation (SF), supernova feedback, and extragalactic and local stellar radiation. I will discuss the results of a suite of hydrodynamic simulations of an isolated gas-rich galaxy at z=3, showing that the model can naturally reproduce the observed correlation between SF and H2 surface densities, without assuming any a priori dependence of SF on the H2 abundance. I will also present a study of the kinematics and dynamics of molecular gas in high-redshift quasars (z=6), where we investigate whether a central accreting black hole (BH) can significantly affect the H2 distribution in the host galaxy and generate molecular outflows. ]  [PDF]  [video]
Alessandro Lupi
12:30 Dark matter or modified gravity?
[abstract An alternative to dark matter, proposed more than 30 years ago by M. Milgrom, is quite successful at galaxy scales, and reproduces the MDAR (Mass Discrepancy-Acceleration Relation) much better than dark matter models. It has however problems at large scales, for clusters and cosmology. I will review its present status, and the various possible observational tests to select among the two models. ]  [PDF]  [video]
Françoise Combes
13:00 Lunch break
14:45 Round table: Ian McCarthy, Marc Huertas-Company, Evan Scannapieco, Rosemary Wyse
  • Is galaxy formation at high redshift qualitatively different from that at low redshift?
  • Do we understand feedback and its impact on galaxy evolution?
  • What are the links between galaxy properties and the cosmic web (beyond the halo mass dependency)?
Session II: Dark Matter
Chair: Emmanuel Moulin
15:30 Particle Candidates for Dark Matter
[abstract Particle candidates for dark matter range in mass from the Planck mass down to fractions of an electron volt. The long-dominant WIMP paradigm is under pressure from the LHC and non-accelerator experiments, but is still an attractive option. There are increasing efforts to frame searches in terms that are as model-independent as possible. In addition to discussing these, I will also present a speculative idea to look for a dark matter signal in the spectrum of gravitational waves from neutron-star mergers. ]  [PPT]  [video]
John Ellis
16:00 Dark Matter Indirect Detection, a review of all the early ideas by Joe that still keep us busy
[abstract I will briefly review the most promising channels for Dark Matter indirect detection (gamma rays, antiprotons, positrons, neutrinos), expressing awe at how the papers by Joe Silk are almost always among the seminal ones in each channel. I will then give a concise presentation of the current status. ]  [PDF]  [video]
Marco Cirelli
16:30 Coffee break
17:00 Nonlinear structure in the dark matter distribution
[abstract I will discuss predictions for nonlinear structure in the dark matter distribution on scales ranging from those of the cosmic web down to those of dark matter detection experiments, covering topics as diverse as the importance of caustics for dark matter annihilation signals, variability in the density and velocity distribution of Galactic dark matter on Solar System scales, and the typical density of dark matter in intergalactic space. ]  [PDF]  [***]
Simon White
17:30 Probing dark matter around black holes at the centers of galaxies
[abstract The dark matter (DM) distribution in the central regions of galaxies remains poorly constrained at present. In particular, DM density profiles may be significantly affected by the presence of central black holes, leading to the possible formation of density spikes. I will discuss different avenues that can shed light on the characteristics of the DM distribution in the cores of galaxies. I will focus on DM annihilation signals in the vicinity of supermassive black holes with the Event Horizon Telescope, and I will describe the signatures of a plausible population of intermediate mass black holes in the Milky Way. I will also argue that one can obtain robust constraints on the DM density profile at the Galactic center by studying the dynamics of stars on close orbits around the central supermassive black hole Sgr A*. ]  [PDF]  [***]
Thomas Lacroix
17:45 Massive Black holes from dissipative Dark Matter
[abstract We show that a subdominant component of dissipative dark matter resembling the Standard Model can form many intermediate-mass black hole seeds during the first structure formation epoch, such that a few percent of all the dwarf galaxies host one. We also observe that, in the presence of this matter sector, the black holes will grow at a much faster rate with respect to the ordinary case. These facts can explain the observed abundance of supermassive black holes feeding high-redshift quasars. ]  [PDF]  [***]
Paolo Panci
18:00 END
18:15 Cocktail

[***] These 3 videos are not available due to the power failure on Monday at the end of the day in the amphitheatre.

Tuesday, 12 december 2017

Session II: Dark Matter (end)
Chair: Bernard Carr
09:30 Round table: Céline Boehm, Stéphane Colombi, Keith Olive, Pierre Salati
  • Where shall we expect to detect DM first: directly, indirectly or in colliders?
  • Do hydrodynamic simulations solve both the cusp/core and over-abundant satellite issues?
  • Which among Joe’s predictions for DM annihilation products will be detected first: photons, anti-protons, or neutrinos?
Session III: Large-scale Structure
Chair: Henry Joy McCracken
10:15 Analytic models of large-scale structure
[abstract I will discuss how techniques from field theory can be used to model the clustering of the matter and biased tracers in large-scale structure and what we can learn from application of these techniques to present and future data sets. ]  [PDF]  [video]
Martin White
10:45 Conference photo & Coffee break
11:15 Connecting the cosmic web to galaxy formation
[abstract A key issue in the context of galaxy evolution is trying to determine the role of the cosmic web in shaping their physical properties and morphology. I will present the current status on this topic. ]  [PDF]  [video]
Christophe Pichon
11:45 Cosmology from large scale structure
[abstract The observed structure in the universe is a treasure-trove of information but accessing it is complicated by non-linear evolution, bias, and systematics. I will discuss approaches to access the abundant fluctuations modes beyond the linear regime to probe the questions of cosmic acceleration, dark matter, and the cosmic beginning. ]  [video]
Benjamin Wandelt
12:15 Lunch break
14:15 Round table: August Evrard, Nick Kaiser, Alexander Szalay
  • Is there any tension between observed LSS and those predicted in the LCDM context? + Is bias a fundamental limitation for doing cosmology with LSS?
  • How do we best extract information beyond the power spectrum?
Session IV: Cosmic Microwave Background
Chair: Jérôme Martin
15:00 Cosmology after Planck
[abstract I will review the main results of the planck satellite mission, at least regarding cosmology, hoping that the final legacy release results will be available by the time of the meeting. In any case, I'll place them in an historical context and will give a view on aspects fo the future with CMB probes. ]  [PDF]  [video]
François Bouchet
15:30 Evidences for inflation, constraints on alternatives
[abstract CMB observations yield the best measurements of cosmological parameters, especially when combined with other observables. Replaced in the $\Lambda$CDM framework, they strongly suggest that the primordial perturbations out of which large scale structures formed originated from the quantum vacuum fluctuations of a single scalar degree of freedom, thereby providing evidence for a phase of inflation before the radiation and matter eras. I will review this evidence supporting inflation, discuss the corresponding constraints on the bouncing alternatives and argue that the most recent data may also support a slight modification of the primordial power spectrum on the largest scales. ]  [PDF]  [video]
Patrick Peter
16:00 The next generation of ground based CMB experiments
[abstract In this talk, I will discuss the most recent results of ground based CMB experiments: ACTPol, SPTPOL, BICEP/Keck array and Polarbear. I will then focus on the next generation of ground based experiments, especially Simons Observatory and CMB S4. ]  [PDF]  [video]
Thibaut Louis
16:15 Coffee break
Chair: Glenn Starkman
16:45 Round table: Nabila Aghanim, Douglas Scott, Naoshi Sugiyama
  • What are the prospects to tackle inflation through the CMB?
  • What can we learn from cross-correlations of the CMB with other tracers?
  • Should we prioritize B-mode polarization or spectral distortions as the next new frontier of the very early Universe?
17:30 Counterfactual universes (Special talk)
[abstract It is well known that the structure of our universe and the emergence of complexity are dependent on the values of a few basic microphysical and cosmological numbers. I will discuss the consequences of tweaking the values of these numbers. This exercise is of course a necessary strand of cosmological research if there are multiple vacuum states or if 'eternal inflation' leads to a multiplicity of big bangs. However, even those who are allergic to the 'A-word' may have their intuition developed by exploring these alternative scenarios -- just as some historians explore 'counterfactual scenarios, such as what might have happened if the Brits had lost at Waterloo. ]  [PPT]  [video]
Martin Rees
18:00 END

Wednesday, 13 December 2017

Session V: Gravitational Waves
Chair: Martin Haehnelt
09:00 Gravitational Wave Astronomy and Cosmology
[abstract Since the first detection of GW150914 two years ago, gravitational wave astronomy has been moving fast. I will review the list of detections so far, what we have learned from them, what new questions they pose, and what we can expect in the near future. The ground-based network will grow in the medium term and begin to explore out to modest redshifts of perhaps 0.5. The LIGO detections plus the dramatic success of the LISA Pathfinder mission last year and this have led to the confirmation of LISA as an ESA mission with NASA participation; LISA will start exploring early cosmology using black hole binaries as a tool after its launch around 2034. ]  [PDF]  [video]
Bernard Schutz
09:30 Gravitational Waves and Theory
[abstract The gravitational wave detectors LIGO/VIRGO have discovered the signals generated by the coalescence of binary black holes at astronomical distances. Theoretical and numerical works on the two-body problem in general relativity (GR) play a very important role when deciphering and interpreting the gravitational wave signals. In this talk we present the state-of-the-art on post-Newtonian (PN) methods in GR, applied to the gravitational waveform generated during the inspiraling phase of coalescing black hole binaries. In particular we discuss recent developments on the fourth post-Newtonian (4PN) approximation. ]  [PDF]  [video]
Luc Blanchet
10:00 Studying stellar evolution with gravitational wave observations
[abstract The first observations of gravitational waves emitted by merging binary black holes demonstrated the existence of black holes more massive than previously observed in X-ray in our Galaxy. Future observations with ground-based interferometers such as LIGO and Virgo will probe the mass and spin distributions of black holes in various galactic environments. Since stellar-mass black holes form at the end of the nuclear lifetimes of massive stars, these measurements will provide a new tool for stellar evolution studies. In this talk I will describe a cosmological framework for modeling the population of black holes and its evolution with redshift. I will then show which features of stellar evolution models can be constrained with future observations of binary black hole mergers. Finally, I will discuss the stochastic gravitational wave background from merging and inspiraling binary black holes. ]  [PDF]  [video]
Irina Dvorkin
10:15 Conference photo & Coffee break
Chair: Maria Angeles Perez-Garcia
10:45 Massive black holes and gravitational waves
[abstract I'll discuss the population of massive black holes evolving in galaxies across cosmic time in the context of gravitational wave observations. I'll highlight the physical processes linking massive black hole evolution to observable gravitational wave events, and then how gravitational wave observations can help us understand in turn massive black hole evolution. ]  [PDF]  [video]
Marta Volonteri
11:15 Round table: Bruce Allen, Enrico Barausse, Guillaume Faye, Samaya Nissanke
  • What are the prospects to tackle inflation through the primordial GW?
  • Can GW observations tell us something about Dark Matter/Dark Energy or about the quantum nature of gravity?
  • What do direct GW observations tell us about whether GR is correct? will they ever be more constraining than binary pulsar observations in this respect?
  • Do massive BH binaries merge?
12:00 Lunch break
Chair: Mario Livio
13:30 LCDM: Much more than we expected, but now less than what we want (Special talk)
[abstract The LCDM cosmological model is remarkable: with just 6 parameters it describes the evolution of the Universe from a very early time when all structures were quantum fluctuations on subatomic scales to the present, and it is consistent with a wealth of high-precision data, both laboratory measurements and astronomical observations. However, the foundation of LCDM involves physics beyond the standard model of particle physics: particle dark matter, dark energy and cosmic inflation. Until this new physics is clarified, LCDM is at best incomplete and at worst a phenomenological construct that accommodates the data. I discuss the path forward, which involves both discovery and disruption, some grand challenges and finally the limits of scientific cosmology. ]  [PDF]  [video]
Michael Turner
Session VI: Limits of Cosmology
14:00 Limits on cosmology as a testable science
[abstract Cosmology faces three fundamental problems: the uniqueness of the universe, limits of what will ever in a practicable sense be possible in observational terms (the visual horizon), and limits on our ability to test the relevant physics that governs its dynamics at early times (the physics horizon). The promise of unifications of different branches of physics (gravitational, atomic, nuclear, particle physics, and quantum gravity) with cosmology has not been fulfilled in the latter two cases. In the cases of the inflaton and dark energy, we are often faced with a form of `saving the phenomenon’ where running the field equations backwards (Synge’s g-method) is used to propose relevant effective potentials for a scalar field (this is possible because the energy conservation equation is in this case equivalent to the Klein-Gordon equation); but the resulting potential has no solid basis in physics, unless the inflaton is the electroweak Higgs. Phenomenological approaches often result in unphysical conclusions such as existence of matter with w < -1. Proposals for how a physical origin of the universe could have taken place run into the limits of testable scientific theorising. The major problems for inflation pointed out strongly by Roger Penrose because gravitational degrees of freedom in the extremely early universe are not excited whereas all the matter degrees of freedom are maximally excited might possibly be met by a quantum gravity theory where gravity is turned off at extremely early times, as suggested by Greene and collaborators. This might be a pointer towards the correct form of quantum gravity. A variety of multiverse proposals generically are untestable by means of astronomical observations. Their prime motivation is often philosophical, in particular explaining the observed value of the cosmological constant and its apparent contradiction with values predicted by quantum field theory; but that contradiction can be resolved by some form of unimodular gravity. Their status as scientific theories is therefore questionable (Ellis and Silk 2014). What are definitely not scientific claims are the variety of statements that there physically exist an infinite number of anything (universes, galaxies, people, whatever) in a Multiverse. ]  [PDF]  [video]
George Ellis
14:30 The future of empirically established cosmology
[abstract Non empirically-established constructs are important in physical science: consider in our subject prescriptions for star formation, sub-grid processes in galaxy formation, and elements of cosmology. Star formation is one of Joe Silk's long-term research interests, and he is the best authority on whether and in what manner star formation theory may become more predictive. The empirically-established LambdaCDM cosmology has inspired a galaxy formation theory that has been tuned to fit an impressively broad variety of observations. It has limited predictive power, but close attention to the growing wealth of observations may lead to a more predictive empirically-established small-scale cosmology, maybe in the process guiding us to an even better theory of large-scale structure. Might inflation be established as predictive? This is challenging, but there have been too many surprising advances in cosmology to rule it out. Might multiverses ever be empirically established, perhaps as a convincing prediction of a convincingly established fundamental theory? This seems really unlikely, and a good example of why we should be clear about the distinction between empirically- and nonempirically-established constructs, in cosmology as in all branches of natural science. ]  [PDF]  [video]
Jim Peebles
15:00 Round table: Alain Blanchard, Katherine Freese, Marc Kamionkowski, Rocky Kolb, Adi Nusser
  • Are there compelling reasons to go beyond LCDM?
  • Does dark energy vary with time and position? What happens when we kill the cosmological principle?
  • Is the concept of multiverse falsifiable?
15:45 Coffee break
Session VII: Conclusions
Chair: Michael Rowan-Robinson
16:15 Conclusion  [PDF]  [video] Joe Silk
16:45 END
19:30 Banquet (La Compagnie des Bateaux-Mouches)