Daniel Pomarède

Numerical simulations are a key tool for deciphering the dynamics of gravitation. Yet, they fail to spatially reproduce the Universe we observe, limiting the comparison between observations and simulations to a statistical level. This is highly problematic for rare, faint, or well-studied nearby objects that are observed in a single environment. The computational cost of recovering this environment in random simulations is prohibitive for most realistic applications. We present ̋amletPM, a method that enables the constraining of initial conditions for cosmological simulations so as to produce evolved numerical universes that can be directly compared to observations of the local Universe. Such simulations are known as constrained simulations. Our method implements the field-level forward modeling of the early-time density field from sparse and noisy measurements of late-time peculiar velocities. The dynamics are integrated with a coarse particle-mesh gravity solver, probing the mildly nonlinear regime. We applied our code to the Cosmicflows-4 compilation of peculiar velocities up to z ). The resulting constrained simulations were analyzed under the framework of the local Universe model as well as an amended version that captures systematic spatial shifts in the scattering of simulated counterparts. (160,h^ -1 ̊m Mpc h^ -1 Mpc ^3 box are presented. These spatially reproduce the observed large-scale structure in the constrained volume. Special attention is given to twelve prominent nearby galaxy clusters, whose simulated counterparts are matched on criteria of mass and separation. We provide a mass estimate constrained by the dynamical environment for each cluster. Field-level forward modeling of the initial conditions can be applied to actual observations so as to produce highly constrained cosmological simulations. In its current form, this method already overtakes in quality the pipeline currently in use in the peculiar-velocity community, although systematic biases still need to be addressed. Furthermore, improving the model can be done easily thanks to the inherent flexibility of the Bayesian approach.

Muon tomography is a consolidated technique to investigate the internal components of high-opacity objects. With the use of detectors with high angular and spatial resolution (usually referred to as “muon telescopes”), the track reconstruction of the atmospheric muons after they have crossed the object provides muography images, also called “muon radiographs.” This work presents the muography (or muon radiography) reconstruction of a French historical UNGG (natural-uranium, graphite, and gas) nuclear reactor: the G3 reactor, located at CEA Marcoule. A measurement campaign involving 46 points of view below the reactor was conducted, providing a set of muography images. The method to analyze these muographies is detailed here. Besides, a method to combine these images to provide three-dimensional (2D) tomography of the reactor is also presented. After a general description of the project, all the methods performed to reconstruct and analyze muographies and the 3D reconstruction are presented. It comprises different tools based on data augmentation and machine learning, which proved to be very efficient on simulated data and increase the quality of the experimental data analysis. All these methods were evaluated with a set of synthetic data, providing a precise evaluation for better identification of the associated systematics. Finally, all the described methods were applied to the experimental data, leading to a full characterization of the reactor, in both two dimensions and three dimensions, and the identification of some discrepancies between the reactor actual structure and the original model. These anomalies are discussed, showing the potential of the technique to evaluate the internal structure of a nuclear reactor. This could have potential applications during the decommissioning phase and the dismantling phase of different kinds of nuclear installation.

Theory of the physics of the early hot universe leads to a prediction of baryon acoustic oscillations (BAOs) that has received confirmation from the pairwise separations of galaxies in samples of hundreds of thousands of objects. Evidence is presented here for the discovery of a remarkably strong individual contribution to the BAO signal at z = 0.068, an entity that is given the name Ho’oleilana. The radius of the 3D structure is 155 h 75 − 1 Mpc. At its core is the Boötes supercluster. The Sloan Great Wall, Center for Astrophysics Great Wall, and Hercules complex all lie within the BAO shell. The interpretation of Ho’oleilana as a BAO structure with our preferred analysis implies a value of the Hubble constant of 76.9 − 4.8 + 8.2 km s − 1 Mpc − 1 .

The inspection of very large or thick structures represents one of the biggest challenges for nondestructive techniques. For such objects, a particularly powerful technique is muography, which makes use of free, natural cosmic-ray muons. Among other applications, this technique has been applied to provide two-dimensional (2D) images of nuclear reactors, pyramids, or volcanos. Recently, 3D algorithms developed for medical applications were adapted to the special case of muon imaging to derive density maps. The main difficulties relate to the size of the object and to the limited number of available projections. Here, we report on the first 3D imaging of a whole nuclear reactor, obtained without any prior information on its structure and using the largest set of muographic projections ever made in this field.

With Cosmicflows-4, distances are compiled for 55,877 galaxies gathered into 38,065 groups. Eight methodologies are employed, with the largest numbers coming from the correlations between the photometric and kinematic properties of spiral galaxies (TF) and elliptical galaxies (FP). Supernovae that arise from degenerate progenitors (type Ia SNe) are an important overlapping component. Smaller contributions come from distance estimates from the surface brightness fluctuations of elliptical galaxies and the luminosities and expansion rates of core-collapse supernovae (SNe II). Cepheid period–luminosity relation and tip of the red giant branch observations founded on local stellar parallax measurements along with the geometric maser distance to NGC 4258 provide the absolute scaling of distances. The assembly of galaxies into groups is an important feature of the study in facilitating overlaps between methodologies. Merging between multiple contributions within a methodology and between methodologies is carried out with Bayesian Markov chain Monte Carlo procedures. The final assembly of distances is compatible with a value of the Hubble constant of H ₀ = 74.6 km s⁻¹ Mpc⁻¹ with the small statistical error of ±0.8 km s⁻¹ Mpc⁻¹ but a large potential systematic error of ∼3 km s⁻¹ Mpc⁻¹. Peculiar velocities can be inferred from the measured distances. The interpretation of the field of peculiar velocities is complex because of large errors on individual components and invites analyses beyond the scope of this study.

The trajectories since z = 4 of systems of galaxies (“halos”) with cz < 8000 km s⁻¹ are found through numerical action reconstructions. A set of 9719 halos from a Two Micron All Sky Survey group catalog and Cosmicflows-3 catalogs are given attention. Present distances are adjusted to minimize departures from observed redshifts. For those with the most precisely determined distances, compromises are made between distance and redshift agreement. H ₀ is varied from 69 to 77 km s⁻¹ Mpc⁻¹, with Ω _m set by the baryon acoustic oscillation constraint from the Planck satellite. A best-fitting amplitude of the mass-to-light relation is found. A uniform density associated with the interhalo medium accounts for the matter not in halos. The solution paths provide the histories of the formation of the nearby large structures and depict how the voids emptied. Assuming no local over/underdensity, the best model has H ₀ = 73 km s⁻¹ Mpc⁻¹, with nearly the same density arising from interhalo matter (IHM) as from halos. We examine local over/underdensities by varying the IHM density and find a valley of best-fit models along H ₀ = 73.0(1 + 0.165δ) km s⁻¹ Mpc⁻¹. Friedmann models with distinct densities internal and external to the study region give a similar relationship. The fraction of matter in the IHM seen in n-body simulations roughly matches that in our H ₀ = 72 scenario. Videos have been created to visualize the complexities of formation of large-scale structures. Standard n-body calculations, starting from the first time steps as tests of the numerical action method solutions and continuing until cosmic scale factor a = 2, provide glimpses into the future.

Velocity and density field reconstructions of the volume of the universe within 0.05c derived from the Cosmicflows-3 catalog of galaxy distances has revealed the presence of a filamentary structure extending across ∼0.11c. The structure, at a characteristic redshift of 12,000 km s⁻¹, has a density peak coincident with the celestial South Pole. This structure, the largest contiguous feature in the local volume and comparable to the Sloan Great Wall at half the distance, is given the name the South Pole Wall.

Tools are provided at the Extragalactic Distance Database website which provide relationships between the distances and velocities of galaxies based on smoothed versions of the velocity fields derived by the Cosmicflows program.