Context. 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. Aims. We present H AMLET -PM, 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. Methods. 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 ∼ 0.05 (160 h −1 Mpc). 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. Results. A series of a hundred dark-matter only cosmological simulations with a resolution of 512 3 particles in a 500 3 [ 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. Conclusions. 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.

The Sloan Great Wall is a dominant structure that is relatively nearby. As well as evident in redshift survey maps, its presence is manifested in distortions to cosmic expansion. Here, Hamiltonian Monte Carlo forward reconstruction in a ΛCDM framework gives probabilistic density and velocity fields constrained by the Cosmicflows-4 compendium of galaxy distances and radial velocities. Streamlines of the reconstructed velocity field started from arbitrary points in space can be followed to sinks, i.e. the minima of the gravitational potential, due to the distribution of mass. A basin of attraction encompasses the volume of all streamlines ending at the same sink. The solution can be assigned probabilities, with uncertainties associated with the imperfect data and the random nature of the ΛCDM model. The Sloan basin of attraction is by far the largest basin in the study region, extending across a diameter of 0.13c. It can be described by velocity streamlines that converge on the Sloan Great Wall, by the reconstructed density field, and by the network of filaments of the V-web, formulated by shear in the velocity field. The discussion of these elements is augmented by a video and interactive models. It is of interest to see the relationship of the Ho`oleilana baryon acoustic oscillation feature with the Sloan basin of attraction.

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. Published by the American Physical Society 2025

The reconstruction of the large scale velocity field from the grouped Cosmicflows-4 (CF4) database is presented. The lognormal bias of the inferred distances and velocities data is corrected by the Bias Gaussianization correction (BGc) scheme, and the linear density and velocity fields are reconstructed by means of the Wiener filter (WF) and constrained realizations (CRs) algorithm. These tools are tested against a suite of random and constrained Cosmicflows-3-like mock data. The CF4 data consists of 3 main subsamples - the 6dFGS and the SDSS data - and the `others'. The individual contributions of the subsamples have been studied. The quantitative analysis of the velocity field is done mostly by the mean overdensity ($\Delta_L(R)$) and the bulk velocity ($V_{\mathrm{bulk}}(R)$) profiles of the velocity field out to $300\, h^{-1}{\rm Mpc}$. The $V_{\mathrm{bulk}}(R)$ and $\Delta_{\mathrm L}(R)$ profiles of the CF4 data without its 6dFGS component are consistent with the cosmic variance to within $1\sigma$. The 6dFGS sample dominates the $V_{\mathrm{bulk}}$ ($\Delta_{\mathrm L}$) profile beyond $\sim120\, h^{-1}{\rm Mpc}$, and drives it to roughly a $3.4\sigma$ ($-1.9\sigma$) excess (deficiency) relative to the cosmic variance at $R\sim250\ (190)\ \, h^{-1}{\rm Mpc}$. The excess in the amplitude of $V_{\mathrm{bulk}}$ is dominated by its Supergalactic X component, roughly in the direction of the Shapley Concentration. The amplitude and alignment of the inferred velocity field from the CF4 data is at $\sim(2\,-\,3)\,\sigma$ discrepancy with respect to the $\Lambda$CDM model. Namely, it is somewhat atypical but yet there is no compelling tension with the model.

Theory of the physics of the early hot universe leads to a prediction of baryon acoustic oscillations that has received confirmation from the pair-wise 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 baryon acoustic oscillation (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} Mpc. At its core is the Bootes supercluster. The Sloan Great Wall, CfA 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+8.2-4.8 km/s/Mpc.

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.

Transmission muography is a non-invasive and non-destructive imaging method which allows to estimate the integrated density of a volume in a given direction (also referred as opacity). It relies on a reconstruction of muons tracks that crossed the studied volume compared to the corresponding open sky expectation. The experimental setup of the muography developed at CEA Irfu consists in portable muon telescopes. Each of these instruments has four Micromegas gaseous detectors, power and acquisition electronics, and an embedded computer connected to the network allowing remote control. It is then well adapted to deploy them in constrained environments. A muography measurements campaign has been carried out in the decommissioned nuclear reactors G2 and G3 at CEA Marcoule (France) which are expected to be dismantled. We were able to conduct 2D muographies and 3D analyses of inside G2’s airtight concrete chamber. In this paper we describe the measurement protocol at the G2 and G3 reactors. We explain what was developed to improve the measurements between G2 and G3 campaigns. It shows how potential improvements were identified in the proof of concept and why we expect a better reconstruction for G3. Among the improvements, we describe how simulations prior to the measurements, denoising and in general automation play an important role to have a precise 3D image in reasonable time.

The WALLABY pilot survey has been conducted using the Australian SKA Pathfinder (ASKAP). The integrated 21-cm HI line spectra are formed in a very different manner compared to usual single-dish spectra Tully-Fisher measurements. It is thus extremely important to ensure that slight differences (e.g. biases due to missing flux) are quantified and understood in order to maximise the use of the large amount of data becoming available soon. This article is based on four fields for which the data are scientifically interesting by themselves. The pilot data discussed here consist of 614 galaxy spectra at a rest wavelength of 21cm. Of these spectra, 472 are of high enough quality to be used to potentially derive distances using the Tully-Fisher relation. We further restrict the sample to the 251 galaxies whose inclination is sufficiently close to edge-on. For these, we derive Tully-Fisher distances using the deprojected WALLABY velocity widths combined with infrared (WISE W1) magnitudes. The resulting Tully-Fisher distances for the Eridanus, Hydra, Norma and NGC 4636 clusters are 21.5, 53.5, 69.4 and 23.0 Mpc respectively, with uncertainties of 5–10%, which are better or equivalent to the ones obtained in studies using data obtained with giant single dish telescopes. The pilot survey data show the benefits of WALLABY over previous giant single-dish telescope surveys. WALLABY is expected to detect around half a million galaxies with a mean redshift of z = 0.05 (200 Mpc). This study suggests that about 200,000 Tully-Fisher distances might result from the survey.

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.

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 that provide relationships between the distances and velocities of galaxies based on smoothed versions of the velocity fields derived by the Cosmicflows program.

This letter presents a new approach using the cosmic peculiar velocity field to characterize the morphology and size of large-scale structures in the local Universe. The algorithm developed uses the three-dimensional peculiar velocity field to compute flow lines, or streamlines. The local Universe is then partitioned into volumes corresponding to gravitational basins, also called watersheds, among the different end points of the velocity flow lines. This new methodology is first tested on numerical cosmological simulations, used as benchmark for the method, and then applied to the Cosmic-Flows project observational data in order to pay particular attention to the nearby superclusters including ours. More extensive tests on both simulated and observational data will be discussed in an accompanying paper.

Cosmicflows-3 distances and inferred peculiar velocities of galaxies have permitted the reconstruction of the structure of overdensities and underdensities within the volume extending to 0.05c. This study focuses on the underdense regions, particularly the Local Void that lies largely in the zone of obscuration and consequently has received limited attention. Major overdense structures that bound the Local Void are the Perseus-Pisces and Norma-Pavo-Indus filaments separated by 8500 km s(-1). The void network of the universe is interconnected and void passages are found from the Local Void to the adjacent very large Hercules and Sculptor voids. Minor filaments course through voids. A particularly interesting example connects the Virgo and Perseus clusters, with several substantial galaxies found along the chain in the depths of the Local Void. The Local Void has a substantial dynamical effect, causing a deviant motion of the Local Group of 200-250 km s(-1). The combined perturbations due to repulsion from the Local Void and attraction toward the Virgo Cluster account for similar to 50% of the motion of the Local Group in the rest frame given by the cosmic microwave background.

To date at least 10 highly flattened planes of dwarf galaxies are claimed to have been discovered in the Local Universe. The origin of these planes of galaxies remains unknown. One suggestion is that they are related to the large-scale structure of the cosmic web. A recent study found that the normal of a number of these dwarf galaxy planes is very closely aligned with the eigenvector of the shear tensor corresponding to the direction of greatest collapse obtained by reconstructing the full velocity field in the linear regime. Here we extend that work by both considering an additional 5 planes beyond the five examined previously and examining the alignment with respect to the quasi-linear field, a more sophisticated reconstruction, which is a better approximation on smaller (quasi-linear) scales. Our analysis recovers the previous result while not finding a significantly tight alignment with the additional five planes. However, the additional five plane normals also do not appear to be randomly oriented. We conclude that this could be due either to the normals of the new planes being poorly defined and described; the quasi-linear field at those locations being poorly constrained; or different formation mechanisms for the orientation of planes of dwarf galaxies.

The local Universe provides a unique opportunity for testing cosmology and theories of structure formation. As the velocities of galaxies respond to the distribution of matter—both visible and dark—the velocity field provides structural information. Here, we present an original method for the reconstruction of the quasi-linear matter density and velocity fields from galaxy peculiar velocities and apply it to the Cosmicflows-2 database of velocites. The method consists of constructing an ensemble of cosmological simulations, constrained by the standard cosmological model and the observational data. The quasi-linear density field is the geometric mean and variance of the fully nonlinear density fields of the simulations. The main nearby clusters (Virgo, Centaurus and Coma), superclusters (Shapley and Perseus–Pisces) and voids (Dipole Repeller) are robustly reconstructed. Galaxies are born ‘biased‘ with respect to the underlying dark matter distribution. Using our quasi-linear framework, we demonstrate that the luminosity-weighted density field derived from the 2M++ redshift compilations is nonlinearly biased with respect to the matter density field. The bias diminishes in the linear regime. Reconstructing matter density from the velocities of local galaxies in a linear manner is standard practice. Averaging over the density fields of an ensemble of nonlinear simulations reveals a stronger galaxy ‘bias’ than in the linear regime, providing insights into the distribution of dark matter and the formation of galaxies.

A toy model of the post-shock region of core-collapse supernovae is used to study the non-linear development of turbulent motions driven by convection in the presence of advection. Our numerical simulations indicate that buoyant perturbations of density are able to trigger self-sustained convection only when the instability is not linearly stabilized by advection. Large amplitude perturbations produced by strong shock oscillations or combustion inhomogeneities before the collapse of the progenitor are efficiently shredded through phasemixing and generate a turbulent cascade. Our model enables us to investigate several physical arguments that had been proposed to explain the impact of the dimensionality on the onset of explosions in global simulations of core-collapse supernovae. Three-dimensional (3D) simulations are found to lead to higher entropy values than two-dimensional (2D) ones. We attribute this to greater turbulent mixing and dissipation of the kinetic energy into heat in 3D. Our results show that the increase of entropy is enhanced with finer numerical resolution and larger perturbation amplitude.

The fully nonlinear gravitationally induced trajectories of a nearly complete set of galaxies, groups, and clusters in the Local Supercluster are constructed in a numerical action method model constrained by data from the CosmicFlows survey and various distance indicators. We add the gravity field due to inhomogeneities external to the sample sphere by making use of larger-scale peculiar flow measurements. Assignments of total masses were made to find the best overall set of mutual attractions, as determined by a goodness criterion based on present-day radial velocities, individually for the Virgo Cluster, M31, and the Milky Way (MW), and via a mass-to-light ratio relationship for other masses. The low median chi-square found indicates that the model fits the present-day velocity flow well, but a slightly high mean chi-square may indicate that some masses underwent complex orbits. The best fit, when setting the value of H-0 to the CosmicFlows value of 75 km s(-1) Mpc(-1) and the WMAP value for Omega(m) = 0.244 consistent with that H-0, occurs with the following parameters: Omega(orphan) = 0.077 +/- 0.016, M/L-K = 40 +/- 2L(10)(0.15) M-circle dot/L-circle dot (L-10 is the K-band luminosity in units of 10(10) L-circle dot), a Virgo mass of 6.3 +/- 0.8 x 10(14) M-circle dot (M/L-K = 113 +/- 15 M-circle dot/L-circle dot), and a mass for the MW plus M31 of 5.15 +/- 0.35 x 10(12) M-circle dot. The best constant mass-to-light ratio is M/L-K = 58 +/- 3 M-circle dot/L-circle dot. The Virgocentric turnaround radius is 7.3 +/- 0.3 Mpc. We explain several interesting trends in peculiar motions for various regions now that we can construct the 3D orbital histories.

Cosmography, the study and making of maps of the universe or cosmos, is a field where visual representation benefits from modern three-dimensional visualization techniques and media. At the extragalactic distance scales, visualization is contributing to our understanding of the complex structure of the local universe in terms of spatial distribution and flows of galaxies and dark matter. In this paper, we report advances in the field of extragalactic cosmography obtained using the SDvision visualization software in the context of the Cosmicflows Project. Here, multiple visualization techniques are applied to a variety of data products: catalogs of galaxy positions and galaxy peculiar velocities, reconstructed velocity field, density field, gravitational potential field, velocity shear tensor viewed in terms of its eigenvalues and eigenvectors, envelope surfaces enclosing basins of attraction. These visualizations, implemented as high-resolution images, videos, and interactive viewers, have contributed to a number of studies: the cosmography of the local part of the universe, the nature of the Great Attractor, the discovery of the boundaries of our home supercluster of galaxies Laniakea, the mapping of the cosmic web, and the study of attractors and repellers.

In the standard (LCDM) model of cosmology the universe has emerged out of an early homogeneous and isotropic phase. Structure formation is associated with the growth of density irregularities and peculiar velocities. Our Local Group is moving with respect to the cosmic microwave background (CMB) with a velocity 631+/-20 km s-1 and participates in a bulk flow that extends out to distances of at least 20,000 km s-1. Since the discovery of the CMB dipole, the implicit assumption was that excesses in the abundance of galaxies induce the Local Group motion. Yet, underdense regions push as much as overdensities attract but they are deficient of light and consequently difficult to chart. It was suggested a decade ago that an underdensity in the northern hemisphere roughly 15,000 km s-1 away is a significant actor in the local flow. Here we report on kinematic evidence for such an underdensity. We map the large scale 3D velocity field using a Wiener filter reconstruction from the Cosmicflows-2 dataset of peculiar velocities, and identify the attractors and repellers that dominate the local dynamics. We show here that the local flow is dominated by a single attractor -associated with the Shapley Concentration- and a single previously unidentified repeller. Multipole expansion of the local flow provides further support for the existence and role played by the attractor and repeller. The bulk flow (i.e. dipole moment) is closely (anti)aligned with the repeller at a distance of 16,000+/-4,500 km s-1. The expansion eigenvector of the shear tensor (quadrupole moment) is closely aligned with the Shapley Attractor out to 7,000 km s-1. The close alignment of the local bulk flow with the repeller provides further support for its dominant role in shaping the local flow. This Dipole Repeller is predicted to be associated with a void in the distribution of galaxies.