arXiv subjects

Albert Bonnefous

Publications and source records attributed to Albert Bonnefous.

4 recordsLinked to original sources

arXiv2026

The kinematic cosmic dipole beyond Ellis and Baldwin

The cosmic dipole anomaly, currently detected at a significance exceeding 5$\sigma$ in several independent survey poses a significant challenge to the standard model of cosmology. The Ellis & Baldwin formula provides a theoretical link between the intrinsic dipole anisotropy in the sky distribution of extragalactic light sources and the observer's velocity relative to the cosmic rest frame, under the assumptions that the sources follow a power-law luminosity function and exhibit power-law spectral energy distributions. In this work, we demonstrate that this relation can be generalized to arbitrary luminosity distributions and spectral profiles. We derive the corresponding expression for the effective spectral index and apply it to a sample of quasars observed in the W1 band of the CatWISE survey. We show that the anomalous cosmic dipole persists beyond the power-law assumption. These results provide a more general and robust framework to interpret measurements of the cosmic dipole in future large-scale surveys.

astro-ph.CO↗

arXiv2026

Weak Lensing Low Multipoles

We analyse the low--multipole components of the weak-lensing convergence field in a FLRW universe. The low--multipole convergence field, encodes the largest-angle coherent potential gradients, essential for assessment of large-angle features in data. To study this large angle signal, we perform a combined analytical, numerical and observational study. Starting from exact analytical expressions for the convergence power spectrum, we quantify how the dipole, quadrupole and octupole build up with source redshift and show that, in $\Lambda$CDM, they saturate at an amplitude of order $10^{-4}$. We then use full-sky, horizon-scale $N$-body simulations (Quijote) to explore the dependence of this signal on the observer's environment, comparing random observers to ``Milky Way--like'' observers. In parallel, we reconstruct the convergence field due to our local Universe with the 2MASS Redshift Survey (2MRS), with proper treatment of incompleteness and galaxy bias. We find that the observed low multipoles from observation is above the $\Lambda$CDM mean predictions, but in full agreement with Milky Way--like observers in the simulation. Finally, by converting the convergence dipole into a number-count dipole, we test whether weak lensing can contribute to the cosmic dipole anomaly, an idea motivated by its natural alignment with the CMB dipole and by the fact that lensing, unlike clustering, cannot be removed by cross-matching surveys and thus survives in all high-redshift catalogues. We show that weak lensing by local structure contributes at most a few percent to this observed anomaly.

astro-ph.CO↗

arXiv2024

Monge-Ampère gravity, optimal transport theory and their link to the Galileons

Mathematicians have been proposing for sometimes that Monge-Ampère equation, a nonlinear generalization of the Poisson equation, where trace of the Hessian is replaced by its determinant, provides an alternative non-relativistic description of gravity. Monge-Ampère equation is affine invariant, has rich geometric properties, connects to optimal transport theory, and remains bounded at short distances. Monge-Ampère gravity, that uses a slightly different form of the Monge-Ampère equation, naturally emerges through the application of large-deviation principle to a Brownian system of indistinguishable and independent particles. In this work we provide a physical formulation of this mathematical model, study its theoretical viability and confront it with observations. We show that Monge-Ampère gravity cannot replace the Newtonian gravity as it does not withstand the solar-system test. We then show that Monge-Ampère gravity can describe a scalar field, often evoked in modified theories of gravity such as Galileons. We show that Monge-Ampère gravity, as a nonlinear model of a new scalar field, is screened at short distances, and behaves differently from Newtonian gravity above galactic scales but approaches it asymptotically. Finally, we write a relativistic Lagrangian for Monge-Ampère gravity in flat space time, which is the field equation of a sum of the Lagrangians of all Galileons. We also show how the Monge-Ampère equation can be obtained from the fully covariant Lagrangian of quartic Galileon in the static limit. The connection between optimal transport theory and modified theories of gravity with second-order field equations, unravelled here, remains a promising domain to further explore.

gr-qc↗

arXiv2024

Velocity recostruction with graph neural networks

In this work, we seek to improve the velocity reconstruction of clusters by using Graph Neural Networks -- a type of deep neural network designed to analyze sparse, unstructured data. In comparison to the Convolutional Neural Network (CNN) which is built for structured data such as regular grids, GNN is particularly suitable for analyzing galaxy catalogs. In our GNNs, galaxies as represented as nodes that are connected with edges. The galaxy positions and properties -- stellar mass, star formation rate, and total number of galaxies within 100~\mpc -- are combined to predict the line-of-sight velocity of the clusters. To train our networks, we use mock SDSS galaxies and clusters constructed from the Magneticum hydrodynamic simulations. Our GNNs reach a precision in reconstructed line-of-sight velocity of $Δv$=163 km/s, outperforming by $\approx$10\% the perturbation theory~($Δv$=181 km/s) or the CNN~($Δv$=179 km/s). The stellar mass provides additional information, improving the precision by $\approx$6\% beyond the position-only GNN, while other properties add little information. Our GNNs remain capable of reconstructing the velocity field when redshift-space distortion is included, with $Δv$=210 km/s which is again 10\% better than CNN with RSD. Finally, we find that even with an impressive, nearly 70\% increase in galaxy number density from SDSS to DESI, our GNNs only show an underwhelming 2\% improvement, in line with previous works using other methods. Our work demonstrates that, while the efficiency in velocity reconstruction may have plateaued already at SDSS number density, further improvements are still hopeful with new reconstruction models such as the GNNs studied here.

astro-ph.CO↗