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Publications
Publications (116)
Modern cosmology is built on the assumption that the Universe is homogeneous and isotropic on large scales - but this is challenged by results of the Ellis-Baldwin test that show an unexplained anomaly in the distribution of distant galaxies and quasars.
We study Monge-Ampère gravity (MAG) as an effective theory of cosmological structure formation through optimal transport theory. MAG is based on the Monge-Ampère equation, a nonlinear version of the Poisson equation, that relates the Hessian determinant of the potential to the density field. We explain how MAG emerges from a conditioned system of i...
We demonstrate the effectiveness of one of the many multitracer analyses enabled by optimal transport (OT) reconstruction. Leveraging a semidiscrete OT algorithm, we determine the displacements between initial and observed positions of biased tracers and the remaining matter field. With only redshift-space distorted final positions of biased tracer...
Mathematicians have been proposing for sometimes that Monge-Amp\`ere 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-Amp\`ere equation is affine invariant, has rich geometric properties, connects to optimal...
The existence of ‘peculiar’ velocities due to the formation of cosmic structure marks a point of discord between the real universe and the usually assumed Friedmann–Lemaítre–Robertson–Walker metric, which accomodates only the smooth Hubble expansion on large scales. In the standard ΛCDM model framework, Type Ia supernovae data are routinely “correc...
The Universe expansion rate is modulated around local inhomogeneities due to their gravitational potential. Velocity waves are then observed around galaxy clusters in the Hubble diagram. This paper studies them in a sim 738 Mpc-wide, 2048$^3$-particle cosmological simulation of our cosmic environment (a.k.a. CLONE: Constrained LOcal Nesting Environ...
Recent research has emphasized the benefits of accurately reconstructing the initial Lagrangian positions of biased tracers from their positions at a later time, to gain cosmological information. A weighted semidiscrete optimal transport algorithm can achieve the required accuracy, provided the late-time positions are known, with minimal informatio...
Recent research has emphasized the benefits of accurately reconstructing the initial Lagrangian positions of biased tracers from their positions at a later time, to gain cosmological information. A weighted semi-discrete optimal transport algorithm can achieve the required accuracy, provided the late-time positions are known, with minimal informati...
The Universe expansion rate is modulated around local inhomogeneities due to their gravitational potential. Velocity waves are then observed around galaxy clusters in the Hubble diagram. This paper studies them in a ~738 Mpc wide, with 2048^3 particles, cosmological simulation of our cosmic environment (a.k.a. CLONE: Constrained LOcal & Nesting Env...
A weighted, semidiscrete, fast optimal transport (OT) algorithm for reconstructing the Lagrangian positions of protohalos from their evolved Eulerian positions is presented. The algorithm makes use of a mass estimate of the biased tracers and of the distribution of the remaining mass (the “dust”) but is robust to errors in the mass estimates. Tests...
In view of the growing tension between the dipole anisotropy of number counts of cosmologically distant sources and of the cosmic microwave background (CMB), we investigate the number count dipole induced by primordial perturbations with wavelength comparable to or exceeding the Hubble radius today. First, we find that neither adiabatic nor isocurv...
We present the first joint analysis of catalogs of radio galaxies and quasars to determine whether their sky distribution is consistent with the standard ΛCDM model of cosmology. This model is based on the cosmological principle, which asserts that the universe is statistically isotropic and homogeneous on large scales, so the observed dipole aniso...
In view of the growing tension between the dipole anisotropy of number counts of cosmologically distant sources and of the cosmic microwave background (CMB), we investigate the number count dipole induced by primordial perturbations with wavelength comparable to or exceeding the Hubble radius today. First, we find that neither adiabatic nor isocurv...
We present the first joint analysis of catalogs of radio galaxies and quasars to determine if their sky distribution is consistent with the standard $\Lambda$CDM model of cosmology. This model is based on the cosmological principle, which asserts that the universe is isotropic and homogeneous on large scales, so the observed dipole anisotropy in th...
Optimal transport theory has recently re-emerged as a vastly resourceful field of mathematics with elegant applications across physics and computer science. Harnessing methods from geometry processing, we report on the efficient implementation for a specific problem in cosmology-the reconstruction of the linear density field from low redshifts, in...
A 10% difference in the scale for the Hubble parameter constitutes a clear problem for cosmology. Here, considering angular distribution of type Ia supernovae (SN) within the Pantheon compilation and working within flat ΛCDM cosmology, we observe a correlation between higher H0 and the CMB dipole direction, confirming our previous results for stron...
A model-independent, weighted semi-discrete, fast optimal transport algorithm to reconstruct the Lagrangian positions of proto-halos from their evolved Eulerian positions is presented. Tests with state-of-art cosmological simulations show that the positions of proto-halos are reconstructed accurately, without having to assume a background cosmology...
Optimal transport theory has recently reemerged as a vastly resourceful field of mathematics with elegant applications across physics and computer science. Harnessing methods from geometry processing, we report on the efficient implementation for a specific problem in cosmology -- the reconstruction of the linear density field from low redshifts, i...
We run high-resolution fully GPU N -body simulations to demonstrate that the dark matter (DM) distribution in M31 is well fitted by a core-like profile. Rich observational data especially on the giant stellar stream provides stringent constraints on the initial conditions of our simulations. We demonstrate that the accretion of a satellite on a hig...
The tension between early and late Universe probes of the Hubble constant has motivated various new FLRW cosmologies. Here, we reanalyse the Hubble tension with a recent age of the Universe constraint. This allows us to restrict attention to matter and a dark energy sector that we treat without assuming a specific model. Assuming analyticity of the...
In the late 1990’s, observations of two directionally-skewed samples of, in total, 93 Type Ia supernovae were analysed in the framework of the Friedmann–Lemaître–Robertson–Walker (FLRW) cosmology. Assuming these to be ‘standard(isable) candles’ it was inferred that the Hubble expansion rate is accelerating as if driven by a positive Cosmological Co...
We leverage powerful mathematical tools stemming from optimal transport theory and transform them into an efficient algorithm to reconstruct the fluctuations of the primordial density field, built on solving the Monge-Ampère-Kantorovich equation. Our algorithm computes the optimal transport between an initial uniform continuous density field, parti...
In the late 1990's, observations of 93 Type Ia supernovae were analysed in the framework of the FLRW cosmology assuming these to be `standard(isable) candles'. It was thus inferred that the Hubble expansion rate is accelerating as if driven by a positive Cosmological Constant $\Lambda$. This is still the only direct evidence for the `dark energy' t...
A 10\% difference in the scale for the Hubble parameter constitutes a clear problem for cosmology. As recently observed \cite{Krishnan:2021dyb}, only a modification to early Universe physics remains as a resolution within Einstein gravity plus the Friedmann-Lema\^itre-Robertson-Walker (FLRW) paradigm, but the current approaches are unconvincing, si...
The tension between early and late Universe probes of the Hubble constant has motivated various new FLRW cosmologies. Here, we reanalyse the Hubble tension with a recent age of the Universe constraint. This allows us to restrict attention to matter and a dark energy sector that we treat without assuming a specific model. Assuming analyticity of the...
We study the large-scale anisotropy of the universe by measuring the dipole in the angular distribution of a flux-limited, all-sky sample of 1.36 million quasars observed by the Wide-field Infrared Survey Explorer (WISE). This sample is derived from the new CatWISE2020 catalog, which contains deep photometric measurements at 3.4 and 4.6 μ m from th...
We leverage powerful mathematical tools stemming from optimal transport theory and transform them into an efficient algorithm to reconstruct the fluctuations of the primordial density field, built on solving the Monge-Amp\`ere-Kantorovich equation. Our algorithm computes the optimal transport between an initial uniform continuous density field, par...
We study the large-scale anisotropy of the Universe by measuring the dipole in the angular distribution of a flux-limited, all-sky sample of 1.3 million quasars observed by the Wide-field Infrared Survey Explorer (WISE). This sample is derived from the new CatWISE2020 catalog, which contains deep photometric measurements at 3.4 and 4.6 um from the...
Using fully GPU N-body simulations, we demonstrate for the first time that subhaloes sink and transfer energy via dynamical friction into the centres of dwarf galaxies. This dynamical heating kicks any central massive black hole out to tens of parsecs, especially at early epochs (z = 1.5–3). This mechanism helps explain the observed off-centre blac...
We study correlated fluctuations of Type~Ia supernova observables due to peculiar velocities of both the observer and the supernova host galaxies, and their impact on cosmological parameter estimation. We demonstrate using the CosmicFlows-3 dataset that at low redshifts the corrections for peculiar velocities in the JLA catalogue have been systemat...
We performed a series of high-resolution N-body simulations to examine whether dark matter candidates in the form of primordial black holes (PBHs) can solve the cusp–core problem in low-mass dwarf galaxies. If some fraction of the dark matter in low-mass dwarf galaxies consists of PBHs and the rest is cold dark matter, dynamical heating of the cold...
Using fully GPU $N$-body simulations, we demonstrate for the first time that subhalos sink and transfer energy via dynamical friction into the centres of dwarf galaxies. This dynamical heating kicks any central massive black hotel (MBH) out to tens of parsecs, especially at early epochs ($z$=1.5-3). This mechanism helps explain the observed off-cen...
We use a fully GPU N-body code to demonstrate that dark matter (DM) minihaloes, as a new component of globular clusters (GCs), resolve both the timing and cusp–core problems in Fornax if the (five or six) GCs were recently accreted (≤3 Gyr ago) by Fornax. Under these assumptions, infall of these GCs does not occur and no star clusters form in the c...
We run state-of-the-art high resolution fully GPU N-body simulations to show that M31 harbours a dark matter core. Observational data in the inner halo of M31 provide stringent constraints on the initial conditions of our simulations. We demonstrate that an infalling satellite on a highly eccentric orbit is at the origin of the giant stellar stream...
We have shown (Colin et al. 2019) that the acceleration of the Hubble expansion rate inferred from Type Ia supernovae is essentially a dipole with 3.9$\sigma$ significance, approximately aligned with the CMB dipole, while its monopole component which may be interpreted as due to a Cosmological Constant (or more generally dark energy) is consistent...
Observations reveal a “bulk flow” in the local Universe which is faster and extends to much larger scales than are expected around a typical observer in the standard ΛCDM cosmology. This is expected to result in a scale-dependent dipolar modulation of the acceleration of the expansion rate inferred from observations of objects within the bulk flow....
We use a high performance fully GPU $N$-body code with adaptive time steps to show that dark matter candidates in the form of primordial black holes can induce a cusp-to-core transition in low-mass dwarf galaxies via dynamical friction by dark matter particles at 1 M$_{\odot}$ mass resolution acting on primordial black holes in the mass range 25 -...
We use a fully GPU $N$-body code to demonstrate that dark matter minihalos, as a new component of globular clusters, resolves both the timing and cusp-core problems in Fornax if the five globular clusters were recently accreted (2-4 Gyr ago) by Fornax. Under these assumptions, the infall of these globular clusters does not occur and no star cluster...
We re-investigate the Fornax cusp-core problem using observational results on the spatial and mass distributions of globular clusters (GCs) in order to put constraints on the dark matter profile. We model Fornax using high-resolution N-body simulations with entirely live systems, i.e. self-gravitating systems composed of stars and dark matter, whic...
Recent observations reveal a bulk flow in the local Universe which is faster and extends to larger scales than is expected around a typical observer in the standard $\Lambda$CDM cosmology. The deceleration parameter $q_0$ derived from local observations is then expected to show a scale-dependent dipolar modulation. From a maximum likelihood analysi...
Fornax is the most massive of the Milky Way dwarf spheroidal galaxies and has five globular clusters orbiting in a dense background of dark matter. Observational analyses suggest that globular clusters were initially much more massive and lost most of their stars to the Fornax field. We re-investigate the Fornax cusp-core problem, to clarify tensio...
Previous studies have shown that our velocity in the rest frame of galaxies at high redshift does not converge to that deduced from the CMB temperature-dipole anisotropy. In this work we determine the dipole in the galaxy catalogue derived from the Wide-field Infrared Survey Explorer (WISE) survey. After reducing star contamination to ~0.1% by reje...
We determine the dipole in the WISE galaxy catalogue. After reducing star contamination to <0.1% by rejecting sources with high apparent motion and those close to the Galactic plane, we eliminate low redshift sources to suppress the non-kinematic, clustering dipole. We remove sources within {\pm}5{\deg} of the super-galactic plane, as well as those...
Previous studies have found our velocity in the rest frame of radio galaxies at high redshift to be substantially larger than that inferred from the CMB temperature dipole anisotropy. We construct a full sky catalogue NVSUMSS, by merging the NVSS and SUMSS catalogues and removing local sources by various means including cross-correlating with the 2...
Previous studies have found our velocity in the rest frame of radio galaxies at high redshift to be substantially larger than that inferred from the CMB temperature dipole anisotropy. We construct a full sky catalogue NVSUMSS, by merging the NVSS and SUMSS catalogues and removing local sources by various means including cross-correlating with the 2...
The existence of critical points for the peculiar velocity field is a natural feature of the correlated vector field. These points appear at the junctions of velocity domains with different orientations of their averaged velocity vectors. Since peculiar velocities are the important cause of the scatter in the Hubble expansion rate, we propose that...
The existence of critical points for the peculiar velocity field is a natural feature of the correlated vector field. These points appear at the junctions of velocity domains with different orientations of their averaged velocity vectors. Since peculiar velocities are the important cause of the scatter in the Hubble expansion rate, we propose that...
The existence of critical points for the peculiar velocity field is a natural feature of the correlated vector field. These points appear in contact zones of the velocity domains with different orientation of the averaged velocity vector. At the same time peculiar velocities are the cause of the scatter of the Hubble expansion rate. We propose that...
We propose a new probe to test the nature of gravity at various redshifts
through large-scale cosmological observations. We use our void catalog,
extracted from the Sloan Digital Sky Survey (SDSS, DR10), to trace the
distribution of matter along the lines of sight to SNe Ia that are selected
from the Union 2 catalog. We study the relation between S...
We propose that the accretion of a dwarf spheroidal galaxy provides a common origin for the giant southern stream and the
warp of M31. We run about 40 full N-body simulations with live M31, infalling galaxies with varying masses and density profiles, and cosmologically plausible
initial orbital parameters. Good agreement with a full range of observ...
We present catalogues of voids for the SDSS DR7 redshift survey and for
Millennium I simulation mock data. We aim to compare the observations with
simulations based on a $\Lambda$CDM model and a semi-analytic galaxy formation
model. We use the void statistics as a test for these models. We assembled a
mock catalogue that closely resembles the SDSS...
We establish a refined version of the Second Law of Thermodynamics for Langevin stochastic processes describing mesoscopic systems driven by conservative or non-conservative forces and interacting with thermal noise. The refinement is based on the Monge-Kantorovich optimal mass transport and becomes relevant for processes far from quasi-stationary...
Dual active galactic nuclei (AGNs) are natural byproducts of hierarchical mergers of galaxies in the ΛCDM cosmogony. Recent observations have shown that only a small fraction (~0.1%-2.5%) of AGNs at redshift z 0.3 are dual with kpc-scale separations, which is rather low compared to the high merger rate of galaxies. Here we construct a phenomenologi...
We carry out fully three-dimensional simulations of evolution from self-similar, spherically symmetric linear perturbations of a cold dark matter (CDM)-dominated Einstein–de Sitter universe. As a result of the radial orbit instability, the haloes which grow from such initial conditions are triaxial with major-to-minor axial ratios of the order of 3...
Dark matter substructures around nearby galaxies provide an interesting opportunity for confusion-free indirect detection
of dark matter. We calculate the boost over a smooth background distribution of dark matter for gamma-ray emission from dark
matter self-annihilations in tidal structures in M31, assuming a cross-section inversely proportional t...
Dual AGNs are natural byproducts of hierarchical mergers of galaxies in the
LambdaCDM cosmogony. Recent observations have shown that only a small fraction
(~ 0.1%-1%) of AGNs at redshift z<~ 0.3 are dual with kpc-scale separations,
which is rather low compared to the high merger rate of galaxies. Here we
construct a phenomenological model to estima...
The question of the transition to global isotropy from our anisotropic local universe is studied using the Union 2 catalogue
of Type Ia supernovae (SNe Ia). We construct a ‘residual’ statistic sensitive to systematic shifts in their brightness in
different directions and use this to search in different redshift slices for a preferred direction on t...
We generate the peculiar velocity field for the Two Micron All-Sky Redshift Survey (2MRS) catalog using an orbit-reconstruction algorithm. The reconstructed velocities of individual objects in 2MRS are well correlated with the peculiar velocities obtained from high-precision observed distances within 3000 km s–1. We estimate the mean matter density...
We develop a theoretical framework for describing the hierarchical structure of the phase space of cold dark matter haloes, due to gravitationally bound substructures. Because it includes the full hierarchy of the cold dark matter initial conditions and is hence complementary to the halo model, the stable clustering hypothesis is applied for the fi...
We simulate the growth of isolated dark matter haloes from self-similar and spherically symmetric initial conditions. Our N-body code integrates the geodesic deviation equation in order to track the streams and caustics associated with individual simulation particles. The radial orbit instability causes our haloes to develop major-to-minor axis rat...
We simulate the growth of isolated dark matter haloes from self-similar and spherically symmetric initial conditions. Our N-body code integrates the geodesic deviation equation in order to track the streams and caustics associated with individual simulation particles. The radial orbit instability causes our haloes to develop major-to-minor axis rat...
Most of the mass content of dark matter haloes is expected to be in the form of tidal debris. The density of debris is not constant, but rather can grow due to formation of caustics at the apocenters and pericenters of the orbit, or decay as a result of phase mixing. In the phase space, the debris assemble in a hierarchy which is truncated by the p...
Motion of a continuous fluid can be decomposed into an “incompressible” rearrangement, which preserves the volume of each infinitesimal fluid element, and a gradient map that transfers fluid elements in a way unaffected by any pressure or elasticity (the polar decomposition of Y. Brenier). The Euler equation describes a system whose kinematics is d...
Anisotropic emission of gravitational waves during the merger of black holes induces a recoil velocity on the centre of mass of the binary and the final merger product can then be ejected from its host galaxy. We consider ejected black holes which stay on bound orbits around their host haloes. A recoiled black hole which moves on an almost radial o...
Anisotropic emission of gravitational waves during the merger or formation of black holes can lead to the ejection of these black holes from their host galaxies. A recoiled black hole which moves on an almost radial bound orbit outside the virial radius of its central galaxy, in the cold dark matter background, reaches its apapsis in a finite time....
Gravitational collapse of dark matter, merger of dark matter haloes and tidal disruption of satellites are among processes
which lead to the formation of fine and dense dark matter shells, also known as dark matter caustics. The putative weakly
interacting species which may form the dark matter are expected to strongly annihilate in these dense reg...
A new scenario for the emission of high-energy gamma-rays from dark matter annihilation around massive black holes is presented. A black hole can leave its parent halo, by means of gravitational radiation recoil, in a merger event or in the asymmetric collapse of its progenitor star. A recoiled black hole which moves on an almost-radial orbit outsi...
The peculiar velocity reconstruction methods allow one to have a deeper insight into the distribution of dark matter: both to measure mean matter density and to obtain the primordial density fluctuations. We present here the Monge-Ampere-Kantorovitch method applied to mock catalogues mimicking in both redshift and distance catalogues. After having...
Lagrangian reconstruction of large-scale peculiar velocity fields can be strongly affected by observational biases. We develop a thorough analysis of these systematic effects by relying on specially selected mock catalogues. For the purpose of this paper, we use the MAK reconstruction method, although any other Lagrangian reconstruction method shou...
Cold dark matter haloes are populated by caustics, which are yet to be resolved in N-body simulations or observed in the Universe. Secondary infall model provides a paradigm for the study of caustics in "typical" haloes assuming that they have had no major mergers and have grown only by smooth accretion. This is a particular characteristic of the s...
The peculiar velocities of a large number of galaxies can be uniquely reconstructed using present redshift surveys. Our method, based on minimisation of Euler‐Lagrange action, performs extremely well when tested against simulations. We apply our method to the updated NBG redshift
galaxy catalogue and reconstruct the peculiar velocity field in our...
Peculiar velocity reconstruction methods allow one to have a deeper insight into the distribution of dark matter: both to measure Omega_m and to characterize the primordial density fluctuations. We present here the Monge-Ampère-Kantorovitch method applied to redshift reconstruction. We show what are the limitations and the problems to overcome to p...
The peculiar velocities of a large number of galaxies can be uniquely reconstructed using present redshift surveys. Our method, based on minimisation of Euler-Lagrange action, performs extremely well when tested against simulations. We apply our method to the updated NBG redshift galaxy catalogue and reconstruct the peculiar velocity field in our l...
Caustics are formally singular structures, with infinite density, that form in collisionless media. The non-negligible velocity dispersion of dark matter particles renders their density finite. We evaluate the maximum density of the caustics within the framework of secondary infall model of formation of dark matter haloes. The result is then used t...
Peculiar velocities thoughout the region of the local supercluster are reconstructed by two different orbit-retracing methods. The requirement of the optimal correlation between the radial components of reconstructed velocities and the observed peculiar velocities derived from our extensive new catalog of distances puts stringent constraints on the...
Caustics are high-density structures that form frequently in collisionless media. Under self-gravity, cold dark matter flows focus onto caustics which are yet to be resolved in numerical simulations and or observed in the real world. If detected, caustics would provide a strong evidence for dark matter and would rule out alternative models such as...