Research

2026 · arXiv preprint

Finding the distribution of matter using lenses – I: deconvolution-based reconstruction with CMB lensing

Dawn, A., Jiang, J.-Q., Hazra, D. K., L'Huillier, B., Shafieloo, A.

We reconstruct the linear matter power spectrum at $z = 0$ from joint Planck PR4, ACT DR6 and SPT-3G CMB lensing, using a modified Richardson–Lucy deconvolution. The result follows the linear prediction on large scales, lies systematically higher for $k \gtrsim 0.1\,\mathrm{Mpc}^{-1}$, and partly preserves the BAO feature.

Large-scale structureGravitational lensingInverse problems

2026 · arXiv preprint

Finding the distribution of matter using lenses – II: deconvolution-based reconstruction with $3\times2$pt measurements

Jiang, J.-Q., Dawn, A., Hazra, D. K., L'Huillier, B., Shafieloo, A.

A regularised Richardson–Lucy framework tests scale-dependent departures from the nonlinear matter power spectrum using galaxy clustering, galaxy–galaxy lensing and cosmic shear. On LSST Year-10-like mocks, oscillations of about 1% or more are recovered over $0.1 \lesssim k \lesssim 0.5\,\mathrm{Mpc}^{-1}$, and a 1% oscillation is detected at about $2.6\sigma$.

Large-scale structureGravitational lensingInverse problems

2026 · A&A 712 (2026) A240

A refined method for measuring cosmological distances using variability and proper motions in AGNs with VLBI-detected counter jets

Hodgson, J. A., Carr, A., Parkinson, D., Statti, M., Myeong, J., L'Huillier, B., Shafieloo, A., Liodakis, I., Oh, S.-H.

We extend the ‘speed-gun’ distance method using the proper motions of both the approaching jet and the counter-jet, which removes its dependence on the Doppler factor. For 3C 84 we obtain an angular-diameter distance of 71–79 Mpc depending on the assumed geometry; the spherical case agrees with SH0ES-calibrated supernovae.

AGN & jetsVLBI

2026 · arXiv preprint

Non-parametric estimation of the baryon gas fraction and the cosmological bias with clusters

Kim, H.†, Wicker, R., L'Huillier, B., Douspis, M., Salvati, L., Shafieloo, A.

From a model-independent expansion history reconstructed from supernovae, we infer with iterative smoothing how the hydrostatic mass bias of galaxy clusters depends on mass and redshift, using X-ray gas fractions. Reconciling clusters with CMB constraints requires this bias to evolve with time.

Galaxy clustersType Ia supernovaeIterative smoothingWith my students

Is the Universe flat? A test with DESI DR2

2026 · JCAP 08 (2026) 016

Model independent test of the FLRW metric and the curvature in light of DESI DR2

Millard, C.†, L'Huillier, B.*, Douspis, M.

We reconstruct distances and the Hubble rate from Pantheon+ and DES supernovae without assuming any dark energy model, and combine them with DESI DR2 baryon acoustic oscillations to test the FLRW metric and measure the spatial curvature. With Pantheon+ and DESI DR2 we find $\Omega_{k,0} = 0.045^{+0.045}_{-0.081}$, consistent with flatness and with Planck 2018.

Key papersCurvature & FLRWType Ia supernovaeBAOIterative smoothingWith my students

Black-hole jets know their host galaxy

2025 · Nature Astronomy 9 (2025) 302

Detection of an orthogonal alignment between parsec-scale AGN jets and their host galaxies

Fernández Gil, D.†, Hodgson, J. A., L'Huillier, B., Asorey, J., Saulder, C., Finner, K., et al.

Comparing VLBI jet directions with optical host-galaxy shapes for about 6,000 AGN, we detect a weak but significant alignment between the parsec-scale jet and the minor axis of its host galaxy: a link between the black hole and its host across three orders of magnitude in scale.

Key papersAGN & jetsGalaxy formationVLBIWith my students

Litmus tests for Rubin and DESI

2025 · JCAP 05 (2025) 030

Litmus tests of the flat ΛCDM model and model-independent measurement of $H_0 r_\mathrm{d}$ with LSST and DESI

L'Huillier, B., Mitra, A., Shafieloo, A., Keeley, R. E., Koo, H.

Reconstructing the expansion history from simulated LSST supernovae and combining it with simulated DESI 5-year BAO, we forecast constraints of up to ±4% on the curvature and ±0.1 on $c/(H_0 r_\mathrm{d})$, without assuming any form of dark energy.

Key papersCurvature & FLRWType Ia supernovaeBAOIterative smoothingFirst author

2024 · Universe 10 (2024) 439

An analysis of variance of the Pantheon+ dataset: systematics in the covariance matrix?

Keeley, R. E., Shafieloo, A., L'Huillier, B.

The best-fit ΛCDM $\chi^2$ of Pantheon+ is unusually small, and its residuals scatter less than the covariance matrix predicts, pointing to errors overestimated by about 7%. After accounting for this, no deviation from ΛCDM is found.

Type Ia supernovae

2023 · MNRAS 521 (2023) L44

Estimating the feasibility of ‘standard speed-gun’ distances

Hodgson, J. A., L'Huillier, B., Liodakis, I., Lee, S.-S., Shafieloo, A.

We extend the ‘standard speed-gun’ distance method by using the maximum intrinsic brightness temperature of a source instead of its Doppler factor. Forecasts show that recovering the cosmology depends mainly on how well this temperature is known and on the number of observed flares.

AGN & jetsVLBI

2023 · Phys. Rev. D 108 (2023) 023504

Joint reconstructions of growth and expansion histories from stage-IV surveys with minimal assumptions. II. Modified gravity and massive neutrinos

Calderón, R., L'Huillier, B., Polarski, D., Shafieloo, A., Starobinsky, A. A.

We reconstruct the effective gravitational coupling $G_\mathrm{eff}(z)$ as a Gaussian process from forecast stage-IV growth data. DESI-like surveys could detect departures from General Relativity if dark energy is well determined; massive neutrinos do not change this, but assuming a ΛCDM expansion biases the inferred $\Omega_\mathrm{m}$ and $\sigma_8$.

Modified gravityGrowth of structure (RSD)Gaussian processes

Gaussian processes, done right

2023 · JCAP 02 (2023) 014

How to use GP: effects of the mean function and hyperparameter selection on Gaussian process regression

Hwang, S.-g.†, L'Huillier, B.*, Keeley, R. E., Jee, M. J., Shafieloo, A.

The choice of mean function and hyperparameters biases Gaussian process reconstructions of supernova distances: a zero mean gives unphysical results and a best-fit ΛCDM mean biases them. Marginalising over a family of mean functions and over the hyperparameters removes the bias, whatever the kernel.

Key papersType Ia supernovaeGaussian processesWith my students

2023 · arXiv preprint

Exploring connections between the VLBI and optical morphology of AGNs and their host galaxies

Fernández Gil, D.†, Hodgson, J. A., L'Huillier, B.

Matching over 9,000 VLBI sources from the Astrogeo catalogue to their optical host galaxies and fitting the hosts' shapes, this first study finds no clear correlation between host morphology and jet direction.

AGN & jetsGalaxy formationVLBIWith my students

2022 · Phys. Rev. D 106 (2022) 083513

Joint reconstructions of growth and expansion histories from stage-IV surveys with minimal assumptions I: dark energy beyond $\Lambda$

Calderón, R., L'Huillier, B., Polarski, D., Shafieloo, A., Starobinsky, A. A.

Gaussian processes reconstruct the dark energy density from forecast stage-IV supernova, BAO and redshift-space-distortion data, assuming only a flat FLRW universe that becomes matter-dominated at high redshift, which also yields the growth history. Several dark energy models can be distinguished from ΛCDM at $2\sigma$ or more.

Dark energyType Ia supernovaeBAOGrowth of structure (RSD)Gaussian processes

2022 · JCAP 03 (2022) 047

Bayesian vs frequentist: comparing Bayesian model selection with a frequentist approach using the iterative smoothing method

Koo, H., Keeley, R. E., Shafieloo, A., L'Huillier, B.

On simulated Roman supernova data, Bayesian model selection finds the true model when it is among the candidates, but otherwise only picks the least wrong one. A frequentist test based on iterative smoothing can instead conclude that all tested models are false.

Dark energyType Ia supernovaeIterative smoothingModel selection

2021 · JCAP 03 (2021) 034

Model selection and parameter estimation using the iterative smoothing method

Koo, H., Shafieloo, A., Keeley, R. E., L'Huillier, B.

Likelihood distributions derived from the non-parametric iterative smoothing method let us test a dark energy model and estimate its parameters without comparing it to an alternative. On WFIRST-like mocks we quantify how confidently different dark energy models can be distinguished.

Dark energyType Ia supernovaeIterative smoothingModel selection

2021 · Phys. Rev. D 103 (2021) 023526

Negative cosmological constant in the dark sector?

Calderón, R., Gannouji, R., L'Huillier, B.*, Polarski, D.

We study dark sectors made of a negative cosmological constant plus a late-time accelerating component. The acceleration is often transient and some models eventually contract; current data show no decisive evidence for a negative $\Lambda$, but the best fits are phantom at $z \gtrsim 1$.

Key papersDark energy

2020 · MNRAS 495 (2020) L27

Using variability and VLBI to measure cosmological distances

Hodgson, J. A., L'Huillier, B., Liodakis, I., Lee, S.-S., Shafieloo, A.

We propose measuring AGN distances by comparing the physical size implied by variability with the angular size measured by VLBI. For 3C 84 we find $D_A = 72^{+5}_{-6}$ Mpc, consistent with other measurements at that redshift.

AGN & jetsVLBI

2020 · ApJ 899 (2020) 9

Model-independent constraints on Type Ia supernova light-curve hyper-parameters and reconstructions of the expansion history of the Universe

Koo, H., Shafieloo, A., Keeley, R. E., L'Huillier, B.

Reconstructing the expansion history from JLA supernovae with iterative smoothing while sampling the light-curve hyper-parameters, we find hyper-parameters consistent with those from ΛCDM, CPL and PEDE fits, and no significant redshift evolution.

Type Ia supernovaeIterative smoothing

2020 · JCAP 04 (2020) 039

Parametrising non-linear dark energy perturbations

Hassani, F., L'Huillier, B., Shafieloo, A., Kunz, M., Adamek, J.

Using k-evolution N-body simulations, we quantify non-linear k-essence dark energy perturbations through an effective modification $\mu$ of the Poisson equation, show that linear theory is accurate at large sound speeds, and propose a simulation-calibrated parametrisation of $\mu$.

Dark energyLarge-scale structureSimulations

2020 · MNRAS 494 (2020) 819

Defying the laws of Gravity I: model-independent reconstruction of the Universe expansion from growth data

L'Huillier, B., Shafieloo, A., Polarski, D., Starobinsky, A. A.

From redshift-space distortion data alone, we reconstruct the growth history with crossing statistics and Gaussian processes, derive the expansion history from it, and fit supernovae to constrain $\Omega_{\mathrm{m},0}$ and $\sigma_{8,0}$. The results are consistent with flat ΛCDM and General Relativity.

Modified gravityGrowth of structure (RSD)Type Ia supernovaeGaussian processesFirst author

2020 · MNRAS 491 (2020) 3983

Debiasing cosmic gravitational wave sirens

Keeley, R. E., Shafieloo, A., L'Huillier, B., Linder, E. V.

Gaussian process regression can remove the bias in reconstructing $H(z)$ from gravitational-wave standard sirens, and combined with supernovae it tests $H_0$ and ΛCDM. Dark-siren redshifts need close to spectroscopic precision to avoid significant bias.

Gravitational wavesType Ia supernovaeGaussian processes

2019 · MNRAS 485 (2019) 2783

Model independent expansion history from supernovae: cosmology versus systematics

L'Huillier, B., Shafieloo, A., Linder, E. V., Kim, A. G.

A model-independent analysis of the Pantheon supernovae shows deviations from ΛCDM at $z \gtrsim 1$. They vanish with a simple Malmquist-like correction, but neither $\chi^2$ tests nor Gaussian processes find that this extra correction is statistically required.

Type Ia supernovaeGaussian processesIterative smoothingFirst author

2018 · Phys. Rev. D 98 (2018) 083526

Falsifying ΛCDM: model-independent tests of the concordance model with eBOSS DR14Q and Pantheon

Shafieloo, A., L'Huillier, B.*, Starobinsky, A. A.

Combining model-independent expansion reconstructions from Pantheon supernovae and BAO, we test the FLRW metric and flatness, and use eBOSS DR14Q growth data to constrain $\Omega_\mathrm{m}$, $\gamma$ and $\sigma_8$. Everything is consistent with a flat FLRW universe, General Relativity and $\Lambda$, with some tension at $z > 1$.

Key papersCurvature & FLRWModified gravityType Ia supernovaeBAOGrowth of structure (RSD)Iterative smoothing

2018 · MNRAS 476 (2018) 3263

Model-independent cosmological constraints from growth and expansion

L'Huillier, B., Shafieloo, A., Kim, H.†

We reconstruct the expansion history from supernovae and fit growth-rate data to constrain $\Omega_\mathrm{m}$, the growth index $\gamma$ and $\sigma_8$ without a dark energy model. The results agree with ΛCDM in General Relativity, and requiring a positive dark energy density tightens them further.

Key papersModified gravityType Ia supernovaeGrowth of structure (RSD)Iterative smoothingFirst authorWith my students

2018 · MNRAS 477 (2018) 2772

Cylinders out of a top hat: counts-in-cells for projected densities

Uhlemann, C., Pichon, C., Codis, S., L'Huillier, B., Kim, J., et al.

Large-deviation statistics predict the one-point PDF and clustering of projected densities in cylinders, in agreement with the Horizon Run 4 simulation to within a few percent in the quasi-linear regime: a tool for photometric surveys such as DES and Euclid.

Large-scale structureSimulations

2018 · MNRAS 477 (2018) 2503

Probing features in the primordial perturbation spectrum with large-scale structure data

L'Huillier, B., Shafieloo, A., Hazra, D. K., Smoot, G. F., Starobinsky, A. A.

Can large-scale structure distinguish primordial power spectra with features that the CMB cannot? With 15 DESI-like N-body simulations, we show that the halo mass function and two-point correlation function cannot, but simple counts-in-cells statistics can.

Early UniverseLarge-scale structureSimulationsFirst author

2018 · MNRAS 473 (2018) 5098

A question of separation: disentangling tracer bias and gravitational non-linearity with counts-in-cells statistics

Uhlemann, C., Feix, M., Codis, S., Pichon, C., Bernardeau, F., et al. (incl. L'Huillier, B.)

We model the bias of tracer densities in spheres with a quadratic bias relation and the large-deviation dark matter PDF, validated on Horizon Run 4 subhaloes. This allows a joint estimate of the non-linear dark matter variance and the bias parameters.

Large-scale structureSimulations

Haloes beyond General Relativity

2017 · MNRAS 468 (2017) 3174

Dark matter haloes in modified gravity and dark energy: interaction rate, small- and large-scale alignment

L'Huillier, B., Winther, H. A., Mota, D. F., Park, C., Kim, J.

In N-body simulations of $f(R)$ gravity, DGP and coupled dark energy, $f(R)$ raises halo spin and weakens the spin alignment of interacting pairs, while only strongly coupled dark energy increases the halo interaction rate and the alignment of halo shapes with the large-scale structure.

Key papersModified gravityDark energyDark matter haloesSimulationsFirst author

2017 · MNRAS 466 (2017) 4875

Ecology of dark matter haloes – II. Effects of interactions on the alignment of halo pairs

L'Huillier, B., Park, C., Kim, J.

In Horizon Run 4, interacting haloes have lower spin and are more spherical than typical haloes. Pairs start with antiparallel spins that gradually become parallel, interactions are mostly radial, and alignments are strongest for massive, close pairs and persist up to $z = 4$.

Galaxy formationDark matter haloesSimulationsFirst author

2017 · MNRAS 466 (2017) 2067

Beyond Kaiser bias: mildly non-linear two-point statistics of densities in distant spheres

Uhlemann, C., Codis, S., Kim, J., Pichon, C., Bernardeau, F., et al. (incl. L'Huillier, B.)

Parameter-free analytic bias functions extend Kaiser bias into the mildly non-linear regime, using large-deviation statistics and spherical collapse. They match Horizon Run 4 at the percent level down to about $10\,h^{-1}\,\mathrm{Mpc}$, and reduce the variance of the estimator about fivefold.

Large-scale structureSimulations

Testing ΛCDM without assuming it

2017 · JCAP 01 (2017) 015

Model-independent test of the FLRW metric, the flatness of the Universe, and non-local measurement of $H_0 r_\mathrm{d}$

L'Huillier, B., Shafieloo, A.

Combining BOSS DR12 BAO with JLA supernovae, we measure $H_0 r_\mathrm{d}$ without assuming a cosmological model and introduce the $\Theta(z)$ diagnostic of the flat-FLRW metric. The results are consistent with a flat FLRW universe within $2\sigma$.

Key papersCurvature & FLRWType Ia supernovaeBAOIterative smoothingFirst author

2015 · J. Korean Astron. Soc. 48 (2015) 213

Horizon Run 4 simulation: coupled evolution of galaxies and large-scale structures of the Universe

Kim, J., Park, C., L'Huillier, B., Hong, S. E.

Horizon Run 4 follows $6300^3$ particles in a $3150\,h^{-1}\,\mathrm{Mpc}$ box, with halo merger trees down to $2.7\times10^{11}\,h^{-1}\,M_\odot$. The halo mass function departs from universality and evolves with redshift, and the BAO peak in mock galaxy correlations broadens and shifts. The data are public.

Key papersLarge-scale structureDark matter haloesSimulations

2015 · MNRAS 451 (2015) 527

The ecology of dark matter haloes – I. The rates and types of halo interactions

L'Huillier, B., Park, C., Kim, J.

Using Horizon Run 4, we measure how often haloes interact as a function of environment, separation, mass ratio, mass and redshift. Most interactions happen at density contrast $\delta \approx 20$, while the interacting fraction peaks at $\delta \approx 1000$; we provide a fitting formula and identify two interaction modes.

Galaxy formationDark matter haloesSimulationsFirst author

2014 · New Astron. 30 (2014) 79

Effects of the initial conditions on cosmological N-body simulations

L'Huillier, B., Park, C., Kim, J.

We test how the pre-initial configuration, the order of Lagrangian perturbation theory and the starting redshift affect N-body results. Glass and grid give similar results at $z \lesssim 2$, while first-order LPT underestimates massive haloes by about 2% and small-scale power by 6%.

Key papersSimulationsFirst author

2012 · A&A 544 (2012) A68

Mass assembly of galaxies: smooth accretion versus mergers

L'Huillier, B., Combes, F., Semelin, B.

Multi-zoom cosmological simulations show that galaxies grow mostly by smooth accretion, which brings on average 77% of their mass, while mergers bring 23%. Only the most massive galaxies grow significantly through mergers.

Key papersGalaxy formationSimulationsFirst author