Key papers

Ok(z) from supernova reconstructions combined with DESI DR2, for three data sets, colour-coded by Deltachi2; right: the resulting likelihoods. — $\mathcal{O}_k(z)$ from supernova reconstructions combined with DESI DR2, for three data sets, colour-coded by $\Delta\chi^2$ ; right: the resulting likelihoods.

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.

DOI · arXiv:2601.20293

Key papers Curvature & FLRW Type Ia supernovae BAO Iterative smoothing With my students

How the jet–host angle is measured: VLBI jet position angle (a) against the minor axis of the optical host in DES, DESI LS, SDSS and SkyMapper (b–e).

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.

DOI · arXiv:2411.09099

Key papers AGN & jets Galaxy formation VLBI With my students

The curvature diagnostic Ok on mocks from four fiducial models: it recovers Omegak,0 = 0.1 when the input is curved. — The curvature diagnostic $\mathcal{O}_k$ on mocks from four fiducial models: it recovers $\Omega_{k,0} = 0.1$ when the input is curved.

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.

DOI · arXiv:2407.07847

Key papers Curvature & FLRW Type Ia supernovae BAO Iterative smoothing First author

Gaussian-process reconstruction of the distance modulus relative to the truth, fully marginalised over hyperparameters and CPL parameters.

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.

DOI · arXiv:2206.15081

Key papers Type Ia supernovae Gaussian processes With my students

Where a universe with a negative cosmological constant still accelerates today (white), accelerated only in the past (dark grey), or never accelerated (light grey), for Omegam = 0.3. — Where a universe with a negative cosmological constant still accelerates today (white), accelerated only in the past (dark grey), or never accelerated (light grey), for $\Omega_\mathrm{m} = 0.3$ .

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 $z \gtrsim 1$ .

DOI · arXiv:2008.10237

Key papers Dark energy

Theta(z) (top) and Ok(z) (bottom) from supernovae and BOSS/eBOSS BAO. A flat FLRW universe gives Theta = 1. — $\Theta(z)$ (top) and $\mathcal{O}_k(z)$ (bottom) from supernovae and BOSS/eBOSS BAO. A flat FLRW universe gives $\Theta = 1$ .

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 $z > 1$ .

DOI · arXiv:1804.04320

Key papers Curvature & FLRW Modified gravity Type Ia supernovae BAO Growth of structure (RSD)Iterative smoothing

Model-independent regions allowed by growth data (blue) compared with ΛCDM (red), at fixed gamma = 0.55 (left) and fixed sigma8 = 0.8 (right). — Model-independent regions allowed by growth data (blue) compared with ΛCDM (red), at fixed $\gamma = 0.55$ (left) and fixed $\sigma_8 = 0.8$ (right).

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.

DOI · arXiv:1712.04865

Key papers Modified gravity Type Ia supernovae Growth of structure (RSD)First author With my students

Halo interaction rate versus mass in ΛCDM and in f(R), DGP and coupled dark energy simulations. — Halo interaction rate versus mass in ΛCDM and in $f(R)$ , DGP and coupled dark energy simulations.

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.

DOI · arXiv:1703.07357

Key papers Modified gravity Dark energy Dark matter haloes Simulations First author

Theta(z) and the curvature diagnostic Ok(z) at the BOSS LOWZ and CMASS redshifts, one point per supernova-based reconstruction. A flat FLRW universe gives Theta = 1 and Ok = 0. — $\Theta(z)$ and the curvature diagnostic $\mathcal{O}_k(z)$ at the BOSS LOWZ and CMASS redshifts, one point per supernova-based reconstruction. A flat FLRW universe gives $\Theta = 1$ and $\mathcal{O}_k = 0$ .

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 $2\sigma$ .

DOI · arXiv:1606.06832

Key papers Curvature & FLRW Type Ia supernovae BAO Iterative smoothing First author

A 7h-1Mpc-thick slice through Horizon Run 4 at z = 0, with two successive zooms onto a galaxy cluster. — A $7\,h^{-1}\,\mathrm{Mpc}$ -thick slice through Horizon Run 4 at $z = 0$ , with two successive zooms onto a galaxy cluster.

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.

DOI · arXiv:1508.05107

Key papers Large-scale structure Dark matter haloes Simulations

Effect of the order of Lagrangian perturbation theory (1LPT vs 2LPT) in the initial conditions on the halo mass function.

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%.

DOI · arXiv:1401.6180

Key papers Simulations First author

Fraction of stellar mass gained through smooth accretion rather than mergers, versus galaxy mass. The mean is 77%.

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.

DOI · arXiv:1108.4247

Key papers Galaxy formation Simulations First author

Publications