With my students

Reconstructed cluster bias, normalised by the Wicker et al. 2023 model, as a function of redshift (left) and mass (right).

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.

arXiv:2603.13763

Galaxy clusters Type Ia supernovae Iterative smoothing With my students

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

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

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). (Figure from the 2025 follow-up paper.)

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.

arXiv:2305.06713

AGN & jets Galaxy formation VLBI With my students

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