arXiv subjects

Alexandra Wernersson

Publications and source records attributed to Alexandra Wernersson.

3 recordsLinked to original sources

arXiv2026

Merlin: Fast and flexible 3x2pt cosmology with simulation-based inference

We present Merlin, a simulation-based inference (SBI) pipeline to perform cosmological analyses of 3x2pt summary statistics: cosmic shear, galaxy clustering and the cross-correlation power spectra. Our approach combines Marginal Neural Ratio Estimation (MNRE) with 3x2pt angular power spectra predictions from the cloelib library, although the pipeline is readily extensible to other cosmology libraries. We demonstrate this pipeline on a realistic setting representative of a Stage-IV photometric survey, with a 50-dimensional parameter space describing cosmology and a wide range of systematic effects. We find posteriors that are in excellent agreement with the nested sampler Nautilus, while reducing the number of required CPU-hours by two orders of magnitude. Because the generation of training data and the training of inference networks are decoupled processes, a single simulation bank can be reused to perform inference under different analysis choices, further improving the simulator-efficiency. We illustrate this flexibility by applying scale cuts, varying the survey area, and removing cosmic shear from the data vector (2x2pt), all at zero extra model evaluations, whereas sampling-based methods need costly re-runs for each case. The Merlin code is publicly available on GitHub.

astro-ph.CO↗

arXiv2025

The impact of binary neutron star outflow mass models on kilonova parameter estimation

The ejecta from binary neutron star mergers, which powers its associated kilonova, can inform us about source properties, merger dynamics, and the dense nuclear matter equation of state. While now in the era of multi-messenger astronomy, we remain more likely to observe purely electromagnetic (or purely gravitational-wave) signals due to the duty cycle and maximum observing distance of gravitational wave detectors. It is thus imperative to be able to perform high-accuracy parameter inference of purely electromagnetic detections. Here, we use a collection of ejecta formulae in an end-to-end analysis to generate mock multi-wavelength kilonova signals and recover the intrinsic merger parameters. By generating mock light curves for a broad range of possible mergers and with a variety of ejecta models, and then performing parameter estimation with different ejecta models, we measure how reliably and consistently we can use the 'map' between intrinsic and outflow properties provided by the formulae to gain source (intrinsic) information from purely electromagnetic detections (observables). We find that the posteriors probability densities are prone to biases when we vary the choice of reference model for our sampler, especially away from the best studied region of the binary parameter space, i.e. near equal mass, low mass binaries. This highlights both the need for improved models for the mass of the ejecta, and the need to exert caution when performing parameter estimation with a single mass model. We also find that most models predict that parameter degeneracies in kilonova light curves are largely orthogonal to those measurement observables, such as the chirp mass, from gravitational wave signals, indicating that kilonovae may provide a lot of additional information even in multi-messenger observations, if better modeled.

astro-ph.HE↗

arXiv2022

Self-Interacting Gravitational Atoms in the Strong-Gravity Regime

We numerically investigate free and self-interacting ultralight scalar fields around black holes in General Relativity. We focus on complex scalar fields $Φ$ whose self-interactions are described by the quartic potential $V \propto λ|Φ|^4$, and ignore the black hole spin in order to disentangle the effects of self interactions on the boson cloud. Using the spectral solver Kadath, we compute quasi-equilibrium configurations of the dominant eigenstates, including their backreaction on the spacetime metric. For scenarios with $- 10^{-2} \lesssim λ\lesssim 10^{-2}$ we find the mass of the self-interacting scalar cloud to be up to $\sim 70\%$ larger than that of a free scalar cloud, though the additional backreaction effect on the spacetime metric is only up to $\sim 1\%$ due to the low-density nature of the bosonic configurations. In this region of parameter space we observe approximate quadratic scalings between the mass of the cloud with $λ$, the scalar field amplitude, and the couplings between these two parameters. For systems with $λ$ beyond this range, the eigenfrequencies differ sufficiently from the known free-test-field values used as inputs in our numerical setup to make the results, though convergent, physically unreliable. This bounds the range of $λ$ in which the free scalar field solution remains a good approximation to self-interacting scalar field configurations. Our work is among the first nonperturbative explorations of self-interacting bosonic clouds around black holes, yielding detailed new insights into such systems in the nonlinear regime, while also overcoming technical challenges and quantifying limitations. Additionally, our results provide useful inputs for fully dynamical numerical relativity simulations and for future explorations of spinning black holes and real scalar fields.

gr-qc↗