arXiv subjects

Linus Thummel

Publications and source records attributed to Linus Thummel.

3 recordsLinked to original sources

arXiv2026

cloelib: A Flexible Python Library for Computing Cosmological Observables in the Euclid Era

cloelib is a Python library developed to compute cosmological observables within the Cosmology Likelihood for Observables in Euclid (CLOE) ecosystem (cloe-org). As cosmology enters a precision era driven by galaxy survey missions such as Euclid, there is a growing need for flexible, efficient, and differentiable software capable of supporting next-generation inference pipelines. cloelib addresses these demands through a modular architecture that interfaces seamlessly with established Boltzmann solvers whilst incorporating JAX-based automatic differentiation to enable gradient-based methods. The library defines consistent protocols for background evolution, perturbations, and non-linear structure formation, and supports a wide range of observables, including photometric and spectroscopic large-scale structure probes, as well as cross-correlations with the Cosmic Microwave Background and galaxy clusters. In its finalised form, cloelib is intended to serve as the reference theory computation infrastructure for Euclid's first cosmological release, bridging traditional numerical cosmology with modern optimisation techniques and emerging machine learning approaches to inference.

astro-ph.CO↗

arXiv2025

Stable Islands of Weak Gravity

We present an exploration of the phenomenology of Horndeski gravity, focusing on regimes that produce weak gravity compared to General Relativity. This letter introduces a novel method to generate models of modified gravity theories that produce a specific observational behaviour while fulfilling stability criteria, without imposing a fixed parametrisation. We start from the inherently stable basis of linear Horndeski theory, implemented in the recently released Einstein-Boltzmann solver mochi_class. The time evolution of the basis functions is designed with Gaussian processes that directly include the stability and phenomenology criteria during the generation. Here, we focus on models with weak gravity that suppress the growth of Large-Scale Structure at late times. To achieve this behaviour, we mainly focus on the design of a dynamical effective Planck mass for theories with a vanishing fifth force. We find a broad range of weak-gravity islands in Horndeski theory space. We also include additional features, like a vanishing modification to gravity at $z=0$, and extend the exploration to islands of gravity with a non-zero fifth force. Finally, we show that replacing the $\Lambda$CDM expansion model by the DESI $w_0w_a$CDM best fit also produces stable islands of weak gravity.

gr-qc↗

arXiv2021

Spin-conserving Boltzmann theory for carriers and excitons in organic semiconductors

The rise of organic electronics calls for versatile modeling tools. In this context, we develop a semiclassical Boltzmann theory that describes transport and excitonic processes in crystalline organic semiconductors on equal footing. The generation of singlet and triplet excitons out of the ground state, their formation from free electrons and holes, the reverse processes, as well as the fusion and fission of excitons are included. The corresponding scattering integrals respect spin conservation, which requires matrix-valued distribution functions. They also include fermionic and bosonic many-particle effects such as Pauli blocking. We employ a multipole expansion of the distribution functions, where quadrupolar terms turn out to be essential for the triplet excitons. This work provides a basis for the modeling of organic solar cells, in which excitonic processes are crucial for the performance. Moreover, the theory is of general interest for transport and transitions of multiple (quasi-) particle species carrying spin in nonequilibrium systems.

cond-mat.mtrl-sci↗