I am a postdoctoral researcher at the Institut d'Astrophysique de Paris studying cosmology with Silvia Galli and the NEUCosmoS team (ERC). I analyse cosmic microwave background data to learn more about the composition and evolution of the universe and develop ways to improve these types of analysis. I am lucky to be a member of the SPT-3G collaboration and have wonderful colleagues and access to the amazing data collected by the South Pole Telescope.
I completed my BSc in Physics at the University of Sussex in the UK in 2016, after which I moved across the world and obtained my MSc and PhD from the University of Melbourne in 2018 and 2023, respectively. After my PhD I moved to Paris to work as a postdoc with Silvia Galli at the IAP.
When my heads spinning, I like to unwind at fencing. I picked the sport up during my undergraduate studies and haven’t been able to put the épée down since! During my MSc and PhD I was supported by the University of Melbourne’s elite athlete programme.
Lennart Balkenhol
Institut d'Astrophysique de Paris
98bis Bd Arago, 75014 Paris
France
lennart.balkenhol@iap.fr
I have a broad interest in all things cosmology with a focus on primary and secondary CMB science. I use the CMB as a tool to pursue hints of new physics and enjoy learning about (or developing) new statistical tools along the way. I am excited about the intersections of cosmology with particle physics and general relativity.
You can see some of my research highlights below and find a full list of publications here.
I developed candl: a JAX-based python likelihood for CMB experiments. Thanks to JAX's automatic differentiation algorithm, calculating the derivate of candl likelihoods is easy, fast, and robust. This gives easy access to Fisher matrices and facilitates new ways of analysing CMB data.
I expanded upon the previous SPT-3G release and added temperature band power measurements to the polarisation data. This work featured applying many Planck-style consistency tests to the measurements, showing agreement across frequencies as well as temperature and polarisation.
Does contemporary CMB data contain signatures of early dark energy (EDE)? Though recent ACT data is known to support EDE, this work shows that Planck polarisation data is also of great interest and lends support to a different EDE model with distinct signatures in the power spectrum. The addition of SPT data further increases the preference for EDE over ΛCDM.
Inhomogeneities sourced by magnetic fields in the early universe may provide an avenue to resolving the Hubble tension. I contributed to an analysis of primordial magnetic fields using Planck, SPT, and ACT data. The focus of this work is on consistency of these datasets, which is explored by discecting individual constraints and studying the expected constraining power using mock data.
I lead the high-level analysis of the first SPT-3G power spectra. This involved computing the covariance of the band powers, writing and verifying the CosmoMC likelihood, running the MCMC chains, and interpreting the results. I not only looked at the constraints placed on ΛCDM, but also studied extensions to the standard model of cosmology with an eye for the Hubble tension.
In CMB analysis, covariance matrices are typically conditioning in order to guarantee robust parameter cocnstraints. In this paper, I benchmarked the performance of various commonplace conditioning strategies using a set of simulations, finding that in some cases minimal conditioning can lead to a 30% increase of the parameter uncertainty.
Antarctica- and space-based CMB experiments rely on satellite links to transmit their data, which are limited by their bandwidth. For my MSc project, I explored a lossy compression scheme for the time-ordered data, which instead of using 32-bit floats, restricts each datapoint to 1-, 2-, or 3-bits. The latter increases the map noise level by <2%.