The CosmoverseLectures@YourDesk is an exciting new online series that aims to build on the CosmoVerse Training Series and to showcase lecture-style talks on the most prominent works related to cosmological tensions. This will provide early stage researchers with an in-depth understanding of the technical details of leading measurements connected with establishing the robustness of the question of cosmic tensions with a real focus on the underlying analysis.
The CosmoVerse Training Series was aimed at the interface of Fundamental Physics and Data Analysis in Cosmology , while here there will be more focus on the effects of Observational Cosmology and systematics and its intersection with these other areas. By evaluating recent works in the measurement of cosmological parameters using Type Ia supernova, Cosmic microwave background, Baryon acoustic oscillations, Large scale structure, Tip of the red-giant branch, and other measurements, the series will cover the core observables in the field.
Speakers
Supernova Cosmology, the nuts and bolts
Thursday - March 27, 2025 @ 10:00am CET
Tamara Davis (University of Queensland)
Tamara Davis is an enthusiastic professor of astrophysics at the University of Queensland. She led the Australian Dark Energy Survey (OzDES) and is Deputy Director of the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav). She ran cosmology analyses for three supernova cosmology teams, ESSENCE (2007), SDSS (2009), and DES (2024). During the 2010s she also worked with the WiggleZ survey doing baryon acoustic oscillation cosmology, and is continuing that now with DESI. She is an avid communicator of science and occasional guest host of astrophysics documentaries, including the episode “Black Hole Hunters” which won the American Institute of Physics Science Communication Award. Amongst her many accolades she received an Order of Australia for “significant service to astrophysical science, to education, and to young astronomers”.
How do type Ia supernovae constrain cosmology? It sounds simple… they’re standard(isable) candles so you use their apparent magnitude as a proxy for their distance and plot that against redshift to make a Hubble diagram. Compare that to theory and voila!!, cosmological parameters. Even though supernovae are one of the simplest possible cosmological probes, in practice there are many subtleties and potential selection effects/systematics that need to be taken into account. This talk will show from the ground up the considerations needed when making a supernova Hubble diagram, and potential pitfalls in its use. It will then explore the latest results from projects such as the Dark Energy Survey and discuss the significance of hints of time-varying dark energy seen in the data.
Here’s the link to the fitting SN Ia for cosmology tutorial: https://astrouq.github.io/PHYS3080/
Here’s the link to the GitHub, which takes you further to the ReadTheDocs: https://github.com/des-science/DES-SN5YR
CMB anisotropies as indirect probe of Hubble parameter
Thursday - March 27, 2025 @ 10:00am CET
Julien Lesgourgues (RWTH Aachen University)
Julien Lesgourgues has been working on many topics in theoretical cosmology, at the interface between theory and observations. He has been a member of the Planck satellite collaboration, and he is currently part of the Euclid satellite collaboration. His research highlights include several seminal publications on the role of neutrinos in the Universe; the developpement of the code CLASS, which simulates the evolution of the Universe on large scales, and is used massively within the international community of cosmology; the coordination of a software package, MontePython, allowing to fit cosmological models to observational data. Julien Lesgourgues also inferred from cosmological observations several important bounds on the properties of dark matter and neutrinos, on scenarios describing the early Universe, and on cosmological models that may account for the ‘Hubble tension’ raised by recent observations. Julien Lesgourgues has been awarded the 2006 Bronze medal for research in Theoretical Physics by CNRS. He shares the 2018 Gruber Cosmology prize, the 2018 Royal Astronomical Society Award and the 2019 European Physical Society Cocconi prize for his contribution to the Planck satellite collaboration. He is regularly invited to teach cosmology at international summer schools in Europe, North America and South America.
Why do CMB maps provide an indirect measurement of H0? We will very briefly summarise the underlying theory and the experimental process that leads to such a measurement. CMB people often say that the determination of the CMB anisotropy spectra is trustable and robust. We will discuss this claim and review the main systematics that could potentially affect the measurements. We will stress that the inference of H0 from CMB spectra is indirect and model-dependent. Finally, we will discuss which theoretical ingredient could shift the CMB prediction for H0.
- Slides
Cosmology with Baryonic Acoustic Oscillations
Thursday - April 10, 2025 @ 2:00pm CET
Hee-Jong Seo (Ohio University)
I am an Associate Professor at Ohio University, with a primary research focus on enhancing the accuracy and precision of clustering measurements obtained from large-scale galaxy surveys. My professional experience includes serving as a co-chair of the DESI Galaxy-Quasar Clustering Science Working Group and then as a co-convener of the DESI Year 1 Baryon Acoustic Oscillation (BAO) Key Project. I have contributed to prominent large-scale galaxy surveys such as the Baryon Oscillation Spectroscopic Survey (BOSS), extended-BOSS (eBOSS), and I am currently involved in the ongoing Dark Energy Spectroscopic Survey (DESI) and Roman Space Telescope Galaxy Redshift Survey
Baryon Acoustic Oscillations are one of the most robust cosmological probes of the expansion history of the Universe. In this lecture, I will summarize the Physics of BAO, BAO as the standard ruler: a Beginner’s Guide to the method, density field reconstruction, how the BAO measurements propagate to the cosmological measurements, and a brief history of the observational BAO measurements, all the way to Dark Energy Spectroscopic Instrument.
Large-Scale Structure from Theory to Observables
Thursday - April 10, 2025 @ 2:00pm CET
Stephen Chen (IAS, Princeton, US)
Stephen Chen is a postdoctoral member at the Institute for Advanced Study. His research interests include building theoretical models for large-scale structure, particularly using techniques from effective field theory, and applying these models to constrain fundamental physics using cosmological datasets. As part of this work, Stephen participated in the Year 1 analyses of the Dark Energy Spectroscopic Instrument (DESI), including both the measurement of baryon acoustic oscillations (BAO) and full-shape galaxy clustering.
Observations of the large-scale structure (LSS) of the universe touch on many of the outstanding questions of fundamental physics including the nature of dark energy, dark matter and neutrinos, the quantum initial conditions of the universe, and gravity. How can we connect the dots between these basic ingredients and the LSS measured through observations as different as spectroscopic surveys, galaxy lensing and the cosmic microwave background? In this lecture I will give an overview of how all these observables can be understood simultaneously within our theories of structure formation, focusing particularly on the effective theory of large-scale structure, and how to connect the underlying physics to data. With these ingredients in hand, we will be able to ask: where is the information coming from, and how do we know if we’ve detected new physics, a systematic, or an astrophysical unknown?
Cepheid variables in the distance ladder
Thursday - April 24, 2025 @ 2:00pm CET
Louise Breuval (Space Telescope Science Institute (STScI))
I’m an ESA Research Fellow at the Space Telescope Science Institute (STScI) in Baltimore (USA), and a member of the SH0ES team. My research focuses on the astronomical distance ladder and the direct measurement of the Hubble Constant H0 with Cepheid variables.
Cepheid variables are the most widely used and the best understood primary distance indicators. In this lecture, I will start by describing how they can be used to measure extragalactic distances and what is their role in the direct measurement of the Hubble constant, H0. The lecture will then cover the most recent updates on the Cepheid and SNIa distance ladder by the SH0ES team, focusing on the first step of the measurement: the geometric calibration of the Cepheid Period-Luminosity relation (Leavitt Law) in nearby galaxies. I will explain how we measure Cepheid distances with the Hubble and James Webb Space Telescopes, and will present important improvements from the Gaia mission. Finally, I will describe how systematic uncertainties are treated in the Cepheid distance ladder.
Tip of the Red Giant Branch Stars as Distance Indicators
Thursday - April 24, 2025 @ 2:00pm CET
Gagandeep Anand (Space Telescope Science Institute (STScI))
Gagandeep Anand is a Senior Staff Scientist at the Space Telescope Science Institute working on the Advanced Camera for Surveys (ACS), which is one of the two operational imagers on the Hubble Space Telescope. His main research area is near-field cosmology with resolved stellar populations, with a particular focus on the extragalactic distance scale.
The tip of the red giant branch (TRGB) marks the brightest ascent of low-mass red giants, and has been used to measure distances to over five hundred nearby galaxies with the Hubble Space Telescope. I will go over the underlying physical basis for the use of the TRGB as a standard candle, the evolution of the technique over time, and the current state of the method. Particular attention will be given to recent developments in the field, especially with the advent of JWST and in the context of the Hubble tension.