Personal website of Simon Birrer


My research focus is to probe fundamental physics on cosmological scales. Among the most striking phenomenas that drive the apperance of our universe on cosmological scales is the observed accelerated expansion of our Universe (dark energy) and a dominated gravitational mass component that does not interact with the regular composition of the standard model of particle physics but drives the gravitational structure formation observed at scales of galaxies and beyond (dark matter).
I am primary using gravitational lensing, a phenomena described by general relativity, causing light to follow curved paths when travelling through inhomogeneous matter distributions. In the strongest regime, gravitational lensing can lead to multiple appearances of the same source and highly distorts images - know as strong gravitational lensing.
Hubble Space Telescope images of 12 quadruply lensed quasars and models thereof performed with the lenstronomy software package, from Shajib, Birrer et al. 2018 (STRIDES collaboration).

Measuring the Hubble constant

The value of the recent expansion rate of the universe (Hubble constant) is the key measurement that anchors the physical scales and age of our universe without relying on a model early universe physics. A precise measurement of the Hubble constant allows to test cosmological models robustly. However, there is a controversy in the precise value of the Hubble constant between local measurements and the model propagated value from the cosmic microwave background. This leaves room for either new physics or unknown systematics in either of the probes. I am spearheading an effort to provide a precision measurement from strong gravitational lensing time-delay cosmography. This technique is independent of the local distance ladder and other cosmological probes such as the CMB. In my PhD, I did an independent reanalysis of one of the previous systems with an emphasis on the treatment of systematics (Birrer et al. 2016, JCAP 08, 020). Within H0LiCOW, I lead the analysis of one doubly imaged quasar (Birrer et al. 2019, MNRAS). . Previously, the H0LiCOW collaboration provided a 2.4% precision with six lenses being analysed (Wong et al. 2019). In the STRIDES collaboration, I co-lead the analysis of the most precise single lens constraint on the Hubble constant (Shajib, Birrer et al. 2019). Recently, I revised the methodology and lead an updated measurement within the TDCOSMO collaboration (Birrer et al. 2020, submitted).

Birrer et al. 2020

TDCOSMO IV: Hierarchical time-delay cosmography -- joint inference of the Hubble constant and galaxy density profiles

Birrer et al. 2019, MNRAS

H0LiCOW - IX. Cosmographic analysis of the doubly imaged quasar SDSS 1206+4332 and a new measurement of the Hubble constant

Quantifying dark matter

The physical nature of the most common type of matter in our universe is still unknown and its potential particle physics nature remains unconfirmed to date. Although a direct or indirect detection of dark matter is missing, we can constrain its physical nature by quantifying its gravitational imprint on our universe. Gravitatioinal lensing probes the total matter distribution and as such provides an unique window into the dark sector. I am using statistical approaches to quantify the imprint of dark matter in gravitational lenses. During my PhD, I was using Hubble Space Telescope imaging data to constrain the thermal relic mass of dark matter Birrer et al. 2017, JCAP 05, 037. Since moving to UCLA, I worked in a team to extend the statistical approach to multiple lensed quasar flux ratios Gilman, Birrer et al. 2018, MNRAS 481, 819, Gilman, Birrer et al. 2019, MNRAS 487, 5721, resulting in tight constraints on the dark matter warmth Gilman, Birrer et al. 2019, MNRAS submitted, and constraints on the mass-concentration relation of low mass dark matter halos Gilman, incl Birrer et al. 2019, MNRAS submitted.

Gilman, Birrer et al. 2019

Warm dark matter chills out: constraints on the halo mass function and the free-streaming length of dark matter with 8 quadruple-image strong gravitational lenses

Linking dark matter and galaxy evolution

In my first science project, Birrer et al. 2014, ApJ 793, 1, 12, I merged the phenomenological galaxy evolution model by Lilly et al. 2013 with the hierarchical formation of dark matter structure. The semi-analytical model allows to self-consistently model the galaxy-dark matter halo connection through cosmic time and reproduces the major evolutionary trends in the galaxy population as well as in the halo population in the full cosmological context. I am involved in studies that probe the connection between supermassive black holes and the host halo at high redshift Ding incl. Birrer et al. 2019, Silverman incl. Birrer et al. 2019.

Strong lensing in the context of large scale structure

Gravitational lensing occurs along the entire path of light, from its emission to the detector. Although the major contribution in the strong lensing regime comes from one single deflector (a galaxy, galaxy group or cluster), the effect of the line of sight can significantly impact observables and thus needs to be taken into account. In Birrer et al. 2017, JCAP 04, 049, we introduced a practical approach to account for multi-plane weak distortion effects in the modelling paired with a galaxy halo rendering framework. The information imprinted in the observables from the line of sight can, in turn, also be used as a cosmological probe. In Birrer et al. 2018, ApJL 852, 1, L14 we proposed using Einstein rings to measure cosmic shear. This approach is complementary to the standard statistical measurement of galaxy distortions.

Developing methods and supporting implementations

To enable the science I am pursuing, we developed a new strong lensing modelling approach with basis sets, Birrer et al. 2015, ApJ 813, 2, 102. In a next step, we made a public release of the software package in Birrer & Amara 2018. I make a big effort in facilitating the distribution and use of the software through packaging, modular built up, documentations and example use cases. My tools are now in wide use for generating simulations for neural network lens finding and modeling, reconstruction of sources that are lensed in galaxy clusters and even for initially unexpected science cases, such as the decomposition of quasars and its host galaxy at high redshift Ding incl. Birrer et al. 2019.