Personal website of Simon Birrer

Research

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).
My research focus is in further quantifying those phenomenas with exquisite data and comparing those precision measurements with different theoretical models.
I am primary using gravitational lensing, a phenomena described by general relativity, causing light to follow curved paths when travelling through inhomogeneous matter. In the strongest regime, gravitational lensing can lead to multiple apperance of the same source and highly distorted images (strong gravitational lensing).
Hubble Space Telescope images of 12 quadruply lensed quasars and models therof performed with the lenstronomy software package, from Shajib, Birrer et al. 2018.

Measuring the Hubble constant

The value of the recent expansion rate of the universe (Hubble constant) is an important measurement that anchores the physical scales and age of our universe. 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 the 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 strongly involved in an effort to provide a precision measurement from strong gravitationl lensing time-delay cosmography. The H0LiCOW collaboration is down to 3% precision with four lenses being analysed (Birrer et al. 2018, MNRAS submitted). 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). This technique is independent of the local distance ladder and other cosmological probes such as the CMB.

Quantifying dark matter

The physical nature of the most commen 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 also dark matter. 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. We are now extending the statistical approach to multiple lensed quasar flux ratios Gilman, Birrer et al. 2018, MNRAS 10, 1093.

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.

The line of sight of gravitational lenses - source of systematics and a cosmological probe at the same time

Gravitational lensing occures 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 pursueing, 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 and Amara 2018, Physics of the Dark Universe, submitted. I make a big effort in facilitating the distribution and use of the software through packaging, modular built up, documentations and example use cases.

Woking in collaborations

I am an active member of the following collaborations: