An overview of the project

Since the discovery of the accelerated expansion of the Universe, major space- and ground-based experiments have been designed to measure cosmological parameters, including the current expansion rate of the Universe, the Hubble constant (H0). Late-universe methods to measure H0 include the cosmic distance ladder (requiring Type Ia supernovae and Cepheid stars), water masers in a handful of nearby galaxies, and gravitationally lensed quasars. The early-universe probes include the Cosmic Microvave Background (CMB) and Baryon Acoustic Oscillations (BAO). Combining independent measurements of H0 in the late- and early-universe, respectively, now yields a tension over 4σ (i.e., the probability that they are compatible is less than 1 in 16 thousand).

The need for an independent measurement of the Hubble constant within 2% is now vital to rule out the possibility of systematic errors, and to verify this tension as real and unexplained by our current models of cosmology. This is the goal of COSMICLENS, using the time-delay method in gravitationally lensed quasars. Each lensed quasar yields an independent measurement of H0, but requires several detailed steps, which we separate into the following work packages (WP):

  1. the first high-cadence photometric monitoring of lensed quasars to measure 50 new time delays
  2. new flexible non-parametric lens models based on sparse regularization of the reconstructed source and lens mass/light distributions
  3. a modular end-to-end simulation framework to mock lensed systems from hydro-simulations and to evaluate in detail the impact of model degeneracies on H0
  4. discovering new suitable lensed quasars in current surveys

Work Package 1

High-cadence Photometric Monitoring of Lensed Quasars

The power of time-delay cosmography relies on the time delays between quasar images scaling inversely with the Hubble constant (H0). Therefore it is crucial to minimise the fractional uncertainty of the measured time delays. Unfortunately, many systems have short time delays and the error budget on the H0 inference can be dominated by the time delay uncertainty (as is the case for two of the five well-studied H0LiCOW quadruply-imaged quasars). A major contribution to the time delay uncertainty comes from the extrinsic variation due to microlensing by stars in the lensing galaxy. These variations must be disentangled from the true source quasar variations. Previous observations every few days over ~10 years have measured time delays to ~5% precision.

However, it has recently been demonstrated that daily observations with high signal-to-noise can lead to 2% precision time delays within a single observing season. This deep, high-cadence strategy overcomes the problems from the long-term, smooth microlensing variations by detecting intrinsic short-term, small-amplitude features in the quasar lightcurve. COSMICLENS uses several telescopes to measure these delays including the 2.56m VLT Survey Telescope, the MPIA 2.2m telescope, and the Swiss Euler telescope, with the aim of measuring 50 lensed quasar time delays each to 2% precision.

Work Package 2

Sparsity-based Lens Modeling Techniques

Lensed quasar time delays are set by cosmological parameters, geometry, and the lensing mass distribution. Once the time delays are measured, any inference on cosmological parameters relies on creating a realistic model of the true mass distribution. Galaxy mass distributions are far from simple however, with a baryonic component embedded within an invisible dark matter halo, with distributions depending on the complex growth history of the galaxy. Current lens modelling techniques model the lensing mass as an analytical profile, perhaps leading to biases, or, as a regularised pixel-based profile, possibly allowing too many degrees of freedom.

This COSMICLENS work package will focus on a new lens modelling approach based on the fact that galaxy mass distributions are sparse in an appropriate space, i.e. any galaxy mass profile can be well-represented by the combination of only a few basis profiles. Such an approach allows for a more physically motivated lens mass representation. Furthermore, the use of sparse basis sets can also be applied to the light distribution of the source quasar host galaxy. Finally, this work package will be closely tied to the results of Work Package 3, as these models will be blindly tested on mock lensed systems with known density profiles and cosmological parameters, in order to identify any systematic biases in the modelling process.

Work Package 3

Detailed Study of the Lensing Degeneracies

Each lens model is constrained by fitting the multiple quasar positions and the lensed quasar host galaxy light distribution. This is done with high-resolution imaging from the Hubble Space Telescope (HST) and/or ground-based adaptive optics imaging. However, there always exists a family of models that can reproduce the data, each leading to different cosmological inferences. This degeneracy can be broken with independent constraints on the lens mass (i.e. from kinematics), however the current methods linking mass and kinematics may introduce biases larger than the statistical uncertainty when eventually combining 50 lenses.

COSMICLENS is shedding light on these biases by analysing realistic mock lensed systems. This involves ray-tracing through state-of-the-art cosmological hydrodynamical simulations, and subsequently creating mock data sets mimicking those currently obtained from HST. Most importantly, the true mass distribution and kinematics are known for these mock lenses. The analysis of these mocks will show how important each dataset is for accuracy and precision in lens modelling, thus informing our future follow-up plan.

Work Package 4

Lens Searches in Wide Field Imaging Surveys

The time-delay cosmography method was proposed in 1964 by Sjur Refsdal, fifteen years before the first example of lensing was discovered. Refsdal originally suggested measuring time delays in lensed supernovae, however, in the following years quasars were realised to be a more common example of a variable source. By the mid-1990s, around 20 examples of lensed quasars had been discovered. During the late 1990s and 2000s, dedicated lensed quasar searches in the radio (JVAS/CLASS) and in spectroscopic quasar samples (SDSS), increased the number of known lensed quasars to over 100. However, only some of these lensed quasars are powerful probes for time-delay cosmography, depending on many factors including number of images, brightness, length of time delay, and even position on the sky.

Only in the past few years has deep optical imaging of the whole-sky become available, thanks to surveys such as Pan-STARRS, the Dark Energy Survey, and Gaia. COSMICLENS uses these all-sky datasets and a variety of techniques (including spectral energy distribution comparison, multi-band pixel-modelling, and machine learning) to discover the remaining bright lensed quasars. The best of these new systems will be monitored (through Work Package 1) to reach our goal of 50 time delays, each to 2% precision.


Denzel et al. 2020

Lessons from a blind study of simulated lenses: image reconstructions do not always reproduce true convergence, arXiv:1911.06218

Harvey et al. 2020

Exploiting flux ratio anomalies to probe warm dark matter in future large-scale surveys, 2019A&A...629A..97B

Chen et al. 2020

A SHARP view of H0LiCOW: H0 from three time-delay gravitational lens systems with adaptive optics imaging, 2019A&A...628L...7T

Millon et al. 2020

TDCOSMO. I. An exploration of systematic uncertainties in the inference of H0 from time-delay cosmography, arXiv:1907.04869

Krone-Martins et al. 2020

Gaia GraL: Gaia DR2 Gravitational Lens Systems. V. Doubly-imaged QSOs discovered from entropy and wavelets, arXiv:1905.09338

Lemon et al. 2020

The STRong lensing Insights into the Dark Energy Survey (STRIDES) 2017/2018 follow-up campaign: Discovery of 10 lensed quasars and 10 quasar pairs, 2019arXiv190908638C

Huber et al. 2020

Strongly lensed SNe Ia in the era of LSST: observing cadence for lens discoveries and time-delay measurements, arXiv:1912.09133

Sluse et al. 2020

H0LiCOW - X. Spectroscopic/imaging survey and galaxy-group identification around the strong gravitational lens system WFI 2033-4723, arXiv:1910.06306

Cornachione et al. 2020

A Microlensing Accretion Disk Size Measurement in the Lensed Quasar WFI 2026-4536, 2020MNRAS.492.3885D

Shajib et al. 2020

STRIDES: A 3.9 per cent measurement of the Hubble constant from the strong lens system DES J0408-5354, 2019MNRAS.490..613S

Arendse et al. 2020

Cosmic dissonance: new physics or systematics behind a short sound horizon?, arXiv:1912.08977

Chan et al. 2020

Twisted quasar light curves: implications for continuum reverberation mapping of accretion disks, 2020MNRAS.491.4247H

Bonvin et al. 2020

COSMOGRAIL. XVIII. time delays of the quadruply lensed quasar WFI2033-4723, arXiv:1912.08027

Taubenberger et al. 2020

The Hubble constant determined through an inverse distance ladder including quasar time delays and Type Ia supernovae, arXiv:1909.07986

Wong et al. 2020

H0LiCOW XIII. A 2.4% measurement of H0 from lensed quasars: 5.3σ tension between early and late-Universe probes, 2019A&A...631A.161H

Rusu et al. 2020

H0LiCOW XII. Lens mass model of WFI2033-4723 and blind measurement of its time-delay distance and H0, 2019MNRAS.490.1743C


COSMOGRAIL data release 1: Database

Conferences and Meetings

ESO Conference: H0 "Assessing Uncertainties in Hubble’s Constant Across the Universe": website


Presentations, videos, and talks for the general public

April 2020

Using gravitational lensing to measure the Hubble Constant

March 2020

La crise de Hubble

February 2020

Les origines de l'Univers

September 2019

50 years of EPFL



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