COSMICLENS

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.

Publications

Savary et al. 2022

Strong lensing in UNIONS: Toward a pipeline from discovery to modeling, 2022A&A...666A...1S

Ertl et al. 2022

TDCOSMO XI. Automated Modeling of 9 Strongly Lensed Quasars and Comparison Between Lens Modeling Software, 2022arXiv220903094E

Gomer et al. 2022

TDCOSMO VIII: A key test of systematics in the hierarchical method of time-delay cosmography, 2022arXiv220902076G

Van de Vyvere
et al. 2022

Consequences of the lack of azimuthal freedom in the modeling of lensing galaxies, 2022A&A...663A.179V

Millon et al. 2022

Evidence for a milli-parsec separation Supermassive Black Hole Binary with quasar microlensing, 2022arXiv220700598M

Schmidt et al. 2022

STRIDES: Automated uniform models for 30 quadruply imaged quasars, 2022arXiv220604696S

Lemon et al. 2022

Gravitationally lensed quasars in Gaia -- IV. 150 new lenses, quasar pairs, and projected quasars, 2022arXiv220607714L

Connor et al. 2022

Gaia GraL: Gaia DR2 Gravitational Lens Systems. VII. XMM-Newton Observations of Lensed Quasars, 2022ApJ...927...45C

Paic et al. 2022

Constraining quasar structure using high-frequency microlensing variations and continuum reverberation, 2022A&A...659A..21P

Van de Vyvere
et al. 2022

TDCOSMO. VII. Boxyness/discyness in lensing galaxies: Detectability and impact on H0, 2022A&A...659A.127V

Chan et al. 2022

Discovery of strongly lensed quasars in the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), 2022A&A...659A.140C

Shajib et al. 2022

TDCOSMO. IX. Systematic comparison between lens modelling software programs: time delay prediction for WGD 2038-4008, 2022arXiv220211101S

Huber et al. 2022

HOLISMOKES. VII. Time-delay measurement of strongly lensed Type Ia supernovae using machine learning, 2022A&A...658A.157H

Lemon et al. 2022

J1721+8842: a gravitationally lensed binary quasar with a proximate damped Lyman-α absorber, 2022A&A...657A.113L

Desira et al. 2022

Discovery of two bright high-redshift gravitationally lensed quasars revealed by Gaia, 2022MNRAS.509..738D

Hartley et al. 2021

Using strong lensing to understand the microJy radio emission in two radio quiet quasars at redshift 1.7, 2021MNRAS.508.4625H

Stern et al. 2021

Gaia GraL: Gaia DR2 Gravitational Lens Systems. VI. Spectroscopic Confirmation and Modeling of Quadruply Imaged Lensed Quasars, 2021ApJ...921...42S

Denzel et al. 2021

A new strategy for matching observed and simulated lensing galaxies, 2021MNRAS.506.1815D

Bayer et al. 2021

HOLISMOKES. V. Microlensing of type II supernovae and time-delay inference through spectroscopic phase retrieval, 2021A&A...653A..29B

Rojas et al. 2021

Strong lens systems search in the Dark Energy Survey using Convolutional Neural Networks, 2021arXiv210900014R

Birrer et al. 2021

lenstronomy II: A gravitational lensing software ecosystem, 2021JOSS....6.3283B

Malbet et al. 2021

Faint objects in motion: the new frontier of high precision astrometry, 2021ExA....51..845M

Ding et al. 2021

Time delay lens modelling challenge, 2021MNRAS.503.1096D

Galan et al. 2021

SLITRONOMY: Towards a fully wavelet-based strong lensing inversion technique, 2021A&A...647A.176G

Chan et al. 2021

Measuring accretion disk sizes of lensed quasars with microlensing time delay in multi-band light curves, 2021A&A...647A.115C

Huber et al. 2021

HOLISMOKES. III. Achromatic phase of strongly lensed Type Ia supernovae, 2021A&A...646A.110H

Ding et al. 2021

Testing the evolution of correlations between supermassive black holes and their host galaxies using eight strongly lensed quasars, 2021MNRAS.501..269D

Suyu et al. 2020

HOLISMOKES. I. Highly Optimised Lensing Investigations of Supernovae, Microlensing Objects, and Kinematics of Ellipticals and Spirals, 2020A&A...644A.162S

Van de Vyvere
et al. 2020

The impact of mass map truncation on strong lensing simulations, 2020A&A...644A.108V

Buckley-Geer et al. 2020

STRIDES: Spectroscopic and photometric characterization of the environment and effects of mass along the line of sight to the gravitational lenses DES J0408-5354 and WGD 2038-4008, 2020MNRAS.498.3241B

Birrer et al. 2020

TDCOSMO. IV. Hierarchical time-delay cosmography - joint inference of the Hubble constant and galaxy density profiles, 2020A&A...643A.165B

Tihhonova et al. 2020

H0LiCOW - XI. A weak lensing measurement of the external convergence in the field of the lensed quasar B1608+656 using HST and Subaru deep imaging, 2020MNRAS.498.1406T

Wong et al. 2020

H0LiCOW - XIII. A 2.4 per cent measurement of H0 from lensed quasars: 5.3σ tension between early- and late-Universe probes, 2020MNRAS.498.1420W

Millon et al. 2020

TDCOSMO. II. Six new time delays in lensed quasars from high-cadence monitoring at the MPIA 2.2 m telescope, 2020A&A...642A.193M

Rusu et al. 2020

H0LiCOW XII. Lens mass model of WFI2033-4723 and blind measurement of its time-delay distance and H0, 2020MNRAS.498.1440R

Millon et al. 2020

COSMOGRAIL. XIX. Time delays in 18 strongly lensed quasars from 15 years of optical monitoring, 2020A&A...640A.105M

Arendse et al. 2020

Cosmic dissonance: are new physics or systematics behind a short sound horizon?, 2020A&A...639A..57A

Millon et al. 2020

TDCOSMO. I. An exploration of systematic uncertainties in the inference of H0 from time-delay cosmography, 2020A&A...639A.101M

Shajib et al. 2020

STRIDES: a 3.9 per cent measurement of the Hubble constant from the strong lens system DES J0408-5354, 2020MNRAS.494.6072S

Cornachio- -ne et al.
2020

A Microlensing Accretion Disk Size Measurement in the Lensed Quasar WFI 2026-4536, 2020ApJ...895..125C

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, 2020MNRAS.494.3491L

Chan et al. 2020

Twisted quasar light curves: implications for continuum reverberation mapping of accretion disks, 2020A&A...636A..52C

Denzel et al. 2020

Lessons from a blind study of simulated lenses: image reconstructions do not always reproduce true convergence, 2020MNRAS.492.3885D

Harvey et al. 2020

Exploiting flux ratio anomalies to probe warm dark matter in future large-scale surveys, 2020MNRAS.491.4247H

Chen et al. 2019

A SHARP view of H0LiCOW: H0 from three time-delay gravitational lens systems with adaptive optics imaging, 2019MNRAS.490.1743C

Krone-Martins et al. 2019

Gaia GraL: Gaia DR2 Gravitational Lens Systems. V. Doubly-imaged QSOs discovered from entropy and wavelets, 2019arXiv191208977K

Sluse et al. 2019

H0LiCOW - X. Spectroscopic/imaging survey and galaxy-group identification around the strong gravitational lens system WFI 2033-4723, 2019MNRAS.490..613S

Bonvin et al. 2019

COSMOGRAIL. XVIII. time delays of the quadruply lensed quasar WFI2033-4723, 2019A&A...629A..97B

Taubenber- -ger et al.
2019

The Hubble constant determined through an inverse distance ladder including quasar time delays and Type Ia supernovae, 2019A&A...628L...7T

Ding et al. 2018

Time Delay Lens Modeling Challenge: I. Experimental Design, 2018arXiv180101506D

Resources

COSMOGRAIL data release 1: Database

Conferences and Meetings

Lensing Odyssey 2021: website

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

Outreach

Presentations, videos, and talks for the general public

March 2021

Pour La Science

March 2021

La Méthode Scientifique

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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