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
TDCOSMO XI. Automated Modeling of 9 Strongly Lensed Quasars and Comparison Between Lens Modeling Software, 2022arXiv220903094E
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
Evidence for a milli-parsec separation Supermassive Black Hole Binary with quasar microlensing, 2022arXiv220700598M
Gravitationally lensed quasars in Gaia -- IV. 150 new lenses, quasar pairs, and projected quasars, 2022arXiv220607714L
Gaia GraL: Gaia DR2 Gravitational Lens Systems. VII. XMM-Newton Observations of Lensed Quasars, 2022ApJ...927...45C
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
Discovery of strongly lensed quasars in the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), 2022A&A...659A.140C
TDCOSMO. IX. Systematic comparison between lens modelling software programs: time delay prediction for WGD 2038-4008, 2022arXiv220211101S
HOLISMOKES. VII. Time-delay measurement of strongly lensed Type Ia supernovae using machine learning, 2022A&A...658A.157H
J1721+8842: a gravitationally lensed binary quasar with a proximate damped Lyman-α absorber, 2022A&A...657A.113L
Discovery of two bright high-redshift gravitationally lensed quasars revealed by Gaia, 2022MNRAS.509..738D
Using strong lensing to understand the microJy radio emission in two radio quiet quasars at redshift 1.7, 2021MNRAS.508.4625H
Gaia GraL: Gaia DR2 Gravitational Lens Systems. VI. Spectroscopic Confirmation and Modeling of Quadruply Imaged Lensed Quasars, 2021ApJ...921...42S
HOLISMOKES. V. Microlensing of type II supernovae and time-delay inference through spectroscopic phase retrieval, 2021A&A...653A..29B
Strong lens systems search in the Dark Energy Survey using Convolutional Neural Networks, 2021arXiv210900014R
SLITRONOMY: Towards a fully wavelet-based strong lensing inversion technique, 2021A&A...647A.176G
Measuring accretion disk sizes of lensed quasars with microlensing time delay in multi-band light curves, 2021A&A...647A.115C
Testing the evolution of correlations between supermassive black holes and their host galaxies using eight strongly lensed quasars, 2021MNRAS.501..269D
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
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
TDCOSMO. IV. Hierarchical time-delay cosmography - joint inference of the Hubble constant and galaxy density profiles, 2020A&A...643A.165B
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
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
TDCOSMO. II. Six new time delays in lensed quasars from high-cadence monitoring at the MPIA 2.2 m telescope, 2020A&A...642A.193M
H0LiCOW XII. Lens mass model of WFI2033-4723 and blind measurement of its time-delay distance and H0, 2020MNRAS.498.1440R
COSMOGRAIL. XIX. Time delays in 18 strongly lensed quasars from 15 years of optical monitoring, 2020A&A...640A.105M
Cosmic dissonance: are new physics or systematics behind a short sound horizon?, 2020A&A...639A..57A
TDCOSMO. I. An exploration of systematic uncertainties in the inference of H0 from time-delay cosmography, 2020A&A...639A.101M
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
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
Twisted quasar light curves: implications for continuum reverberation mapping of accretion disks, 2020A&A...636A..52C
Lessons from a blind study of simulated lenses: image reconstructions do not always reproduce true convergence, 2020MNRAS.492.3885D
Exploiting flux ratio anomalies to probe warm dark matter in future large-scale surveys, 2020MNRAS.491.4247H
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
H0LiCOW - X. Spectroscopic/imaging survey and galaxy-group identification around the strong gravitational lens system WFI 2033-4723, 2019MNRAS.490..613S
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
Resources
Conferences and Meetings
ESO Conference: H0 "Assessing Uncertainties in Hubble’s Constant Across the Universe": website
Outreach
Presentations, videos, and talks for the general public
Constante de Hubble : « La diversité des méthodes de mesure enrichit le débat »
Pour sortir de la crise provoquée par la mesure de la constante de Hubble, de nombreux chercheurs développent des méthodes de mesure indépendantes de celles utilisées jusqu’ici. Tour d’horizon avec Frédéric Courbin, astrophysicien à l'EPFL, en Suisse.
Trouvez l'article ici.
Expansion de l'univers : de l'importance d'être constante
Que nous indique la constante de Hubble sur l’univers et son expansion ? Comment arrive-t-on à la définir et comment peut-on l’interpréter ? Pourquoi, selon les méthodes employées, les chercheurs arrivent à des résultats différents de la valeur de cette constante ?
Trouvez l'article ici.
Using gravitational lensing to measure the Hubble Constant
Gravitationally Lensed Quasars
How do astronomers measure the expansion rate of the Universe, otherwise known as the Hubble Constant? One method is to use gravitational lenses and the measurement of time delays between multiple images of quasars that are billions of light years away. This short video from EPFL's Astrophysics Laboratory explains how it works.
Click here to watch a video of how gravitationally lensing works.
Constante de Hubble : « La diversité des méthodes de mesure enrichit le débat »
Pour sortir de la crise provoquée par la mesure de la constante de Hubble, de nombreux chercheurs développent des méthodes de mesure indépendantes de celles utilisées jusqu’ici. Tour d’horizon avec Frédéric Courbin, astrophysicien à l'EPFL, en Suisse.
Cliquez ici pour lire un article sur la crise de Hubble, écrit par Frédéric Courbin pour "Pour La Science".
Les origines de l'Univers
Frédéric Courbin on Radio Télévision Suisse
The "Big Bang" is a cosmological model that describes the origin and evolution of the Universe. How did scientists come up with this theory? Why is it said that "the Universe is expanding"? Bastien Confino sheds light on the mysteries of the creation of the Universe.
With Frédéric Courbin, professor at the Astrophysics Laboratory of the Swiss Federal Institute of Technology in Lausanne (EPFL).
Download the mp3 here, or listen to the interview on the RTS website here.
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