Tdcosmo

Abstract

The importance of alternative methods for measuring the Hubble constant, such as time-delay cosmography, is highlighted by the recent Hubble tension. It is paramount to thoroughly investigate and rule out systematic biases in all measurement methods before we can accept new physics as the source of this tension. In this study, we perform a check for systematic biases in the lens modelling procedure of time-delay cosmography by comparing independent and blind time-delay predictions of the system WGD 2038−4008 from two teams using two different software programs: glee and lenstronomy. The predicted time delays from the two teams incorporate the stellar kinematics of the deflector and the external convergence from line-of-sight structures. The un-blinded time-delay predictions from the two teams agree within 1.2σ, implying that once the time delay is measured the inferred Hubble constant will also be mutually consistent. However, there is a ∼4σ discrepancy between the power-law model slope and external shear, which is a significant discrepancy at the level of lens models before the stellar kinematics and the external convergence are incorporated. We identify the difference in the reconstructed point spread function (PSF) to be the source of this discrepancy. When the same reconstructed PSF was used by both teams, we achieved excellent agreement, within ∼0.6σ, indicating that potential systematics stemming from source reconstruction algorithms and investigator choices are well under control. We recommend that future studies supersample the PSF as needed and marginalize over multiple algorithms or realizations for the PSF reconstruction to mitigate the systematics associated with the PSF. A future study will measure the time delays of the system WGD 2038−4008 and infer the Hubble constant based on our mass models.

Tdcosmo

“…The modeling results do not agree as well with the Lenstronomy values, and are closer to the GLEE values. This comparison shows that although subsampling in the IR band is important (Suyu et al 2012, Shajib et al 2022, the modeling results depend on the way in which the PSF is subsampled. It should be done during the PSF reconstruction process, and not as a simple interpolation after the corrections of the PSF.…”

“…In contrast, in this work, we used an subsampled PSF estimated from images of stars in the field, without additional corrections. As shown by Shajib et al (2022), the PSF is a crucial ingredient for cosmography-grade inference.…”

“…We note that our approach and that of S22 are similar, but not identical. Important differences include the modeling of the PSF, which is known to be crucially important (Shajib et al 2022), and the strategy adopted to cope with insufficient data quality (S22 imposed informative priors, but we do not). Therefore, we do not expect the two efforts to agree within the formal uncertainties, which are only a representation of the statistical error and not of the systematics, which are notoriously dominant.…”

Holismokes

“…Gravitational lensing, which means that the light of a background object is gravitationally deflected by a massive object in the foreground, gives us the opportunity to probe the Universe in various aspects. This includs the study of high-redshift systems (e.g., Dye et al 2018;Lemon et al 2018;McGreer et al 2018;Rubin et al 2018;Salmon et al 2018;Shu et al 2018), the study of the nature and distribution of dark matter (e.g., Schuldt et al 2019;Baes & Camps 2021;Basak et al 2022;Gilman et al 2021;Shajib et al 2021;Wang et al 2022), and cosmological parameter measurements (e.g., Refsdal 1964;Chen et al 2019;Birrer et al 2020;Millon et al 2020;Shajib et al 2020Shajib et al , 2022Wong et al 2020).…”

Holismokes

“…In special cases where a variable source, such as a quasar (e.g., Lemon et al 2017Lemon et al , 2018Lemon et al , 2019Ducourant et al 2019;Khramtsov et al 2019;Chan et al 2020;Chao et al 2020Chao et al , 2021 or supernova (SN), is present as a background object (Kelly et al 2015;Goobar et al 2017;Rodney et al 2021), one can measure the time delays (e.g., Millon et al 2020a,b;Huber et al 2022) and then constrain the Hubble constant H 0 (e.g., Refsdal 1964;Chen et al 2019;Rusu et al 2020;Wong et al 2020;Shajib et al 2020Shajib et al , 2022, helping to clarify the current discrepancy between the cosmic microwave background (CMB) measurements (Planck Collaboration VI 2020) and measurements using the local distance ladder (e.g., the SH0ES project, Riess et al 2019Riess et al , 2021. Furthermore, if one detects a lensed SN on the first image, and predicts the location and time for the next appearing image(s), the SN can be observed at earlier phases than without lensing, which would help to shed light on open questions regarding the SN progenitor system(s).…”