H0LiCOW XIII. A 2.4% measurement of $H_{0}$ from lensed quasars: $5.3σ$ tension between early and late-Universe probes (1907.04869v2)

Abstract: We present a measurement of the Hubble constant ($H_{0}$) and other cosmological parameters from a joint analysis of six gravitationally lensed quasars with measured time delays. All lenses except the first are analyzed blindly with respect to the cosmological parameters. In a flat $\Lambda$CDM cosmology, we find $H_{0} = 73.3_{-1.8}^{+1.7}$, a 2.4% precision measurement, in agreement with local measurements of $H_{0}$ from type Ia supernovae calibrated by the distance ladder, but in $3.1\sigma$ tension with $Planck$ observations of the cosmic microwave background (CMB). This method is completely independent of both the supernovae and CMB analyses. A combination of time-delay cosmography and the distance ladder results is in $5.3\sigma$ tension with $Planck$ CMB determinations of $H_{0}$ in flat $\Lambda$CDM. We compute Bayes factors to verify that all lenses give statistically consistent results, showing that we are not underestimating our uncertainties and are able to control our systematics. We explore extensions to flat $\Lambda$CDM using constraints from time-delay cosmography alone, as well as combinations with other cosmological probes, including CMB observations from $Planck$, baryon acoustic oscillations, and type Ia supernovae. Time-delay cosmography improves the precision of the other probes, demonstrating the strong complementarity. Allowing for spatial curvature does not resolve the tension with $Planck$. Using the distance constraints from time-delay cosmography to anchor the type Ia supernova distance scale, we reduce the sensitivity of our $H_0$ inference to cosmological model assumptions. For six different cosmological models, our combined inference on $H_{0}$ ranges from $\sim73$-$78~\mathrm{km~s^{-1}~Mpc^{-1}}$, which is consistent with the local distance ladder constraints.

• The paper achieves a 2.4% precision measurement of H0 by analyzing time delays in six gravitationally lensed quasars.
• It applies advanced time-delay methods and detailed lens modeling to mitigate biases from mass distributions and line-of-sight effects.
• The study uncovers a significant 5.3σ tension between early-Universe and late-Universe measurements, challenging standard ΛCDM assumptions.

Insights into the Measurement of the Hubble Constant from Lensed Quasars

The paper presented conducts a detailed analysis with the goal of obtaining a precise measurement of the Hubble constant ($H_0$) via gravitationally lensed quasars. Using a sample of six such systems, combined with measured time delays, the paper reaches a 2.4% precision on $H_0$, uncovering a significant 5.3\sigma tension between early-Universe and late-Universe methods of measuring $H_0$.

Key Contributions

1. Lensed Quasar Sample: The paper focuses on the H0LiCOW (Hubble Constant from Lenses in COSMOGRAIL's Wellspring) collaboration sample, which includes six gravitationally lensed quasars. The detailed analysis involves measuring their time-delays to derive constraints on cosmological parameters.
2. Time-Delay Methodology: The methodology builds on the Refsdal model (1964) whereby time delays between multiple quasar images provide insights into the mass distribution of the lensing galaxy and, crucially, $H_0$. The authors utilize advanced techniques to mitigate potential biases and error sources including lens model uncertainties, complex line-of-sight adjustments, and galaxy environment considerations.
3. Lensing Distance Analysis: By modeling the mass distribution across each lens system, the paper reveals constraints on the time-delay distances, which correlate inversely with $H_0$. This emphasizes the accuracy of gravitational lensing as an independent tool to probe probe cosmological scales, further strengthening collaborations with complementary frameworks.

Results and Discussion

• Measurement of $H_0$: The reported value of $73.3 \pm 1.7$ km/s/Mpc aligns with local (late-Universe) estimates of $H_0$ determined from type Ia supernovae but shows a distinct discrepancy when compared with early-Universe values derived from Planck satellite measurements ($67.4 \pm 0.5$ km/s/Mpc). This inconsistency suggests either unknown systematic errors or the need for new physics to bridge early and late-Universe observations.
• Complementarity: The paper highlights the utility of combining time-delay observations with cosmic microwave background (CMB) data and baryonic acoustic oscillations (BAO) analyses. Despite some uncertainties in cosmological parameters like spatial curvature or dark energy dynamics ($w$), joint analyses consistently enhance precision and minimize model dependence.

Theoretical and Practical Implications

The striking tension between the $H_0$ estimates invites further scrutiny and may challenge the robustness of the standard $\Lambda$CDM cosmological model, potentially hinting at new physics beyond it. Practically, the paper sets a precedent for leveraging gravitational lensing as a highly precise, independent tool for measuring cosmic distances, supporting its role in upcoming observational missions and campaigns. The paper's rigorous methodology in handling uncertainties and bias also serves as a benchmark for future investigations in cosmology.

Future Prospects

As the paper's findings underline, resolving the $H_0$ tension will not only deepen our understanding of cosmic history but may necessitate reconsideration of $\Lambda$CDM assumptions. Advances in observational technologies and interdisciplinary synergies between gravitational lensing, supernovae research, and large-scale surveys will likely drive next-generation inquiries into the cosmic expansion rate, dark energy, and other fundamental aspects of Universe physics. This emphasizes the ongoing need for refined measurements and the exploration of new theoretical pathways in cosmology.