Study Identifies Key Systematic Uncertainties in Using Gravitational Lenses to Measure the Hubble Constant
A new study accepted for publication in The Astrophysical Journal quantifies systematic uncertainties in velocity dispersion measurements that affect Hubble constant estimates derived from eight time-delay gravitational lenses. The research finds that factors including PSF characterization, anisotropy model choice, and stellar mass distribution modeling can each introduce biases of several percent in the squared velocity dispersion, which directly propagates into errors on H₀. These findings matter because achieving the 2% precision in H₀ needed to resolve the ongoing 'Hubble tension' requires controlling each of these systematics to sub-percent levels.
Researchers have conducted a systematic investigation into the sources of measurement error affecting Hubble constant (H₀) estimates derived from gravitational lens time delays, a method considered independent of the cosmic distance ladder. The study examines eight time-delay lenses used in cosmological analyses and finds that achieving 2% precision on H₀ demands that fractional uncertainties in squared velocity dispersion (Δσ²/σ²) remain below 2%, a threshold that several identified systematics can individually exceed. Point spread function (PSF) characterization in ground-based observations can introduce biases of 1–5% in Δσ²/σ², while the choice of stellar orbital anisotropy models can cause variations as large as 18%. Differences between the true stellar mass distribution and the modeled light profile contribute an additional 1–10% bias, with radial color gradients adding further complications. The authors also highlight that the difference between the observationally measured velocity dispersion and the mean-square velocity required by the Jeans equations introduces a 2–6% systematic. A key recommendation is that many dynamically related parameters should be marginalized over the full lens sample rather than on a lens-by-lens basis, exploiting the relative homogeneity of early-type galaxy populations.
What's missing
The study focuses on identifying and quantifying systematics but does not provide updated H₀ measurements with these corrections applied to the full lens sample. It is also unclear how these findings compare quantitatively to systematic floors in competing H₀ measurement methods (e.g., the cosmic distance ladder or CMB-based estimates), which would be relevant for assessing their impact on the Hubble tension debate. The paper does not discuss whether future space-based observations could substantially reduce the PSF-related systematics identified.
What different sources said
- arXiv astro-phCenter
Dynamical Systematics for Time Delay Lenses and the Impact on the Hubble Constant
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