Astronomy
Cosmological Constraints on the Hubble Constant from Time-Delay Lensing of Quasars
Quick fact
Time-delay lensing measurements of quasars provide a one-step geometric measurement of the Hubble constant, free from the distance ladder's step-by-step calibration, and have measured H0 around 70-75 km/s/Mpc, contributing to the 'Hubble tension'.
Why this is interesting
Imagine seeing the same quasar at four different points in the sky, each flashing at a slightly different time. This cosmic mirage can reveal how fast the universe is expanding—but how?
Read the full explanation
Understanding Cosmological Constraints on the Hubble Constant from Time-Delay Lensing of Quasars
A foreground galaxy can act as a gravitational lens, bending light from a background quasar. Because the paths to each image have different lengths and pass through different gravitational potentials, light arrives at different times. If the quasar varies in brightness, we can measure these delays—hours to months apart. Combined with a precise model of the lens galaxy's mass distribution, these delays directly relate to the relative distances involved, giving us a measure of cosmic distances and the expansion rate (H0).
A deeper explanation
The time delay between two lensed images depends on the difference in the Fermat potential (a combination of geometric path length and gravitational time dilation) and on the angular diameter distances between observer, lens, and source. By modeling the lens mass distribution—including dark matter—and measuring the time delays, we can constrain the 'time-delay distance', which is inversely proportional to H0. This method is independent of the cosmic distance ladder and provides a direct geometric measurement. Recent campaigns (e.g., H0LiCOW) using lensed quasars like HE 0435-1223 have yielded H0 ≈ 73 km/s/Mpc, highlighting the tension with Planck's cosmic microwave background measurement (≈67 km/s/Mpc). Thus, time-delay lensing is a crucial independent test of our cosmological model.