Astronomy
Measuring the Expansion Rate with Gravitational Lensing Time Delays
Quick fact
Gravitational lensing time delays can measure the Hubble constant with about 10% precision, offering an independent check on the expansion rate derived from supernovae and the cosmic microwave background.
Why this is interesting
You've probably seen the same star appear twice in a photo of a distant galaxy – but what if those twin images could tell us how fast the universe is expanding?
Read the full explanation
Understanding Measuring the Expansion Rate with Gravitational Lensing Time Delays
Imagine a massive galaxy acting like a giant magnifying glass. Light from a background quasar (an extremely bright galactic core) travels along different curved paths around this lens, so we see multiple images. Because these paths have different lengths and the light passes through different depths of the gravitational field, the images do not arrive at Earth at the same moment. The time delay between the images is set by the distances involved and the mass of the lensing galaxy. By measuring the delay (from brightness variations of the quasar) and modeling the lens's mass, astronomers can calculate a key cosmic quantity: the Hubble constant, which tells us how fast the universe is expanding.
A deeper explanation
The time delay between lensed images arises from two effects: the geometric path length difference (a longer path takes more time) and the gravitational time dilation (light climbing out of a deeper gravitational well is delayed). Combined, these give a time delay proportional to the 'time delay distance', which depends on the angular diameter distances between observer, lens, and source. Since these distances depend on the expansion history of the universe, measuring the time delay and the lens mass lets us infer the Hubble constant. This method is powerful because it does not rely on the cosmic distance ladder – it uses geometry and gravity directly. However, it requires precise modeling of the lens's mass distribution and the presence of a variable quasar to track the delay. Multiple lensed systems observed across the sky improve the precision, and ongoing surveys are refining this technique to help settle the 'Hubble tension' – the discrepancy between different expansion-rate measurements.