What observation method can scientists use to test for directional preferences in the expansion rate across different lines of sight?

Observing Type Ia supernovae as standard candles to measure distances and expansion.

To confirm isotropy, scientists look for directional asymmetry in the expansion rate. A direct method involves using Type Ia supernovae because they function as 'standard candles,' providing a reliable way to measure distances across the cosmos. If the universe were expanding faster along one axis than another, supernovae observed along those different paths—at the same look-back time—would yield discrepant measured distances or redshift values. Any systematic difference in these measurements along orthogonal axes would imply a violation of the assumption that expansion velocity is independent of direction.