In general, the cosmological redshift of an object changes with time. Amazingly, current technology allows us to observe this change over a period of a couple of decades for objects at a redshift of a few, rather than having to wait for ten million years, as Sandage envisaged just 60 years ago. But we can do better: rather than waiting n years, observe the same object via different images in a strong gravitational-lens system with a time delay of n years.
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