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.
This is a pre-copyedited, author-produced version of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record is available at the URLs mentioned on the abstract page, in my publication list, and at the basic-information page for this work.
Note that there is a minor erratum.
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