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They are cosmic time inferred from redshift, anchored by independently measured distances. Redshift does two things at once. It tells us how much the universe expanded since emission, scale factor then = 1 / (1+z) And it tells us when the light was emitted, because we know how old the universe was at each scale factor if we assume a specific expansion history. Sounds circular but it isn’t, because the expansion history is solved for by fitting many observations at once. So, the timestamps are emission time = age of universe at that redshift, and arrival time = now. These timestamps are inferred consistently across thousands of objects. Redshift itself does not tell us where along the path the stretching occurred. If most redshift had occurred early, the curve would bend the opposite way. So the curve only fits if expansion accelerated late, not early. The missing piece isn’t intuition, it’s realizing that distance measurements are the timestamps. (Wrote this in public with Siri, so feel free to ask or if there’s typos. People with inane conversations about salads lol)