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Timing dark energy (lemmy.world)
submitted 11 months ago by to c/askscience@lemmy.world
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The way I understand it is the farther away a object is the faster it is moving away from us, but also the farther away something is the older it is. So could that mean things were moving apart faster in the past but are slowing down?

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[–] [S] 2 points 11 months ago (1 child)

I guess that's what im getting at. What are the timestamps?

If we see light from really far away, and it has red-shifted, then are we assuming the stretching is consistent for the whole distance or is there more stretching at the beginning or end of the trip? If the expansion is accelerating then more of the red-shift happened recently when it got closer to us since farther away also means older.

My thought was what if most of the red-shift happened when it was still far away from us thus meaning it happened a long time ago.

Like you mentioned some kind of timestamp would tell us, I just don't know what those timestamps would be. I'm sure there's something obvious I'm missing, but the only way I can think of to measure the difference in red-shift from the same light source at different points in its journey is to measure it from two vastly distant perspectives.

I guess if you measure red-shift from closer objects and compare it to the farther objects, and its not linear based on distance. Accelerated expansion would mean closer objects have a higher ratio of red-shift to distance than the farther objects?

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  • [–] 10 points 11 months ago

    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)

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