Challenge, tell me what the u is about in ten easy steps

[wolram]

Main Question or Discussion Point

Can any one take up this challenge?

 

[marcus]

Can any one take up this challenge?

Sure. Five steps. The universe is what cosmologists study. So I just have to tell you what Cosmology is about.

1. Cosmology is about fitting data to the Friedman 1922 model.
The more precision data you fit, the more refined estimates you get of the model parameters. You expect to keep refining until eventually you find a discrepancy that forces you to change the model.

2. The Friedman model is basically a distance function (metric) in which spatial distances depend on a time-varying scalefactor a(t), and the growth of this scalefactor a(t) is governed by two simple equations. (Look up "Friedmann equations" in wikipedia, so I don't have to write them down.)

3. I have to tell you what the time is, that the Friedman equations work by, and according to which the scalefactor a(t) grows. And I have to say what spatial distances are, that have this factor in them.

4. Friedman time is what observers measure who are at rest with respect to the Background. No doppler hotspot in any direction. (Some uniformity assumptions, homog and iso, help out here.) Observers are at the same universe-time if they see the same Background temperature. At present the Background is an almost perfectly uniform thermal soup of microwave, the CMB.

5. Spatial distances are those measured at some specified time between observers at rest. This can only be done approximately since any real measurement has some duration, even if it is broken up into small simultaneously measured increments. (You didn't ask about technicalities but I'll throw that one in for free )

Wolram, I was encouraged to choose this way to tell the story by a comment that Redbelly made, essentially about this very issue:
https://www.physicsforums.com/showthread.php?p=2157775#post2157775
Have a look at his comment. I think he shows excellent judgment, and not only on this occasion.