
#1
Oct112, 04:47 PM

P: 281

Cosmology isn't my thing but a friend showed me this and I thought it was great so I'll leave it here too for anyone interested:




#3
Oct112, 06:08 PM

Astronomy
Sci Advisor
PF Gold
P: 22,797

It is indeed cool!
But towards the end, where it talks about the stretching out of wavelengths (along with distances) remember that in Hubble Law expansion there is a pattern of distances increasing without anybody going anywhere as would happen in ordinary motion. So they really shouldn't call it motion without some explanation. In modern understanding of geometry (GR is a theory of dynamic geometry) the distances between everybody can increase without anybody getting anywhere. Without the relative position of neighbors changing etc. No approach to any destination. No transmission of info. Just everybody getting farther apart. That's not the only seemingly weird thing, the angles of a triangle can change what they add up to. Geometry can interact with matter. And geometry can just...well...expand. But GR also explains for us why in our situation under normal conditions geometry is at least very close to Euclidean. GR explains what CAUSES it to actually be very close to what we used to think it always was. What's more, GRs geometric effects have been extremely well tested. It is our currently accepted Law of Gravity and How Geometry Works. So in Jesse's animation of why night is dark, towards the end when they say wavelengthening is because stuff is MOOOVING away from USSS, be a little careful and don't entirely swallow that. It's an over simplification. Geometric expansion affects both wavelengths and largescale distances between disconnected observers in the same way. (Pairs of observers both at rest relative to ancient light, not part of part of some fixedsize thing bound together by its own internal forces. Coherent things ike that don't experience Hubble Law expansion.) 



#4
Oct212, 10:34 AM

P: 108

Olber's Paradox Cartoon
Not to worry Marcus! Having read through your posts in the balloon analogy sticky thread, when the narrator of the otherwise excellent video said "moving" I automatically substituted the word "receding".



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