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I wonder if it is worth mentioning that there is a difference in saying that
the *Universe* is expanding (which presumably Doppler shifts the wavelength)
and saying that *space* is expanding (so the wavelength lengthens as well.).
Yes, most might say they amount to the same result, but our students have a
hard time seeing the difference.
Indeed, I find myself wondering what measurement can distinguish between a
distant galaxy's fleeing ours (*into* space) with some Doppler velocity and
a "stationary" distant galaxy with the space expanding between us and it.
Is there any more than the Hubble Red Shift?
And why can't a distant galaxyIt can. For the most part, these local motions are negligible and
have a velocity within the Universe *and* the space of the Universe expand?
How can I tell the difference? I wonder if this is a meaningful question. (:-)
Again, it is not a matter of something expanding outward from a
And there was a question of why the background radiation was visible today:
Mechanism 3) is responsible for red-shifting the wavelengths of the cosmic
background 1000-fold since those photons decoupled from the matter which
produced them so that they have a meager intensity in the microwave band
rather than a bright intensity in the visible one. Without this red-shift
the sky would be very bright everywhere from the cosmic backgound in the
visible wavelengths whether or not there are enough point sources (stars,
galaxies, quasars, etc.) to also make the sky bright.
Well yes, David, that is why the background is at 3K, but why can we *see*
it?. I think the answer is that, if we assume that c has been constant since
the BB (we don't have much of a choice but to assume this), that space has
been expanding with the Universe (an unnerving thought), and that the
expansion of space has been <c (but what if it hasn't?), then the photons
have had more than enough time to fill the Universe no matter what the
mechanism of the BB.
BUT this doesn't answer for me why I can see quasars (or Q's flashlight) --
which is where we were in the Old Star thread. (Maybe I have one of those
senility caused blocks like the one I still have re retrograde motion (:-))