Interesting site about balloon analogy in cosmology:

http://www.phinds.com/balloonanalogy/

<< It is DISTANCE that is changing, not space.>>

Call me simple-minded, but it seems to me that something as small as an individual photon should not be affected by expansion of the universe. And yet, it is accepted that photons are redshifted as time goes by--and of course there is a gigantic amount of data from astronomical devices like microwave telescopes to back this up.

Originally posted by Paul Dirac II
<< It is DISTANCE that is changing, not space.>>

Call me simple-minded, but it seems to me that something as small as an individual photon should not be affected by expansion of the universe. And yet, it is accepted that photons are redshifted as time goes by--and of course there is a gigantic amount of data from astronomical devices like microwave telescopes to back this up.

It is stretched out to the extent of the expansion of the universe, so the further back in time you go to the beginning, it can get stretched out 8 times its original wavelength so a photon at 100 nm ends up at 800 nm.

Now can anyone answer in simple terms why the universe as a whole does not lose energy in spite of the apparent loss of energy inherent in a stretched photon?

My guess is you are comparing apples to oranges, the original wavelength was 100 nm in a universe 8 times smaller so as a percentage of the volume of space at that time Vs now we have a longer wavelength filling a larger universe so the percentage of volume remains the same so the total energy remains the same.

Does that make sense or am I just spouting nonsense?

Originally posted by sonhouse
... it can get stretched out 8 times its original wavelength so a photon at 100 nm ends up at 800 nm.

In the balloon-with-coins analogy, the coins are like individual galaxies. I can imagine an ant crawling on the expanding balloon, and the ant not getting stretched, just as the coins don't get stretched. If the ant is playing the role of photon, then the photon does not get stretched, eh?

But as I admitted before, the experimental evidence is there that the photons do get redshifted, so my ant analogy must be misleading.

None of this cosmological stuff is intuitive to me.

Originally posted by sonhouse
Now can anyone answer in simple terms why the universe as a whole does not lose energy in spite of the apparent loss of energy inherent in a stretched photon?

You would probably find this page interesting:

http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

The key statement in it is:
"When the space through which particles move is changing, the total energy of those particles is not conserved."