# Future of the cosmic microwave background pattern

Does the comoving size and the observed angular diameter of CMB hot and cold patches change with time in the future?

• FWIW, here is a good article about the CMB and cosmic structure from the ESA site. May 3 '19 at 8:30

Yes, the CMB will look different in the future, but that will be in the far future.

Basically the CMB radiation that reaches us today is the CMB radiation that was sent out at a specific radius from us (where the photons had just enough time to reach us today). If we wait longer, then we will recieve the CMB radiation from a slightly larger distance from us.

If we wait long enough then the CMB sky we see will be completely different from the CMB sky we see today. However, the underlying statistics in comoving space (power spectrum etc.) of the CMB will be the same, we will just see another sample drawn from the same statistical distribution.

So yes, the distribution of the comoving sizes of hot and cold patches will stay the same (this follows from the CMB power spectrum being the same), but the positions of hot and cold clumps on the sky will be different.

Edit: Changed the initial phrasing where I incorrectly stated that the angular sizes stay the same. Comoving sizes of the hot and cold-patches will stay the same, but the angular sizes will decrease as the distance from us to the observed CMB will grow. The CMB angular power spectrum will thus be shifted towards higher values of $$l$$.

• Ok, I see what you are asking. Yes, the distribution of the angular sizes of hot and cold patches will stay the same (this follows from the CMB power spectrum being the same), but the positions of hot and cold clumps on the sky will be different.
– Ihle
May 3 '19 at 8:28
• I was a little surprised at this. The CMB always shows the universe as it was at the point of recombination (when photons and matter decoupled). Obviously, once the universe is 20Gy old, we will be looking at different hot and cold patches than we are now at 13Gy. Those patches will however have been (roughly) the same absolute size - because the universe is homogeneous at a large enough scale. /cont May 3 '19 at 8:41
• You might expect that the patches would have a smaller angular size in the future because we are seeing the same size features from further away. However that is to ignore the expansion of the universe. It will have had longer to operate, and will expand the patches back to the same angular size. May 3 '19 at 8:43
• No, your initial statement was correct, the patches will be smaller, see edit. Thanks for the comment!
– Ihle
May 3 '19 at 8:57
• @Martin Good point! The comoving sizes will be the same (of course), but the angular sizes should shrink. I think. I'm getting dizzy trying to visualize this. ;) May 3 '19 at 9:03