# How do CMB photons 'gain energy when they pass through normal regions of space with matter' and 'lose energy when they pass through voids'?

The Space.com article Huge Hole Found in the Universe says:

The gargantuan hole was found by examining observations made using the Very Large Array (VLA) radio telescope, funded by the National Science Foundation.

There is a "remarkable drop in the number of galaxies" in a region of sky in the constellation Eridanus, Rudnick said.

The region had been previously been dubbed the "WMAP Cold Spot," because it stood out in a map of the Cosmic Microwave Background (CMB) radiation made by NASA's Wilkinson Microwave Anisotopy Probe (WMAP) satellite. The CMB is an imprint of radiation left from the Big Bang, the theoretical beginning of the universe.

"Although our surprising results need independent confirmation, the slightly colder temperature of the CMB in this region appears to be caused by a huge hole devoid of nearly all matter roughly 6 to 10 billion light-years from Earth," Rudnick said.

Photons of the CMB gain a small amount of energy when they pass through normal regions of space with matter, the researchers explained. But when the CMB passes through a void, the photons lose energy, making the CMB from that part of the sky appear cooler. (emphasis added)

Question: How do CMB photons gain energy when they pass through "normal regions of space with matter" and lose energy when they pass through voids?

• I'm pretty sure there's already an answer to this question, but maybe it's on Physics.SE. Basically, photons (like everything else) gain energy as they fall into gravity wells and lose energy as they climb out of gravity wells. And then you need to consider the expansion, which is greatest in voids & virtually non-existent inside galaxy clusters. – PM 2Ring Jun 14 '19 at 3:57
• @PM2Ring would passing from "normal" through a void and back to "normal" again have no net gravitational effect? And despite several questions of my own (1, 2, 3) and reading others, I am still confused by the idea that metric expansion only happens in "empty" space (e.g. voids, etc.) If that's what's happening in this case, then I'm a goner and I'll never understand this. – uhoh Jun 14 '19 at 4:37
• would passing from "normal" through a void and back to "normal" again have no net gravitational effect It would, if there were no metric expansion. But since there is expansion, the net effect is to increase the redshift. – PM 2Ring Jun 14 '19 at 4:45
• Wouldn't photons falling into a gravity well and then climbing out again have a greater distance to travel and therefore be slowed down?. After all,the speed of c is constant,so greater distance = greater time. Would this result in 'tired light' if the photons came from distant quasars? They may have had to negotiate numerous gravitational wells on their journey. – Michael Walsby Jun 14 '19 at 10:43
• @MichaelWalsby "After all,the speed of c is constant,so greater distance = greater time." The speed of light is only $c$ locally. Your statement has very limited application in GR. As for "tired light", I have explained how the late time integrated Sachs Wolfe effect results in a net (small) blueshift with respect to the average photon path. – Rob Jeffries Jun 25 '19 at 6:22