<p>It is the <a href="https://en.m.wikipedia.org/wiki/Sachs%E2%80%93Wolfe_effect" rel="noreferrer">late time integrated Sachs Wolfe effect</a>.</p>

<p>As they travel towards us, apart from the general expansion, photons from the CMB <em>gain</em> energy when they fall into potential wells (where matter is). Of course, they lose it again as they emerge on the other side of the well, but the cosmic expansion means that the well isn't quite as deep by the time that happens. As a result, photons gain a little energy <em>compared to the average CMB sightline</em>.</p>

<p>The opposite happens for voids. Energy is required to enter a void (the potential is higher than average), but all this energy is not retrieved because the potential flattens due to expansion as the photon traverses it. Hence the directions of voids would be a bit cooler.</p>

CC BY-SA 4.0
2019-06-14T12:19:04.020Z
Okay I have my homework for the weekend, thanks! No more cosmology questions for me until I catch up.
uhoh
CC BY-SA 4.0
2019-06-14T16:44:24.533Z
"As a result, photons gain a little energy compared to the average CMB sightline." CMB has less energy now then when it was released shortly after the big bang. So is the situation technically that CMB that traveled though areas with lots of matter lost less energy than CMB that didn't, rather than gained energy?
Acccumulation
CC BY-SA 4.0
2019-06-25T04:50:14.603Z
Excellent answer, read further, brain expanding... thanks!
uhoh
CC BY-SA 4.0
2019-06-14T06:57:41.170Z
2019-06-14T06:57:41.170Z
ProfRob
<p>The Space.com article <a href="https://www.space.com/4271-huge-hole-universe.html" rel="noreferrer">Huge Hole Found in the Universe</a> says:</p>
<blockquote>
<p>The gargantuan hole was found by examining observations made using the Very Large Array (VLA) radio telescope, funded by the National Science Foundation.</p>
<p>There is a &quot;remarkable drop in the number of galaxies&quot; in a region of sky in the constellation Eridanus, Rudnick said.</p>
<p>The region had been previously been dubbed the &quot;WMAP Cold Spot,&quot; 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.</p>
<p>&quot;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,&quot; Rudnick said.</p>
<p><strong>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</strong>, making the CMB from that part of the sky appear cooler. (emphasis added)</p>
</blockquote>
<hr />
<p><strong>Question:</strong> How do CMB photons gain energy when they pass through &quot;normal regions of space with matter&quot; and lose energy when they pass through voids?</p>

CC BY-SA 4.0
2019-06-14T03:57:24.980Z
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
CC BY-SA 4.0
2019-06-14T04:37:41.743Z
@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](https://physics.stackexchange.com/q/251088/83380), [2](https://physics.stackexchange.com/q/268353/83380), [3](https://space.stackexchange.com/q/14921/12102)) 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
CC BY-SA 4.0
2019-06-14T04:45:21.450Z
*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
CC BY-SA 4.0
2019-06-14T10:43:57.240Z
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
CC BY-SA 4.0
2019-06-14T12:54:40.503Z
@MichaelWalsby see the answer from Rob Jeffries. You have the right two things interacting -- expansion of the universe and gravitational potential wells, but it turns out when you do the numbers that the effect is the opposite of what you suggest. The expansion "smooths out" these very large shallow potential wells (and hills) so that coming out of them has less total effect than going in did.
Steve Linton
CC BY-SA 4.0
2019-06-25T06:22:19.937Z
@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.
ProfRob
<cosmic-microwave-background>
How do CMB photons 'gain energy when they pass through normal regions of space with matter' and 'lose energy when they pass through voids'?
uhoh
# How do CMB photons 'gain energy when they pass through normal regions of space with matter' and 'lose energy when they pass through voids'?

(Asked by: username_0 on 2019-06-14)

The Space.com article [Huge Hole Found in the Universe](https://www.space.com/4271-huge-hole-universe.html) 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?

**Tags:** cosmic-microwave-background
> **Comments:**
> **username_1**: 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.  
> **username_0**: @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](https://physics.stackexchange.com/q/251088/83380), [2](https://physics.stackexchange.com/q/268353/83380), [3](https://space.stackexchange.com/q/14921/12102)) 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.  
> **username_1**: *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.  
> **username_2**: 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.  
> **username_3**: @MichaelWalsby see the answer from Rob Jeffries. You have the right two things interacting -- expansion of the universe and gravitational potential wells, but it turns out when you do the numbers that the effect is the opposite of what you suggest. The expansion "smooths out" these very large shallow potential wells (and hills) so that coming out of them has less total effect than going in did.  
> **username_4**: @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.  

---

## 1 answers

### Answer by username_4 on 2019-06-14. Score: 20 | Accepted answer

It is the [late time integrated Sachs Wolfe effect](https://en.m.wikipedia.org/wiki/Sachs%E2%80%93Wolfe_effect).

As they travel towards us, apart from the general expansion, photons from the CMB *gain* energy when they fall into potential wells (where matter is). Of course, they lose it again as they emerge on the other side of the well, but the cosmic expansion means that the well isn't quite as deep by the time that happens. As a result, photons gain a little energy *compared to the average CMB sightline*.

The opposite happens for voids. Energy is required to enter a void (the potential is higher than average), but all this energy is not retrieved because the potential flattens due to expansion as the photon traverses it. Hence the directions of voids would be a bit cooler.

> **Comments:**
> **username_0**: Okay I have my homework for the weekend, thanks! No more cosmology questions for me until I catch up.  
> **username_5**: "As a result, photons gain a little energy compared to the average CMB sightline." CMB has less energy now then when it was released shortly after the big bang. So is the situation technically that CMB that traveled though areas with lots of matter lost less energy than CMB that didn't, rather than gained energy?  
> **username_0**: Excellent answer, read further, brain expanding... thanks!  

---

