<p>This is kind of old, but I've been reading a bit about CMB and stumbled on this question. If you go back to the original discovery of the CMB, they basically pointed their antenna at the sky and measured the microwave signal. They found one that was "isotropic, unpolarized, and free from seasonal variation" having run the antenna from July 1964 to April 1965. To calibrate the instrument they had a liquid helium cooled reference that they could switch into the receiving circuit. Blocking the CMB signal would be difficult because it's at 2.7 K, and most anything you would block it with would be a larger source (i.e. warmer). Instead, they used a liquid He reference source as a calibration, and then accounted for all other sources of microwave signal. The biggest "other" source was from looking through the atmosphere. You can read about this here:
<a href="http://adsbit.harvard.edu/full/seri/ApJ../0142//0000420.000.html" rel="noreferrer">http://adsbit.harvard.edu/full/seri/ApJ../0142//0000420.000.html</a>
<a href="https://www.wmrc.edu/resources/docs/ch1.pdf" rel="noreferrer">https://www.wmrc.edu/resources/docs/ch1.pdf</a></p>

<p>To really see what they did, you'd have to read more about that antenna, and there are references in the paper. The original publication is just a letter and so it is terse and refers to previous results instead of offering a detailed description.</p>

<p>...but if you're in orbit, it's easier to make something cold. For FIRAS on COBE, they actually had a device called XCAL (External Calibrator, I guess) that fit into the CMB receiving antenna like a trumpet mute. It looks like they held it at 2.7K and used it to calibrate by moving it into and out of the microwave receiving horn. That's pretty close to blocking the measurement signal. Here's the reference:
<a href="https://www.researchgate.net/publication/2669262_Calibration_of_the_COBE_FIRAS_instrument" rel="noreferrer">https://www.researchgate.net/publication/2669262_Calibration_of_the_COBE_FIRAS_instrument</a></p>

CC BY-SA 4.0
2020-01-23T04:51:58.657Z
I just ran across your answer today, this is really cool! (pun intended)
uhoh
CC BY-SA 4.0
2018-11-11T18:05:53.380Z
2018-11-11T19:02:33.663Z
2018-11-11T19:02:33.663Z
Mark_Mark_InstrumentTech
<p>I think this is a good question, not necessarily doubting the experimental results, but asking if a simple sanity check that one would do with other kinds of measurements could be done with CMB. </p>

<p>It's really difficult. For light you could wave any convenient-sized blocker in front, even your hand for small apertures. But for this test you'd have to wave something colder than the <a href="https://en.wikipedia.org/wiki/Cosmic_microwave_background" rel="nofollow noreferrer">CMB</a>, which is only about 2.73 degrees above absolute zero. That particular test would be hard.</p>

<p>You could cool something to 4K with boiling liquid helium at 1 atmosphere, then hold it in front and see a <em>uniform</em> temperature appear, and that probably was done during the testing phase before satellites were launched.</p>

<p>However, the CMB has both coarse and fine structure, and so one can point the detector to different places and and see different patterns. On the rotating Earth, you'd have to keep moving the detector to see the same pattern. The pattern correlates only with the direction with respect to the celestial sphere, not with respect to the telescope or Earth. </p>

<p>That's not the same test as blocking the aperture and seeing the signal going away, but it makes a strong case.</p>

<p><em>note:</em> current CMB imaging is done with imaging telescopes such as the <a href="https://en.wikipedia.org/wiki/South_Pole_Telescope" rel="nofollow noreferrer">South Pole Telescope</a> for example (see <a href="https://astronomy.stackexchange.com/a/19984/7982">@zephyr's answer</a>), unlike the early measurements that were done by measuring a few points at a time (per wavelength band), for example <a href="https://en.wikipedia.org/wiki/Cosmic_Background_Explorer" rel="nofollow noreferrer">COBE</a>, <a href="https://en.wikipedia.org/wiki/Wilkinson_Microwave_Anisotropy_Probe" rel="nofollow noreferrer">WMAP</a> and <a href="https://en.wikipedia.org/wiki/Planck_(spacecraft)" rel="nofollow noreferrer">PLANCK</a>.</p>

CC BY-SA 3.0
2017-04-27T19:47:02.427Z
I'm not sure I understand your second paragraph. Why does the thing you're "waving" have to be colder than the CMB to block it? Additionally, you say for light you just wave something in front of your aperture, such as a hand. But the CMB *is* light. What's wrong with just using a hand in that case?
zephyr
CC BY-SA 3.0
2017-04-27T20:16:19.127Z
@zephyr Things that are hotter will likely produce more light at all wavelengths. It would be more like shining a flashlight down the telescope than it would be like holding your hand in front of it, and the resulting measurement would show much more intensity that could simply mask an instrumental artifact, whereas true blocking with something colder than CMB should produce a drop in intensity, showing that the signal wasn't produced internally. Useful shutters should be darker than the scene, not brighter.
uhoh
CC BY-SA 3.0
2017-04-27T20:21:01.960Z
So? The question is simply about showing the CMB is not instrumental noise. Even if you get a signal from your blocking object, it's pretty easy to show this signal is not the CMB as it will have a significantly different look.
zephyr
CC BY-SA 3.0
2017-04-27T20:24:36.897Z
@zephyr it depends whom you are trying to convince. Anyway I am just sticking with the spirit of the question, not saying what I would do if I wanted to do a test.
uhoh
CC BY-SA 3.0
2017-04-27T15:57:25.920Z
2017-04-28T07:45:46.057Z
2017-04-28T07:45:46.057Z
uhoh
<p>It has been tested as being from the sky, by blocking the signal from all other directions than that being measured, by comparing results of directions and by triangulating output form different receivers. When testing a directional signal, changing the direction of the receiver is in effect blocking the signal from the entire sky except for one aperture. They then compare 1.5 years of results.</p>

<p>Different observations have correlated the same patterns in the same zones of the sky, so it's the same as pointing two cameras at the same object and questioning if the object exists or if it's an artifact of the camera.</p>

<p>what is interesting is to wonder what the sea is like, if our own CMB is only one atom of the sea.</p>

<p>The receiver equipment was also carefully calibrated using this kind of terminology:
<a href="https://arxiv.org/abs/1505.08022" rel="nofollow noreferrer"><em>Planck 2015 results. V. LFI calibration(150 additional authors not shown)</em></a>
We present a description of the pipeline used to calibrate the Planck Low Frequency Instrument (LFI) timelines into thermodynamic temperatures for the Planck 2015 data release, covering four years of uninterrupted operations. As in the 2013 data release, our calibrator is provided by the spin-synchronous modulation of the cosmic microwave background dipole, but we now use the orbital component, rather than adopting the Wilkinson Microwave Anisotropy Probe (WMAP) solar dipole. This allows our 2015 LFI analysis to provide an independent Solar dipole estimate, which is in excellent agreement with that of HFI and within 1σ (0.3% in amplitude) of the WMAP value. This 0.3% shift in the peak-to-peak dipole temperature from WMAP and a global overhaul of the iterative calibration code increases the overall level of the LFI maps by 0.45% (30 GHz), 0.64% (44 GHz), and 0.82% (70 GHz) in temperature with respect to the 2013 Planck data release, thus reducing the discrepancy with the power spectrum measured by WMAP. We estimate that the LFI calibration uncertainty is now at the level of 0.20% for the 70 GHz map, 0.26% for the 44 GHz map, and 0.35% for the 30 GHz map. We provide a detailed description of the impact of all the changes implemented in the calibration since the previous data release.</p>

<p><a href="https://i.sstatic.net/glNrR.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/glNrR.jpg" alt=""></a></p>

CC BY-SA 3.0
2017-04-29T09:39:55.030Z
2017-04-29T09:39:55.030Z
bandybabboon
<p>COBE, WMAP and Planck are all spacecraft that have measured the same cosmic microwave background (in increasing detail) from <em>outside</em> the Earth's atmosphere. The same background is found, with similar anisotropies, from different spacecraft using different telescopes, detectors and analysis techniques to remove non-cosmological sources.</p>

CC BY-SA 4.0
2018-11-11T19:08:34.047Z
2018-11-11T19:08:34.047Z
ProfRob
<p>A rather stupid question but I was just curious if the radiation could be virtual.  From the instrument....sort of like virtual particles. In other words if you just build a sensitive microwave detector and stick it in a South African Durban Deep gold mine 5 miles under the earth would it do the same thing? </p>

CC BY-SA 3.0
2017-04-27T14:05:55.073Z
I doubt you'll find a recorded case where someone tested this, mainly because its such a trivial test, its not worth recording. On the flip side, we can say we *know* the CMB is not just instrumental noise because we can measure it from many different instruments over many different times and it still looks the same. If it was just instrument noise, it'd be a colossal coincidence that all that noise looked the same from all those instruments.
zephyr
CC BY-SA 3.0
2017-04-27T19:49:20.927Z
FYI, what you're essentially referring to is taking a [Dark Frame](https://en.wikipedia.org/wiki/Dark-frame_subtraction).
zephyr
<radio-astronomy>
Has the microwave background radiation ever been blocked in order to prove it's really from the sky?
user16724
# Has the microwave background radiation ever been blocked in order to prove it's really from the sky?

(Asked by: username_0 on 2017-04-27)

A rather stupid question but I was just curious if the radiation could be virtual. From the instrument....sort of like virtual particles. In other words if you just build a sensitive microwave detector and stick it in a South African Durban Deep gold mine 5 miles under the earth would it do the same thing?

**Tags:** radio-astronomy

> **Comments:**
> **username_1**: I doubt you'll find a recorded case where someone tested this, mainly because its such a trivial test, its not worth recording. On the flip side, we can say we *know* the CMB is not just instrumental noise because we can measure it from many different instruments over many different times and it still looks the same. If it was just instrument noise, it'd be a colossal coincidence that all that noise looked the same from all those instruments.  
> **username_1**: FYI, what you're essentially referring to is taking a [Dark Frame](https://en.wikipedia.org/wiki/Dark-frame_subtraction).  

---

## 4 answers

### Answer by username_2 on 2018-11-11. Score: 5

This is kind of old, but I've been reading a bit about CMB and stumbled on this question. If you go back to the original discovery of the CMB, they basically pointed their antenna at the sky and measured the microwave signal. They found one that was "isotropic, unpolarized, and free from seasonal variation" having run the antenna from July 1964 to April 1965. To calibrate the instrument they had a liquid helium cooled reference that they could switch into the receiving circuit. Blocking the CMB signal would be difficult because it's at 2.7 K, and most anything you would block it with would be a larger source (i.e. warmer). Instead, they used a liquid He reference source as a calibration, and then accounted for all other sources of microwave signal. The biggest "other" source was from looking through the atmosphere. You can read about this here:
<http://adsbit.harvard.edu/full/seri/ApJ../0142//0000420.000.html>
<https://www.wmrc.edu/resources/docs/ch1.pdf>

To really see what they did, you'd have to read more about that antenna, and there are references in the paper. The original publication is just a letter and so it is terse and refers to previous results instead of offering a detailed description.

...but if you're in orbit, it's easier to make something cold. For FIRAS on COBE, they actually had a device called XCAL (External Calibrator, I guess) that fit into the CMB receiving antenna like a trumpet mute. It looks like they held it at 2.7K and used it to calibrate by moving it into and out of the microwave receiving horn. That's pretty close to blocking the measurement signal. Here's the reference:
<https://www.researchgate.net/publication/2669262_Calibration_of_the_COBE_FIRAS_instrument>

> **Comments:**
> **username_3**: I just ran across your answer today, this is really cool! (pun intended)  

---

### Answer by username_3 on 2017-04-27. Score: 4

I think this is a good question, not necessarily doubting the experimental results, but asking if a simple sanity check that one would do with other kinds of measurements could be done with CMB.

It's really difficult. For light you could wave any convenient-sized blocker in front, even your hand for small apertures. But for this test you'd have to wave something colder than the [CMB](https://en.wikipedia.org/wiki/Cosmic_microwave_background), which is only about 2.73 degrees above absolute zero. That particular test would be hard.

You could cool something to 4K with boiling liquid helium at 1 atmosphere, then hold it in front and see a *uniform* temperature appear, and that probably was done during the testing phase before satellites were launched.

However, the CMB has both coarse and fine structure, and so one can point the detector to different places and and see different patterns. On the rotating Earth, you'd have to keep moving the detector to see the same pattern. The pattern correlates only with the direction with respect to the celestial sphere, not with respect to the telescope or Earth.

That's not the same test as blocking the aperture and seeing the signal going away, but it makes a strong case.

*note:* current CMB imaging is done with imaging telescopes such as the [South Pole Telescope](https://en.wikipedia.org/wiki/South_Pole_Telescope) for example (see [@username_1's answer](https://astronomy.stackexchange.com/a/19984/7982)), unlike the early measurements that were done by measuring a few points at a time (per wavelength band), for example [COBE](https://en.wikipedia.org/wiki/Cosmic_Background_Explorer), [WMAP](https://en.wikipedia.org/wiki/Wilkinson_Microwave_Anisotropy_Probe) and [PLANCK](https://en.wikipedia.org/wiki/Planck_(spacecraft)).

> **Comments:**
> **username_1**: I'm not sure I understand your second paragraph. Why does the thing you're "waving" have to be colder than the CMB to block it? Additionally, you say for light you just wave something in front of your aperture, such as a hand. But the CMB *is* light. What's wrong with just using a hand in that case?  
> **username_3**: @username_1 Things that are hotter will likely produce more light at all wavelengths. It would be more like shining a flashlight down the telescope than it would be like holding your hand in front of it, and the resulting measurement would show much more intensity that could simply mask an instrumental artifact, whereas true blocking with something colder than CMB should produce a drop in intensity, showing that the signal wasn't produced internally. Useful shutters should be darker than the scene, not brighter.  
> **username_1**: So? The question is simply about showing the CMB is not instrumental noise. Even if you get a signal from your blocking object, it's pretty easy to show this signal is not the CMB as it will have a significantly different look.  
> **username_3**: @username_1 it depends whom you are trying to convince. Anyway I am just sticking with the spirit of the question, not saying what I would do if I wanted to do a test.  

---

### Answer by username_4 on 2017-04-29. Score: 2

It has been tested as being from the sky, by blocking the signal from all other directions than that being measured, by comparing results of directions and by triangulating output form different receivers. When testing a directional signal, changing the direction of the receiver is in effect blocking the signal from the entire sky except for one aperture. They then compare 1.5 years of results.

Different observations have correlated the same patterns in the same zones of the sky, so it's the same as pointing two cameras at the same object and questioning if the object exists or if it's an artifact of the camera.

what is interesting is to wonder what the sea is like, if our own CMB is only one atom of the sea.

The receiver equipment was also carefully calibrated using this kind of terminology:
[*Planck 2015 results. V. LFI calibration(150 additional authors not shown)*](https://arxiv.org/abs/1505.08022)
We present a description of the pipeline used to calibrate the Planck Low Frequency Instrument (LFI) timelines into thermodynamic temperatures for the Planck 2015 data release, covering four years of uninterrupted operations. As in the 2013 data release, our calibrator is provided by the spin-synchronous modulation of the cosmic microwave background dipole, but we now use the orbital component, rather than adopting the Wilkinson Microwave Anisotropy Probe (WMAP) solar dipole. This allows our 2015 LFI analysis to provide an independent Solar dipole estimate, which is in excellent agreement with that of HFI and within 1σ (0.3% in amplitude) of the WMAP value. This 0.3% shift in the peak-to-peak dipole temperature from WMAP and a global overhaul of the iterative calibration code increases the overall level of the LFI maps by 0.45% (30 GHz), 0.64% (44 GHz), and 0.82% (70 GHz) in temperature with respect to the 2013 Planck data release, thus reducing the discrepancy with the power spectrum measured by WMAP. We estimate that the LFI calibration uncertainty is now at the level of 0.20% for the 70 GHz map, 0.26% for the 44 GHz map, and 0.35% for the 30 GHz map. We provide a detailed description of the impact of all the changes implemented in the calibration since the previous data release.

[![](https://i.sstatic.net/glNrR.jpg)](https://i.sstatic.net/glNrR.jpg)


---

### Answer by username_5 on 2018-11-11. Score: 1

COBE, WMAP and Planck are all spacecraft that have measured the same cosmic microwave background (in increasing detail) from *outside* the Earth's atmosphere. The same background is found, with similar anisotropies, from different spacecraft using different telescopes, detectors and analysis techniques to remove non-cosmological sources.


---

