See subject. It would be cool (I think) to have in our bodies such receptors.
Paging Dr. Mary
Back in 2002 some astronomers announced that the Universe as a whole averages out to a kind of turquoise color. Alas, it turns out they’d miscalculated, and it’s actually beige. Judging from this Astronomy Picture of the Day page, it’s a pretty pale beige. (Ecru? Eggshell? Apparently God is an Interior Designer.)
The rationale for this color choice is well-detailed in [I]The Cosmic Spectrum and the Color of the Universe*, by Karl Glazebrook & Ivan. They held a contest for what to name the color (I liked Skyvory).
But they smooched together all the spectra, which had been de-redshifted to boot.
But aren’t stars relatively minuscule matter in the vastness of the Universe? Doesn’t the overall frequency keep on humming?
Band name!
It looks like Skyvory is based solely on visible light.
However, the OP mentions background radiation, which is not visible. It doesn’t make much sense to talk about the “color” of gamma rays, but is there any analysis that includes non-visible wavelengths?
Well, sure. People talk about the color temperature of light sources all the time. This notion of the “color” of EM waves is pretty easily extended out of the visible spectrum; one could therefore say that the color temperature of the cosmic microwave background is 2.7 Kelvin. (Of course, you still can’t see very much of it at all with your eyes, but hey, at least we’ve quantified it.)
According to Dr. SETI, the background cosmic radiation is 279.5 GHz and the visible color spectrum ends at about 430 Thz so the wavelength is about 1300 time too long to see as color and about 3000 times to short to hear on your FM radio. This puts it smack dab in the middle of microwaves.
However, we could derive a theoretical color from the thermal radiation function I(L) = (2hc^2) / (L^5 x [e^(hc/kLt) - 1). This is pretty close to 0 so the light intensity is very small. Making a quick excel spreadsheet from 400 to 800 Thz, the intensity goes from 8.73 ^ 10^-73 J for near infrared to 5.46 x 10^-74 J for near ultraviolet and if I took the average correctly, that would be 535 Thz which is
green just starting to trend to yellow
The original determination of the star-spectrum-smooched-together JHU team was a greenish hue. I don’t know what caused them to change their result to Skyvory.
But I think Saint Cad’s analysis has more validity. Doesn’t it?
Or is one derivation independent of the other entirely? Does Saint Cad’s figures (which were requested in OP) include/disregard/conflict with JHU’s?
Of course it has a “color” in the generalized sense, because that just means the distribution of energy in electromagnetic modes isn’t uniform. It has at least one peak at a certain wavelength and at least one valley at another.
For this to not be true would require the universe to be either black – no radiation at all – or white – equal radiation at every wavelength, which is impossible if it is to have a finite energy.
As for what that color is, it depends on how you do your averaging. The simplest way would be to average over the surface of the sky, in which case the color is “microwave,” corresponding to the color of a black body at 2.7K. I realize “microwave” is a strange color name, and many of you Earthlings cannot experience colors at such low frequencies, but that’s the color it is, or rather, that’s the English translation of fnrzork, which is the name of the color in the language of a species that can experience it with their electromagnetic receptors.
IANAQM but my belief is that a blackbody object gives off EM across a spectrum of frequencies according to Plank’s Law. That being the case, 2.7K does give off some EM in the visible spectrum range but at such a low intensity that we cannot see it.
Assuming that, the question then become that knowing the equation of the curve at the visible spectrum, what color would it look like. I actually feel my solution i too simplistic and that to talk about what color we would see, that curve would have to be run through the filters of our cones and the RGB receptors (and for the tetrachromatic women RGBV) and then average in the same manner our brain averages out the three signals.
However I think my post is enough to answer the question “Does the cosmic background radiation have a color?” Yes, but our eyes are not sensitive enough to see it.
So… now we know what HP Lovecraft’s Color Out of Space was. Just good old mind-bendingly evil microwaves 
Only blackbody sources will really have a well-defined “color temperature”, though. The cosmological background is an excellent blackbody, and thus can be said to have a very definite color temperature of approximately 2.7 K, but if you take the total photon content of the Universe, the spectrum is not a blackbody (it’s a combination of many different blackbodies, plus some non-thermal components like synchrotron radiation), and so its overall color temperature would depend on how you define color temperature. The usual method in astronomy is to record intensities in two different filters, take the ratio of those two intensities, and consider the “color temperature” to be the temperature of a blackbody which would produce that same ratio, but if your source is anything other than a true blackbody, the temperature you get out of this will depend on which two filters you happen to choose.
And “color” is not a complete description of the spectrum of a light source, anyway. A mixture of a monochromatic red and a monochromatic green light source would look the same (to human eyes, at least) as a single beam of monochromatic yellow, but would have a completely different spectrum. True spectra inhabit an infinite-dimensional “color space”, but the color space humans perceive is only three-dimensional, and can be regarded at best as a projection of that infinite-dimensional space. You know those optical illusions that ask “Is this the underside or overside of a staircase”? That’s the kind of ambiguity you can get just from stepping down one dimension in a projection-- Imagine the ambiguities in a step of an infinite number of dimensions.
Cronos, I don’t think I understand your color apprehension/staircas metaphor (although being monocular–like this ;)–, the staircase phenomenon is routine for me).
Is your post a generalization of the JHU “result”? Sol is gold/yellowish to our eye instruments at 1AU, but not to instruments on other stars. Or am I wrong? Otherwise, what gives?
So JHU took whose instruments to shmoosh all these frequencies from a giant survey of stars, and step them down to our eye instruments? This follows from an assumption that a red giant will be “red” in our eye instruments at 1(?)AU from it?
Second, JHU and 2.7 K (bearing in mind the thrust of your post) are apples and oranges?
I now see what I’m dealing with in the larger question I am (poorly) trying to articulate, and perhaps via these two cases–ways of “looking”–I will dip my toe into Olber’s Paradox.
I am an absolue ignoramus, but the Skyvory stuff doesn’t sit well with me.