What Color is the Sky in Your World?

That used to be my favorite expression to tell someone I thought they were full of it. Turns out it's a scientifically valid question after all. Only instead of the sky, astronomers are interested in determining the color of the entire Universe. This may seem like asking what does the Moon taste like, but I can assure you that the collective astronomy community has not spontaneously developed a case of synesthesia.

The medium astronomers work with most is light. In fact, it's pretty much the only medium we have to work with. Everything in the Universe emits some form of electromagnetic radiation and some of that radiation makes it way to Earth and is detected by our telescopes. In addition to the stars in the night sky, there are an unfathomable number of other astronomical objects out there that we have detected and observed in some detail - most notably hundreds of thousands of galaxies, made up themselves of billions of stars and numerous conglomerations of gas and dust known as nebulae.

On a bit of a lark, astronomers decided to take a representative sample of 200,000 galaxies to determine the dominant wavelength of their composite light. The phenomenon we call light comes in a wide range of frequencies known as the electromagnetic spectrum. It runs the gauntlet from low energy, long wavelength radio waves through the canonical rainbow to gamma rays, the highest form of energy currently detectable. However, color is a property unique to optical wavelengths.

So to determine the color of the Universe, astronomers looked at this sample of galaxies through just the narrow window of the visible spectrum and obtained an aggregate signal that can be represented by either a plot of the spectral energy distribution, or by the crowd favorite: a rainbow.



Astronomers prefer the former, but we definitely appreciate the appeal and the accessibility of the latter.

However, rainbows can be misleading. If your eyes were to take in all the light from these 200,000 galaxies at once, you would not see a rainbow, but rather the sum of its constituent wavelengths. It's like how we view sunlight; it appears essentially white as it pours in through a window, but place a prism in just the right spot on the window sill and you can extract its signature rainbow.

Now, imagine a room where the light of the Universe shines through the window. Your eyes would most likely interpret the color of the Universe as white as well, but us astronomers prefer to be more specific. Calculating the exact location of the color in a CIE chromaticity diagram, an international standard for primary colors, prescribes the color of the Universe to be...

...........drumroll please...........

BEIGE! Beige? Yes, beige.


I admit I was hoping for a color with a bit more pizzazz. But then again, the pizzazz is that I no longer ask the question in jest. The sky in my world is beige, thank you very much.

(Image credits:http://www.pha.jhu.edu/~kgb/cosspec/)