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When you look up at the night sky, it's easy to think that the universe is a never-ending sea of blackness. But if you measured the visible light from all of the luminous celestial bodies out there, what would the average color of the universe be?
Let's get this out of the way first: It's not black.
"Black is not a color," Ivan Baldry, a professor at the Liverpool John Moores University Astrophysics Research Institute in the U.K., told Live Science. "Black is just the absence of detectable light." Instead, color is the result of visible light, which is created throughout the universe by stars and galaxies, he said.
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In 2002, Baldry and Karl Glazebrook, a distinguished professor at the Centre for Astrophysics and Supercomputing at the Swinburne University of Technology in Australia, co-led a study published in The Astrophysical Journal that measured the light coming from tens of thousands of galaxies and combined it into a singular spectrum that represented the entire universe.
In doing so, the pair and their colleagues were able to work out the average color of the universe.
The cosmic spectrum
Stars and galaxies emit waves of electromagnetic radiation, which is separated into different groups based on the length of the waves emitted. From shortest to longest wavelength, the groups include gamma-rays, X-rays, ultraviolet light, visible light, infrared radiation, microwaves and radio waves.
Visible light makes up a tiny portion of the electromagnetic spectrum in terms of the range of wavelengths, but it is the only part the naked eye can see. What we perceive as colors are actually just different wavelengths of visible light; reds and oranges have longer wavelengths, and blues and purples have shorter wavelengths.
The visible spectrum of a star or a galaxy is a measure of the brightness and wavelengths of light that the star or galaxy emits, which, in turn, can be used to determine the average color of the star or galaxy, Baldry said.
In 2002, Australia's 2dF Galaxy Redshift Survey — which was the largest survey of galaxies ever carried out at the time — captured the visible spectra of more than 200,000 galaxies from across the observable universe. By combining the spectra of all these galaxies, Baldry and Glazebrook's team was able to create a visible light spectrum that accurately represented the entire universe, known as the cosmic spectrum.
The comic spectrum "represents the sum of all the energy in the universe emitted at different optical wavelengths of light," Baldry and Glazebrook wrote in a separate non-peer-reviewed online paper in 2002 based on their discovery. The cosmic spectrum, in turn, allowed them to determine the average color of the universe.
Color conversion
The researchers used a color-matching computer program to convert the cosmic spectrum into a single color visible to humans, Baldry said.
Our eyes have three types of light-sensitive cones, each of which helps us perceive a different range of visible light wavelengths. This means that we have certain blind spots where we cannot properly register certain colors of wavelengths between these ranges, Baldry and Glazebrook wrote in their online paper. The colors we see also depend on what our reference for white light is as we are observing an object. For instance, the color of an object may appear different in a brightly lit room compared with the outdoors on an overcast day.
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However, the CIE color spaces, created by the International Commission on Illumination in 1931, compensate for our visual limitations by attributing a color to different wavelength combinations as seen by a standardized human observer, which is what the team's computer models used.
The team determined that the average color of the universe is a beige shade not too far off from white. Although this is a rather boring finding, it is not a surprising one, considering that white light is the result of combining all the different wavelengths of visible light and the cosmic spectrum includes such a wide range of wavelengths.
The new color was eventually named "cosmic latte," based on the Italian word for milk, after a poll of the whole research team. Other suggestions included cappuccino cosmico, Big Bang beige and primordial clam chowder.
Unshifting the red
A key concept of the cosmic spectrum is that it represents the light of the universe "as originally envisaged," Balrdy and Glazebrook wrote in their online paper. This means that it represents the light as it was emitted throughout the universe, not just as it appears to us on Earth today.
Like all waves, light gets stretched over vast distances because of the Doppler effect. As light gets stretched, its wavelength increases and its color moves toward the red end of the spectrum, known by astronomers as redshift. This means that the light we see is not the same color it was when it was first emitted.
"We removed the effect of redshift from the spectra of the galaxies," Baldry said. "So, it is the spectra of the galaxies when they emitted the light."
Cosmic latte is, therefore, the color you would see if you could look down on the universe from above and see all the light coming from every galaxy, star and gas clouds all at once, Baldry said.
Originally published on Live Science.