Looking up at the night sky, it’s easy to think that the universe is an infinite black expanse. But if you were to measure the light coming from all the shining celestial objects, what color would the universe be?
Not Black
“Black is not a color,” says Ivan Baldry, a professor at the Liverpool John Moores University’s Astrophysics Research Institute in the UK. “Black is simply the absence of light. Instead, color is the result of visible light, generated throughout the universe by stars and galaxies.”
In 2002, Baldry, along with Karl Glazebrook, a distinguished professor at the Centre for Astrophysics and Supercomputing at Swinburne University of Technology in Australia, led a study published in the Astrophysical Journal. They measured the light from tens of thousands of galaxies and combined it into a peculiar spectrum representing the entire universe. By doing so, they claimed to have determined the average color of the universe.
The Cosmic Spectrum
Stars and galaxies emit electromagnetic radiation, which is divided into different groups based on the wavelength of the emitted waves. From the shortest to the longest wavelength, these groups include gamma rays, X-rays, ultraviolet, visible light, infrared radiation, microwaves, and radio waves.
Visible light makes up a tiny portion of the electromagnetic spectrum, but it is the only part that our eyes can see. What we perceive as colors are different wavelengths of visible light. Red and orange have longer wavelengths, while blue and violet have shorter wavelengths.
The visible spectrum of a star or galaxy is a measure of the brightness and wavelength of the light it emits and can be used to determine the average color of that star or galaxy.
In 2002, Australia’s 2dF Galaxy Redshift Survey, the largest survey of galaxies at the time, captured the visible spectra of over 200,000 observable galaxies from across the universe.
By combining the spectra of all these galaxies, Baldry and Glazebrook’s research group were able to create a single visible light spectrum accurately representing the entire universe, referred to as the cosmic spectrum.
Baldry and Glazebrook wrote in an online paper in 2002 based on their discovery, discussing how the cosmic spectrum allowed them to determine the average color of the universe.
Color Transformation
Researchers used a computer program to convert the cosmic spectrum into a single color visible to humans.
In fact, our eyes have three types of cone-shaped light-sensitive cells, each of which helps us perceive a range of different wavelengths of visible light. This means there are certain wavelengths between these ranges that we cannot accurately register.
The color we see also depends on our reference to white light when observing an object. For example, an object’s color may appear different in a brightly lit room compared to outdoors on a cloudy day.
However, the CIE color space, created by the International Commission on Illumination in 1931, compensates for these limitations in our vision by assigning a color to different combinations of different wavelength colors as seen by a standard human observer, which is what the research group’s computer models used.
The research group identified the average color of the universe to be a color close to but not far from white. While it may be a somewhat unexciting discovery, it’s not surprising since white light is the result of combining all the different wavelength colors of visible light, and the cosmic spectrum includes a range of such wavelengths.
Shifting to Red
A key concept in the cosmic spectrum is that it represents the light of the universe “as originally expected,” as mentioned by Baldry and Glazebrook in their paper.
Like all waves, light is stretched over large distances due to the Doppler effect. As light is stretched, its wavelength increases, and its color shifts toward the red end of the spectrum, known as redshift. This means that the light we see is not the same color as it was when it was first emitted.
Baldry explains: “We removed the effect of redshift from the galaxy spectra. So, that’s the spectrum of galaxies when they emit light.”
Therefore, according to Baldry, Cosmic Latte is the color you would see if you could look down upon the universe from above and view all the light coming from every galaxy, star clusters, and gas clouds at once.