Gazing at the night sky or witnessing images sent from space, we see a deep, dark expanse adorned with shimmering stars. But why is it so black? Why isn’t space vibrant with colors, just like Earth’s sky?
Many might attribute it to the universe being dimly lit, not bathed in the sun’s radiance. However, the color of space is less related to the lack of light than one might think.
“You’d think there are billions of stars in our galaxy. They reflect light and shine brightly when we look at the night sky,” explained Tenley Hutchinson-Smith, a graduate student in astronomy and astrophysics at the University of California, Santa Cruz (UCSC). “But it’s actually quite dark.”
Hutchinson-Smith referred to this paradox as Olbers’ Paradox, recognized by physicists and astronomers.
It is explained through the theory of space-time expansion, suggesting that “our universe is expanding faster than the speed of light.” This means that light emitted by the sun, or from galaxies receding from us, transforms into infrared, ultraviolet, and radio waves—things our human eyes can’t see. This is why space appears dark to our naked eyes.
Miranda Apfel, another graduate student in astronomy and astrophysics at UCSC, believes that if humans could somehow see ultraviolet or infrared rays, space would undoubtedly be filled with light.
This explanation is due to the presence of protons and electrons from the universe’s early days, scattered by the Big Bang, still filling up all of space.
Another reason the space between stars and planets appears quite dark is because it’s close to being a perfect vacuum.
In case you didn’t know, Earth’s sky is blue due to the atmospheric molecules (mostly nitrogen and oxygen) scattering many wavelengths of light, primarily the blue and violet components, from sunlight in all directions.
However, in the absence of matter, light would travel in a straight line from its source. Since space is an almost perfect vacuum—meaning it contains extremely few matter molecules—it’s devoid of a “scattering medium” to disperse light to our eyes. In this scenario, we only perceive a single color: black.
This hypothesis was supported by a study published in early 2021 in the Astrophysical Journal, suggesting that space may not be as dark as we’ve been led to believe.
Through NASA’s New Horizons mission to Pluto and the Kuiper Belt, researchers were able to observe space without the interference of Earth’s or the sun’s light.