Look up at the night sky with your own eyes, or marvel
at images
of the universe online, and you'll see the same thing: the inky,
abysmal blackness of space, punctuated by bright stars, planets or spacecraft.
But why is it black? Why isn't space colorful, like the blue daytime sky
on Earth?
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| The dark night sky, as seen from Arches National Park in Utah (Credit: Pascal Fraboil / EyeEm via Getty images) |
Surprisingly, the answer has little to do with a lack of
light.
"You would think that since there are billions of stars
in our galaxy, billions of galaxies in the universe and other objects, such as
planets, that reflect light, that when we look up at the sky at night, it would
be extremely bright," Tenley Hutchinson-Smith, a graduate student of
astronomy and astrophysics at the University of California, Santa Cruz (UCSC), said. "But instead, it's actually really
dark."
Hutchinson-Smith said this contradiction, known in physics
and astronomy circles as Olbers' paradox, can be explained by the theory
of space-time
expansion — the idea that "because our universe is expanding
faster than the speed of light … the light from distant galaxies might be
stretching and turning into infrared waves, microwaves and radio waves,
which are not detectable by our human eyes."
And because they are undetectable, they appear dark (black) to the naked eye.
Miranda Apfel, who is also a graduate student of astronomy
and astrophysics at UCSC, agreed with Hutchinson-Smith. "Stars give off
light in all colors, even colors not visible to the human eye, like ultraviolet or
infrared," she said. "If we could see microwaves, all of
space would glow." Apfel said this is because the cosmic microwave
background — light energy from the Big Bang that
was scattered by protons and electrons existing during the early universe —
still fills all of space.
Another reason interstellar and interplanetary space appear
dark is that space is a nearly perfect vacuum. Recall that Earth's sky is blue
because molecules that make up the atmosphere, including nitrogen and oxygen, scatter a lot of
visible light's component blue and violet wavelengths from the sun in all
directions, including toward our eyes. However, in the absence of matter, light
travels in a straight line from its source to the receiver. Because space is a
near-perfect vacuum — meaning it has exceedingly few particles — there's
virtually nothing in the space between stars and planets to scatter light to
our eyes. And with no light reaching the eyes, they see black.
That said, a 2021 study in The
Astrophysical Journal suggests that space may not be as black as
scientists originally thought. Through NASA's
New Horizons mission to Pluto and the Kuiper Belt, researchers have
been able to see space without light interference from Earth or the sun. The
team sifted through images taken by the spacecraft and subtracted all light
from known stars, the Milky
Way and possible galaxies, as well as any light that might have leaked
in from camera quirks. The background light of the universe, they found, was
still twice
as bright as predicted.
The reasons for the additional brightness, which remain unknown, will be the focus of future studies. Until then, one thing seems likely: Space could very well be more "charcoal" than pitch-black.