Topics Space and the universe

Why is space dark?

Space looks dark because it is almost empty. Light only becomes visible to your eyes when it travels into them, either straight from a source or after bouncing off something. On Earth, air molecules scatter sunlight in all directions, and blue light scatters the most (this is called Rayleigh scattering), so the daytime sky glows blue. In space there is almost nothing to scatter the light, so sunlight passes through unseen. You see the Sun as a bright disk and the sky around it as black.

There is a second, older question hiding inside this one: why is the night sky dark when there are so many stars? If the universe were infinite, unchanging and eternal, every line of sight would end on a star, and the sky would blaze. This is known as Olbers' paradox. The main resolution is that the universe has a finite age, so light from beyond a certain distance has not had time to reach us. The expansion of the universe also stretches and dims the light, but that is a smaller, secondary effect.

An episode on this would separate the two puzzles, explain why stars vanish from sunlit photos, and say where details are still debated. bre's hosts are AI, so they can make mistakes, and you can press Talk to ask a question mid-episode.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. Light you can only see when it landsSeeing light needs it to reach your eye. Beams crossing empty space are invisible from the side, which is why a flashlight shows a beam in fog but not in a vacuum.
  2. Why Earth's sky is blueAir scatters sunlight, and short blue wavelengths scatter most. Remove the air and the glow disappears, leaving black around the Sun.
  3. Astronauts, cameras and missing starsPhotos taken in sunlight often show no stars because the exposure is short, set for bright surfaces. The stars are still there; the camera simply did not collect enough of their faint light.
  4. Olbers' paradoxIn an endless, eternal universe full of stars, the night sky should be bright everywhere. Why it is not was a real puzzle for centuries.
  5. A universe with a beginningThe universe has a finite age, so light from very distant stars has not arrived yet. This is the main reason the night sky is dark.
  6. Expansion stretches the lightAs space expands, light is redshifted and dimmed. This adds to the darkness, but it is a smaller effect than the finite age.
  7. What is not fully settledDetails of the early universe and how much light the first stars produced are still studied, and the hosts say so plainly.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Arlo. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Here's a question that sounds silly until you try to answer it. The Sun is blasting out light, so why is the space around it black?

  2. ArloAI host

    Nothing to hit.

  3. breAI host

    Right, but say more. Light doesn't glow on its own as it travels. You only see it when it lands in your eye.

  4. ArloAI host

    So the beam's invisible. Unless something bounces it.

  5. breAI host

    On Earth, air does the bouncing. Blue light scatters most, so the whole sky glows blue. Take the air away and you get a bright Sun in a black sky.

  6. ArloAI host

    And the stars? Why can't I see them in astronaut photos?

  7. breAI host

    Short exposure. The camera is set for bright, sunlit surfaces, so faint stars don't register. They're there.

  8. ArloAI host

    Fine. But the night sky. Billions of stars. Why isn't it bright?

  9. breAI host

    Okay, here's the part nobody tells you: the answer depends on the universe having a beginning. That's where we're headed.

Questions people also ask

Why is space black if the Sun is so bright?
Light is only visible when it enters your eye, either directly from its source or after scattering off something. Space is a near-vacuum, so there is almost nothing to scatter sunlight. You see the Sun itself, but the space around it looks black.
Why is the night sky dark when there are so many stars?
The main reason is that the universe has a finite age, so light from beyond a certain distance has not reached us yet. Expansion also redshifts and dims the light, but that is a smaller effect. Together they keep the sky from blazing.
Why can't you see stars in photos taken in space?
Stars are usually missing from sunlit photos because of camera exposure. The settings are short, to capture bright objects like the Moon or a spacecraft without overexposing them, so faint starlight does not register. The stars are still there.
What is Olbers' paradox?
It is the puzzle that an infinite, eternal, unchanging universe full of stars should have a bright night sky, since every line of sight would end on a star. The universe's finite age is the main resolution, with expansion playing a smaller part.

Related topics

More: all 300 topics, space and the universe, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.