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Why is space so dark even though the universe is filled with stars?

The Milky Way as seen from the Southern Hemisphere.
The Milky Way. (Image credit: Luke Peterson Photography/Getty Images)

People have been asking why space is dark despite being filled with stars for so long that this question has a special name — Olbers' paradox.

Astronomers estimate that there are about 200 billion trillion stars in the observable universe. And many of those stars are as bright as, or even brighter than, our sun. So, why isn’t space filled with dazzling light?

I am an astronomer who studies stars and planets — including those outside our solar system — and their motion in space. The study of distant stars and planets helps astronomers like me understand why space is so dark.

Related: Is there anything beyond the universe?

You might guess it’s because a lot of the stars in the universe are very far away from Earth. Of course, it is true that the farther away a star is, the less bright it looks — a star 10 times farther away looks 100 times dimmer. But it turns out this isn’t the whole answer.

Imagine a bubble

Pretend, for a moment, that the universe is so old that the light from even the farthest stars has had time to reach Earth. In this imaginary scenario, all of the stars in the universe are not moving at all.

Picture a large bubble with Earth at the center. If the bubble were about 10 light-years across, it would contain about a dozen stars. Of course, at several light-years away, many of those stars would look pretty dim from Earth.

If you keep enlarging the bubble to 1,000 light-years across, then to 1 million light-years, and then 1 billion light-years, the farthest stars in the bubble will look even more faint. But there would also be more and more stars inside the bigger and bigger bubble, all of them contributing light. Even though the farthest stars look dimmer and dimmer, there would be a lot more of them, and the whole night sky should look very bright.

It seems I'm back where I started, but I'm actually a little closer to the answer.

Age matters

In the imaginary bubble illustration, I asked you to imagine that the stars are not moving and that the universe is very old. But the universe is only about 13 billion years old.

Galaxies as they appeared approximately 13.1 billion years ago, in a photo captured by the James Webb Space Telescope. (Image credit: NASA/ESA/CSA/STScI/Handout from Xinhua News Agency via Getty Images)

Even though that’s an amazingly long time in human terms, it’s short in astronomical terms. It’s short enough that the light from stars more distant than about 13 billion light-years hasn’t actually reached Earth yet. And so the actual bubble around Earth that contains all the stars we can see only extends out to about 13 billion light-years from Earth.

There just are not enough stars in the bubble to fill every line of sight. Of course, if you look in some directions in the sky, you can see stars. If you look at other bits of the sky, you can’t see any stars. And that’s because, in those dark spots, the stars that could block your line of sight are so far away their light hasn’t reached Earth yet. As time passes, light from these more and more distant stars will have time to reach us.

The Doppler shift

You might ask whether the night sky will eventually light up completely. But that brings me back to the other thing I told you to imagine: that all of the stars are not moving. The universe is actually expanding, with the most distant galaxies moving away from Earth at nearly the speed of light.

Because the galaxies are moving away so fast, the light from their stars is pushed into colors the human eye can’t see. This effect is called the Doppler shift. So, even if it had enough time to reach you, you still couldn’t see the light from the most distant stars with your eyes. And the night sky would not be completely lit up.

If you wait even longer, eventually the stars will all burn out — stars like the sun last only about 10 billion years. Astronomers hypothesize that in the distant future — a thousand trillion years from now — the universe will go dark, inhabited by only stellar remnants like white dwarfs and black holes.

Even though our night sky isn’t completely filled with stars, we live in a very special time in the universe’s life, when we’re lucky enough to enjoy a rich and complex night sky, filled with light and dark.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

The views expressed are those of the author and do not necessarily reflect the views of the publisher.

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Associate Professor of Astronomy, Boise State University

I am an associate professor teaching astronomy in the Physics Department at Boise State University.

Before coming to Boise State, I was a postdoctoral fellow at the Carnegie Institution of Washington’s Dept. of Terrestrial Magnetism in Washington DC and before that, at NASA’s Goddard Space Flight Center in Greenbelt MD.

I earned my PhD in Planetary Science from the University of Arizona‘s Lunar and Planetary Laboratory in Tucson AZ and my BS in Physics from Georgia Tech in Atlanta GA.

My research focuses primarily on orbital dynamics and transit observations of extrasolar planets, planets outside of our solar system. I also do some planetary science field work, notably on Death Valley’s Racetrack Playa and on terrestrial and Martian dust devils.

When I’m not chasing my daughter around the house, I enjoy running around Boise and learning classical guitar.

  • rod
    "People have been asking why space is dark despite being filled with stars for so long that this question has a special name — Olbers' paradox."

    My notes. Could axion decay underlie excess cosmic optical background?,
    ref - New Horizons Observations of the Cosmic Optical Background,, 11-January-2021. "Abstract We used existing data from the New Horizons Long-range Reconnaissance Imager (LORRI) to measure the optical-band (0.4 ≲ λ ≲ 0.9 μm) sky brightness within seven high–Galactic latitude fields..."

    My observation. To calculate excess cosmic optical background, various sources that emit optical light must be calculated too and how much optical light they would release, including factoring redshift too. From the reference paper link, "1. How Dark Does the Sky Get? The simple fact that it is dark at night, known as "Olber's paradox", argues that the universe is finite in time or space (Harrison 1987)..."

    My observation. How dark the night sky is points to a finite size universe with a finite age, thus a distinct beginning.
  • rod
    My note, the article states, "we live in a very special time in the universe’s life". Yes indeed, otherwise we would not see the Universe astronomers do today. Consider cosmology, the beginning features inflation and the inflaton or origin of the solar system in astronomy. A lot of violent catastrophism operating during the beginning of our solar system, for example giant impact with Theia for the origin of the Moon. We would not be here today *living in those times*, so we live in a *very special time* to see nature today.
  • Pogo
    if there were infinite stars and no gas and dust, and no Redshift, the sky would be infinitely bright; solid bodies would probably not form due to the high heat.
    But, there are not an infinite number of stars within the distance of zero or small redshift. Beyond the distance of significant redshift, the light emissions of stars have shifted below the visible spectrum.
  • EmmBee
    This is the old 'Olbers Paradox', and we now know Space is filled with dust and yet undefined mass apparent. Dedicated research is ongoing and theories are several. You may do your own R & D for the fun of discovery !
    You're welcome...:cool:
  • Pogo
    This paradox was referred to in an old astronomy text from 1948 I had when I was a kid. It stated exactly that, infinite stars would necessarily result in infinite light, therefore the sky is finite. It didn’t need to take into account dust and gas, and Redshifting.
  • Questioner
    If the furthest stars/galaxies from us are traveling at near luminal speed wouldn't even further (unseen) stuff be traveling at superluminal speed, making the luminal speed limit a local phenomenon?
  • Classical Motion
    "Why is space so dark even though the universe is filled with stars?"

    1. Space is larger than you can imagine.

    2. All electric fields are self repulsive. This causes light to diverge. It spreads, loses density and loses intensity. It dissolves into space. Light has limited range. Limited range on it's own, no expanding space needed.

    And that's why it looks dark. Actually it's quite bright........because it's a multi-exposure. If we could only see the present would probably be quite dark. And extremely lonely.