Why Is Space Black? Olber’s Paradox and the Cosmic Night Sky

Why Is Space Black Olber's Paradox
Space is black because not all of the light from the stars reaches us, due to the expanding universe and the fact that it is finite.

Have you ever pondered why space is black? This profound question lies at the heart of a centuries-old enigma known as Olber’s Paradox. Olber wondered, if the universe is infinite and full of stars, why we don’t see a uniformly bright night sky. Olber’s question led to investigations into the nature of the Universe that resolved his paradox. Basically, we don’t see a sky filled with light because the universe is not infinite.

Olber’s Paradox

The story of Olber’s Paradox takes us back to the early 19th century, to a German astronomer named Heinrich Wilhelm Matthias Olber. Although the concept was discussed by earlier thinkers like Johannes Kepler and Edmond Halley, it was Olber who popularized the paradox. His inquiry revolved around the seeming inconsistency of a night sky filled with infinite stars and the darkness we observe. If the universe is infinite and filled with an infinite number of stars, every line of sight should eventually end on a star, making the night sky uniformly bright.

However, the night sky is not a bright tapestry of infinite stars. It is predominantly black, punctuated only by the distant light of far-off stars and galaxies. To understand this, we delve into the concepts of the age and expansion of the universe.

Why Is Space Black? The Reasons

Key reasons for the blackness of space include:

  • The finite age and size of the universe
  • The expansion of the universe
  • The Doppler effect and cosmic microwave background radiation
  • The insufficiency of interstellar dust to light up the universe

A Finite Universe

Olber thought the universe was infinite, but it turns out it’s not. The universe is finite in terms of its age; it’s about 13.8 billion years old. Consequently, we only see objects as far away as the distance light has been able to travel during this time, effectively creating an observable “sphere” around us. This limits the number of stars we observe and thus the amount of starlight reaching us.

An Expanding Universe

Next, consider the universe’s continual expansion since the Big Bang. This expansion redshifts the light from distant galaxies due to the Doppler effect, moving it to longer wavelengths. Much of the distant starlight shifts into the infrared spectrum, which is invisible to the human eye. This phenomenon further decreases the brightness of the sky.

There Is Light, but We Can’t See Microwaves

Cosmic microwave background radiation (CMBR) also plays a crucial role. This remnant energy from the Big Bang exists everywhere in the universe and it provides a cosmic backdrop to all other stellar observations. However, this radiation exists in the microwave part of the spectrum and is thus invisible. If we look at space in the microwave portion of the spectrum, it actually is illuminated in all directions.

Dust Doesn’t Help

You might think that the interstellar dust scattered throughout space reflects enough starlight to light up the sky. However, dust absorbs more light than it scatters. Even if there were enough dust to scatter light uniformly in all directions, it would dim distant stars more than it would illuminate the sky, maintaining a dark cosmos.

In summary, the enigma of a black space or Olber’s Paradox is reconciled by the finite age and size of the universe, the universe’s expansion causing redshift, the cosmic microwave background radiation, and the insufficiency of interstellar dust. The paradox serves as a fascinating gateway to understanding the large-scale structure and history of the universe, and the reasons why, when we look up at the sky, we are greeted by the awe-inspiring spectacle of a black, star-studded night.

Why Space Is Black Around the Sun

A related question people ask is why space is black surrounding the Sun. Once we get beyond the Earth’s atmosphere, we intuitively expect to see a bright sky filled with sunlight. However, space around the Sun appears black. The reason for this is actually rather simple.

The key to understanding this phenomenon lies in the nature of light and how we perceive it. For us to see light, it either enters our eyes directly or else gets reflects off a surface into our eyes. In space, there’s no atmosphere or large amounts of particulate matter to scatter sunlight like there is on Earth. On Earth, the blue sky is the result of this scattering phenomenon, known as Rayleigh scattering. In Rayleigh scattering the shorter (bluer) wavelengths of light scatter more than the other colors as sunlight passes through the atmosphere.

But. in space there’s nothing substantial for the sunlight to bounce off of before reaching our eyes. Therefore, even though space is full of sunlight, it appears black because there’s no medium like an atmosphere to scatter the light in all directions for us to see. In essence, if you’re in space and you look away from the Sun, you’re looking into the vast universe. You see blackness until your eyes land on a distant star or planet that’s reflecting sunlight.


  • Harrison, Edward Robert (1987). Darkness at Night: A Riddle of the Universe. Harvard University Press. ISBN 9780674192713.
  • Unsöld, Albrecht; Baschek, Bodo (2001). The New Cosmos: An Introduction to Astronomy and Astrophysics. Physics and Astronomy Online. Springer. ISBN 9783540678779.
  • Wesson, Paul (1991). “Olbers’ paradox and the spectral intensity of the extragalactic background light”. The Astrophysical Journal. 367: 399–406. doi:10.1086/169638
  • Zamarovský, Peter (2013). Why is it Dark at Night? Story of Dark Night Sky Paradox. AuthorHouseUK. ISBN 978-1491878804.