Starts With A Bang

Ask Ethan: Why does space appear black? (Synopsis)

The night sky as seen from Earth. Image credit: Wikimedia Commons user ForestWander, under a c.c.a.-s.a.-3.0 license.

“When you get just a complete sense of blackness or void ahead of you, that somehow the future looks an impossible place to be, and the direction you are going seems to have no purpose, there is this word despair which is a very awful thing to feel.” -Stephen Fry

Perhaps the most fundamental difference between day and night is the difference between light and dark that our eyes perceive. While everything is illuminated during the day, the night sky is completely dark, with the sole exception of the stars, galaxies and objects reflecting sunlight back at our world.

The full UV-visible-IR composite of the XDF; the greatest image ever released of the distant Universe. Image credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI).

You might intuit that this is simply because we can’t gather enough light to see the most distant objects in the Universe, but even if we gather arbitrarily large amounts of light, there are still dark spaces between the galaxies, where no shining objects exist. Indeed, there’s a mathematical theorem that if the Universe were of infinite size and a uniform (even if small) density, every direction you looked would eventually end on a light source.

An illustration of “Olbers’ Paradox”, and how given a uniformly dense Universe, you’d run into an infinite amount of starlight in any direction. Image credit: Wikimedia Commons user Htkym, under a c.c.a.-s.a.-3.0 license.

The resolution lies in two sources: the Big Bang and the limitations of our vision’s wavelength perception. Go get the whole story on this week’s Ask Ethan!