4

The sky at night is rather black. If it were completely black, it would correspond to a black body at absolute zero. But the sky is not completely black. Is there a way to assign a temperature value to the actual black night sky?

The question is not about the temperature of the air of the atmosphere, nor that of the stars in the sky. The question is about the the temperature that corresponds to the blackness seen by, say, the Hubble telescope; the question is about the pure blackness of the night sky, between the stars, outside the atmosphere of the Earth.

Nela

Nela
  • 43
  • Sort of related: https://physics.stackexchange.com/questions/153839/what-is-the-temperature-of-the-clear-night-sky-from-the-surface-of-earth – PM 2Ring May 01 '19 at 10:58

1 Answers1

11

It depends on what you mean by “sky”. The Earth’s atmosphere has a large range of temperatures at various altitudes, generally getting colder and colder from the surface temperature as you go up.

If what you are actually asking is the temperature of deep space, then it is 2.725 K, the temperature of the cosmic microwave background (CMB), a remnant of the Big Bang that permeates the entire universe. Every cubic centimeter of space has about 400 microwave-frequency photons with the spectrum of a 2.725 K blackbody. This radiation from about 380,000 years after the Big Bang is now very cold (just three degrees above abdolute zero) because the universe has expanded tremendously since then.

The CMB was predicted in 1948 and detected in 1964 by scientists who didn’t know about the prediction and weren’t looking for it.

G. Smith
  • 51,534