Why’s space black instead of pink?
●﹏● OK, let’s go. First, in relation to pink color, this depends on where the starlight would reflect, for example: “Why is the sky blue here on Earth?”. The daytime sky is blue because the sunlight nearby, beats Earth’s atmosphere molecules and disperses in all directions. The blue color of the sky is the result of this dispersal process. At night, when the part of the Earth is far from the Sun, the space looks black, because there is no source of bright light near, like the Sun, to be spread. If you were on the moon, which has no atmosphere, the sky would be black both at night and day.
So the black color usually signals the absence of light, and most of the universe is empty, the outer space looks black. However if the universe is full of stars, why does not the light from all of them add up to make the whole sky bright all the time? It turns out that if the universe was infinitely large and infinitely old, then we would expect the night sky to be bright from the light of all those stars. Every time you look in space you would be looking at a star. Yet we know from experience that space is black! This paradox is known as the Olbers’ Paradox. It is a paradox because of the apparent contradiction between our expectation that the night sky is bright and our experience that it is black.
Many different explanations have been put forward to resolve Olbers’ Paradox. The best solution at present is that the universe is not infinitely old; it is somewhere around 13,8 billion years old. That means we can only see objects as far as the light can travel in 13,8 billion years. The light from stars farther than that has not yet had time to reach us and can not contribute to making the sky bright.
This is an artist’s concept of the metric expansion of space, where space (including hypothetical non-observable portions of the universe) is represented at each time by the circular sections. Note on the left the dramatic expansion (not to scale) occurring in the inflationary epoch, and at the center the expansion acceleration. The scheme is decorated with WMAP images on the left and with the representation of stars at the appropriate level of development. Credit: NASA
Another reason that the sky may not be bright with the visible light of all the stars is because the light source is moving away from you, the wavelength of that light is longer the light from stars that are moving away from us will become shifted towards red, and may shift so far that it is no longer visible at all. (Note: You hear the same effect when an ambulance passes you, and the pitch of the siren gets lower as the ambulance travels away from you; this effect is called the Doppler Effect).