In fact, Olbers’ paradox is not explained by finite size but a combination of finite age and expansion. Olbers’ paradox states that if the universe is infinite, any line of light would hit a star at some (possibly very long) distance, and therefore the whole sky should appear as hot as the surface of stars.

The first part of the solution is that light travels at finite speed and the universe has a finite age. Therefore when we look far enough we see no more stars because we are looking at such early times that they had not been formed yet. The Hubble space telescope can actually see some of the first galaxies in the universe, and beyond that there is darkness.

However, if you look even farther, you will eventually reach the time when the universe was around 400000 years old. Its temperature was then roughly 4000 degrees, and at that temperature electrons are no longer bound to atoms and therefore the universe would no longer be transparent. Therefore, in places where our line of sight is not blocked by a star, which should see this hot plasma. Essentially we would therefore be back with Olbers’ paradox. The sky should be very hot.

The ultimate reason why it isn’t is that the universe has been expanding all the time. It is more than thousand times bigger than it was when it was 400000 years old. Because of this, the light coming from that time has been redshifted by a factor of 1000, and consequently the temperature if the plasma appears thousand times lower. The light coming from it is therefore no longer visible to the human eye because it is now microwave radiation, but this “cosmic microwave background” can be detected and measured by telescopes (or in fact analog TVs).

## Comments

eigenvectorcommented on :But we know the universe is finite due to Olbers’ paradox. Surely any 3D object like the universe has a centre?

eigenvectorcommented on :Don’t the words isotropic and finite condradict each other?

strangenesscommented on :… or does the Universe wrap back round on itself? 😀

Arttucommented on :In fact, Olbers’ paradox is not explained by finite size but a combination of finite age and expansion. Olbers’ paradox states that if the universe is infinite, any line of light would hit a star at some (possibly very long) distance, and therefore the whole sky should appear as hot as the surface of stars.

The first part of the solution is that light travels at finite speed and the universe has a finite age. Therefore when we look far enough we see no more stars because we are looking at such early times that they had not been formed yet. The Hubble space telescope can actually see some of the first galaxies in the universe, and beyond that there is darkness.

However, if you look even farther, you will eventually reach the time when the universe was around 400000 years old. Its temperature was then roughly 4000 degrees, and at that temperature electrons are no longer bound to atoms and therefore the universe would no longer be transparent. Therefore, in places where our line of sight is not blocked by a star, which should see this hot plasma. Essentially we would therefore be back with Olbers’ paradox. The sky should be very hot.

The ultimate reason why it isn’t is that the universe has been expanding all the time. It is more than thousand times bigger than it was when it was 400000 years old. Because of this, the light coming from that time has been redshifted by a factor of 1000, and consequently the temperature if the plasma appears thousand times lower. The light coming from it is therefore no longer visible to the human eye because it is now microwave radiation, but this “cosmic microwave background” can be detected and measured by telescopes (or in fact analog TVs).