Isaac Newton believed gravity demands that the Universe be without a centre or an edge, and of infinite extent in all directions. According to Newton, a finite and bound Universe would 'fall down into the middle of the whole space, and there compose one great spherical mass'. In an infinite Universe, he believed, 'the fixed stars, being equally spread out in all points of the heavens, cancel out their mutual pulls by opposite attractions.'

Olbers' paradox

The Universe as Newton saw it gave rise to a paradox, known as Olbers' paradox after Heinrich Olbers, who raised the issue in 1823. He deduced that in an infinite Universe of infinite age, there would be an infinite number of stars. If you were, therefore, to look in any direction in the sky, your line of sight would eventually hit on a star's surface. Since every direction would lead to a star, and given the absolute luminosity of a star and the inverse square law for the dimming of light with distance, the night sky would be infinitely bright. Olbers' paradox asks the question "why is the night sky dark?"

Solutions to Olbers' paradox

The German astronomer Johannes Kepler had first pondered the problem of the dark night sky in 1610, and came up with his own solution to it: that the Universe of stars extends only to a finite distance, beyond which the viewer encountered only empty space. Of course, this solution prompts the questions: how far away is the boundary and what lies beyond it?

Olbers himself suggested that starlight is gradually absorbed while travelling through space, and that this cuts off the light from any stars beyond a certain distance. This does not, however, solve the problem, because any absorbing gas or dust would simply heat up until the starlight it had absorbed would be reradiated. Ultimately, the energy we would detect as light from Earth would be the same.

It was the American poet Edgar Allan Poe who came up with one of the first scientifically reasonable solutions to the paradox. He suggested that the Universe is not old enough to fill the night sky with light. He reasoned that while the Universe may be infinite in size, there has not been enough time since it first came into being for starlight to reach us from the farthest corners of space.

Astronomers now conclude that the Universe began some 12-15 billion years ago. We are, however, only seeing the part of it that lies within 12-15 billion light years from us, with the rest of the stars beyond our sight. The number of stars whose light reaches us is not enough to fill the sky with light. In addition, astronomers now argue that while the Universe is infinite, there are a finite number of stars filling it, and the expansion of the Universe explains the lack of absolute starlight in the night sky.

Redshift