| What is Olbers' Paradox, and why was it considered paradoxical? | [2] |
| Olbers' Paradox is that the sky is dark at night. | [0.5] |
| This is paradoxical if the universe is infinitely large, infinitely old, and static, because in such a case one would expect every line of sight eventually to intersect a star. | [1.5] |
| Compare and contrast the resolution of Olbers' Paradox in the Big Bang and Steady State cosmological models. | [3] |
| In both the Big Bang and the Steady State (compare), the universe is expanding, and therefore the "static" condition is not met. Light from distant stars will be redshifted to lower temperatures and so will not make a bright night sky. In the Steady State this is the whole explanation, whereas (contrast) in the Big Bang it applies mainly to the cosmic background radiation. | [2] |
| In the Big Bang, but not the Steady State (contrast), the universe is finite in age, and therefore the "infinitely old" condition is not met. Light from distant stars will not yet have had time to reach us. This is the main effect which prevents starlight from making a bright night sky in the Big Bang model. | [1] |
(2006 Q1.)