article_snippet.sent: For this reason this reason can ‘sap’ the energy of light that is passing through this reason.
article_snippet.sent: To explain how that works, imagine the void to be a hill.
article_snippet.sent: As light enters the void and travels towards its middle, its can be said to be climbing a hill, using up energy.
article_snippet.sent: Once its passes its centre, its would begin ‘descending’ a hill and regain the energy a hill expended to get there.
article_snippet.sent: But as the universe is expanding, by the time the light gets to its centre, the light route out has become ‘shallower’ - or less dense.
article_snippet.sent: This means the light cannot regain as much energy as the light lost initially and leaves the void with less energy, therefore a longer wavelength, which corresponds to a colder temperature.