If approaching rulers look longer than they are (Fig. 1, left), then they must also appear to be faster than they are: Imagine that we take a second picture, on which all rulers have advanced by one place in the line. Then the approaching rulers appear to have moved a longer distance than the...

To be observable at that distance, the energy output of quasars dwarfs every other astronomical event. The most luminous quasars radiate at a rate that can exceed the output of average galaxies, equivalent to one trillion (1012) suns.

There are probably more than 100 billion (1011) galaxies in the observable universe.Most galaxies are 1,000 to 100,000parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs). Intergalactic space (the space between galaxies) is filled with a...