Google is one of the biggest tech companies in existence, but its empire is built on one little white search bar, and the US Justice Department has just launched what might be one of the biggest challenges it’s ever faced. In one of the largest antitrust trials in recent memory, the government argues that Google owes its dominance not simply to a good design but to a series of coercive deals that have let the search engine market stagnate — while Google complains it’s being punished…