A US judge ruled on Monday that Google is a monopoly that has been using its search dominance to stifle competition in a major decision that could shake up big tech. The judge’s decision came nearly a year after the start of a trial pitting regulators against Google in the country’s biggest antitrust showdown in decades. “After having carefully considered and weighed the witness testimony and evidence, the court reaches the following conclusion: Google is a monopolist, and it has acted as one to maintain its monopoly,” US district judge Amit Mehta wrote in a 277-page ruling. It represents a ma…