I am looking for the author and title of a story about a phase ship that departs the Earth against orders. They visit a few planets meeting alien races and empty planets and then return home to possible court martial.
This is Mission to Universe by Gordon R. Dickson.
This review mentions the "phase ship" and a hijacking.
Four! Billion! People! share an overcrowded Earth. More than fifty nations have armed themselves with nuclear weapons of stupendous power. Nobody wants a nuclear war, but nobody can see a way to safely disarm. All they can do is watch each other nervously while waiting for some crisis to trigger the final war.
Benjamin Shore thinks America’s newly developed phase ship, able to bridge interstellar distances in an instant, could be humanity’s salvation. Habitable worlds around other stars could provide a much needed release valve for Earth’s population. Shore’s superiors disagree; they worry that the phase ship could itself be the spark needed to set off World War Three.
Following the delivery of covert orders from Washington, Shore hastily assembles a crew and takes the Phase Ship Mark III into deepest space. Once there, he reveals a heavily redacted copy of their orders: to boldly go search the sky until the Phase Ship Mark III finds a new home for humanity!
What Shore’s crew cannot know is that Shore’s redactions conceal the truth; the President never ordered the ship into space. The mission is a fraud.